LNG Cargo Properties Calculator

Calculation Standards — ISO vs GPA

This calculator supports two measurement-standard families, selectable via the Calculation Standard dropdown:

• ISO — International uses ISO 6578:2017 (liquid density via Klosek-McKinley) together with ISO 6976:2016 (molar gross calorific values, summation factors, and real-gas compression factor). The combustion and metering reference temperatures are user-selectable, matching the flexibility required by international LNG Sale and Purchase Agreements. This is the correct standard for cargoes discharged or loaded at most non-US terminals.
• GPA — USA & North America uses GPA Midstream Standard 2145-16 (Table of Physical Properties for Hydrocarbons and Other Compounds of Interest to the Natural Gas and Natural Gas Liquids Industries, Revised 2016), in which all gas-phase properties are referenced to 60 °F (15.55 °C) and 14.696 psia. This is the industry-standard reference set used by US LNG export terminals (Sabine Pass, Corpus Christi, Cameron, Freeport, Cove Point, Plaquemines, Elba Island, Port Arthur) and by downstream US gas trading, as prescribed by ASTM, API, and NAESB. Volumetric heating values and Wobbe Index are reported natively in BTU/SCF, and the total energy is reported in MMBTU.

Both standards use the same Klosek-McKinley method and the same ISO 6578 Table B.2 orthobaric molar volumes for LNG density calculation — GPA 2145-16 does not provide cryogenic liquid-phase data at LNG temperatures, so the ISO liquid density tables are used regardless of the selected standard. The practical differences between ISO and GPA therefore arise from:

1. Ideal-gas gross calorific values (Hvᵢ). GPA 2145-16 lists values in Btu/ft³ at 60 °F, 14.696 psia, based on NIST REFPROP 9.1 and WTT data. Converting to kJ/mol and comparing to ISO 6976:2016 Table 3 at 15.55 °C shows differences of typically 0.01–0.03 % per component. These small differences compound into a real MMBTU difference at cargo scale (tens to a few hundred MMBTU on a typical 3.8 million-MMBTU cargo).
2. Summation factors for the compression factor Z. GPA gives sᵢ values (unitless form Z = 1 − (P/P₀)·b²) at 60 °F that differ slightly (third/fourth significant figure) from ISO 6976 Table 2 at 15.55 °C, producing a marginally different Zmix and therefore a marginally different real-gas Wobbe Index.
3. Reference conditions. GPA fixes combustion and metering at 60 °F. ISO allows any combination of 0, 15, 15.55, 20, and 25 °C combustion references with 0, 15, 15.55, or 20 °C metering references.

Molar masses are derived from IUPAC 2007 atomic weights (C = 12.0107, H = 1.00794, N = 14.0067, O = 15.9994, S = 32.065) in both standards and are identical to five decimal places.

When reconciling a US terminal Certificate of Quality, select GPA. When reconciling a European, Asian, or Middle Eastern Certificate of Quality, select ISO and confirm the combustion/metering reference temperatures specified in the applicable Sale and Purchase Agreement.

This tool does not reproduce or distribute the underlying standards documents. Users requiring the full text and formal specifications should obtain ISO 6578:2017, ISO 6976:2016, and GPA Midstream 2145-16 from the respective publishers.

Overview

This calculator determines the properties of an LNG cargo from three measured inputs: liquid temperature, liquid volume, and chemical composition. The core calculation — density, mass, and energy content — follows ISO 6578:2017 using the Klosek-McKinley method for liquid density and physical property data from ISO 6976:2016 (default) or GPA 2145-16 (when GPA is selected). The calculator additionally provides the Equilibrium Vapor Pressure, normal boiling point at 1 atm, compression factor, real-gas Wobbe Index, total gas volume equivalent (Sm³ or SCF) produced on regasification, and the LNG-to-gas expansion ratio. The mass-basis gross heating value (Hm) is reported in the unit systems most commonly used in LNG commercial contracts: MJ/kg, kWh/kg, MMBTU/tonne, and Btu/lb.

Two reference temperature selections control the gas quality outputs: the combustion reference temperature determines the gross calorific value used in energy and Wobbe calculations, while the metering reference temperature determines the gas volume basis and the compression factor used in Wobbe Index and volumetric GHV. Neither affects the liquid-phase calculations (density, mass, EVP). When GPA is selected, both references are fixed at 60 °F per the standard.

Step 1 — Average Molar Mass

The molar mass of the mixture is the mole-fraction weighted sum of each component's molar mass (ISO 6976:2016, Formula 5):

Mmix = Σ(xᵢ · Mᵢ)

where xᵢ is the mole fraction and Mᵢ is the molar mass (kg/kmol) of component i. Molar masses are derived from IUPAC 2007 atomic weights (C = 12.0107, H = 1.00794, N = 14.0067, O = 15.9994, S = 32.065) and carried to 5 decimal places. For a typical LNG mixture dominated by methane, the average molar mass falls in the range of 16–20 kg/kmol.

Step 2 — Molar Volumes at Cargo Temperature

Each component's orthobaric (saturated liquid) molar volume at the observed cargo temperature is obtained by linear interpolation from ISO 6578:2017, Table B.2. This table covers temperatures from −167.15 °C to −155.15 °C at 2 K intervals for methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane, and nitrogen.

For hexane and heavier components (C6+), Table B.2 does not provide molar volume data. This calculator extrapolates from the n-alkane homologous series using V(C6) ≈ 2·V(nC5) − V(nC4), which exploits the regular CH₂ increment (~0.0146 m³/kmol) between successive n-alkanes. At typical C6+ concentrations (0–0.1 mol%), this introduces negligible error (< 0.005% on density).

CO₂ is solid at LNG temperatures (it freezes at −56.6 °C) and O₂ is liquid (boiling point −183 °C). Neither is covered by Table B.2. Their molar volumes are estimated from solid CO₂ density (~1,562 kg/m³) and liquid O₂ density (~1,050 kg/m³). These components are typically present at < 0.05 mol% in commercial LNG, so their density contribution is negligible regardless of the estimate used.

Nitrogen's molar volume is notably more sensitive to temperature than the hydrocarbons, reflecting its proximity to the critical point at LNG conditions.

Step 3 — Volume Correction (Klosek-McKinley)

The revised Klosek-McKinley method (ISO 6578:2017, §8.3) accounts for the non-ideal reduction in volume when LNG components are mixed. Real liquid mixtures occupy less volume than the simple sum of their pure-component volumes — this excess volume must be corrected to obtain accurate density.

The correction uses two factors, k₁ and k₂, obtained by bilinear interpolation from Tables C.1 and C.2 based on the mixture's average molar mass and cargo temperature:

C = k₁ + (k₂ − k₁) · (x₂ / 0.0425)

Vc = C · x₁

where x₁ = methane mole fraction, x₂ = nitrogen mole fraction, and Vc is the volume correction in m³/kmol. The factor k₁ captures the effect of heavier hydrocarbons; k₂ adds the additional correction due to nitrogen. The reference value 0.0425 is the nominal upper limit of nitrogen mole fraction for which the correlation was developed.

Step 4 — LNG Density

Density at the observed cargo temperature (ISO 6578:2017, Formula 9):

ρₜ = Σ(xᵢ · Mᵢ) / [ Σ(xᵢ · Vᵢ) − Vc ]

The numerator is the average molar mass; the denominator is the corrected average molar volume. Result is in kg/m³. Typical LNG densities range from 420 kg/m³ (very lean, high-methane LNG) to 480 kg/m³ (rich LNG with significant ethane/propane content).

Step 5 — Cargo Mass

Mass is calculated from volume and density (ISO 6578:2017, Formula 1):

m = V · ρₜ

where V is the measured liquid volume in m³ at the observed cargo temperature (not corrected to standard conditions) and ρₜ is the density at that same temperature. Result is in kg.

The calculator uses the full unrounded density value from Step 4 in this multiplication, consistent with ISO 6578:2017 §8.2 and the GIIGNL LNG Custody Transfer Handbook: intermediate values in a calculation chain must carry full precision, and rounding occurs only at the final reporting stage. A 145,000 m³ cargo at 428.3154 kg/m³ yields a mass of 62,105.7 tonnes; if the density were instead rounded to the displayed 428.3 kg/m³ before multiplication, the mass would change by about 2.2 tonnes — a deviation without physical basis.

Step 6 — Gross Heating Value (GHV)

The molar gross calorific value of the mixture at the selected combustion reference temperature is (ISO 6578:2017, §9.1):

Hc = Σ(xᵢ · Hvᵢ)

where Hvᵢ is the ideal-gas molar gross calorific value of component i in kJ/mol. In ISO mode it comes from ISO 6976:2016 Table 3 at the selected combustion reference temperature; in GPA mode it comes from GPA Midstream 2145-16 ("Fuel as Ideal gas" column, Btu/ft³ at 60 °F, 14.696 psia) after conversion to kJ/mol using the molar volume at 60 °F. The "gross" (or "superior") calorific value includes the latent heat recovered by condensing all water vapour produced during combustion back to liquid. Non-combustible components (N₂, CO₂, O₂) have Hvᵢ = 0; they dilute the mixture's heating value.

From this common molar GHV, the calculator derives two commercially important expressions of the heating value, each with a dedicated result card:

Mass basis (Hm) — the heating value per unit mass of liquefied cargo (ISO 6578:2017, Formula 12):

Hm = Hc / Mmix

The SI result in MJ/kg is equivalently expressible in the unit systems commonly used in LNG trade:

• Hm (MJ/kg) — SI form, the basis for all further conversions
• Hm (GJ/tonne) — numerically identical to MJ/kg (1 MJ/kg = 1 GJ/tonne), commonly quoted in European gas markets
• Hm (kWh/kg) — MJ/kg ÷ 3.6, used in electricity-equivalent energy accounting
• Hm (MMBTU/tonne) — MJ/kg × 0.947817, the figure most commonly quoted by LNG brokers, traders, and US-style cargo invoices. Typical commercial LNG ranges from about 51 MMBTU/tonne (very lean, high-methane) to 54 MMBTU/tonne (rich, with higher C₂–C₅ content).
• Hm (Btu/lb) — MJ/kg × 429.9226, used in some US process-engineering contexts

All five expressions describe the same physical quantity — the ideal-gas gross heating value per unit mass of liquefied cargo — differing only in the unit system in which the value is reported.

Volumetric basis, real-gas (Hv) — the heating value per unit volume of regasified gas at the metering reference conditions (ISO 6976:2016, Formula 10):

Hv = Hc / (Vm · Zmix)

where Vm = R·T/P is the ideal-gas molar volume at the selected metering reference temperature and Zmix is the mixture compression factor (Step 8). The Zmix term corrects the ideal-gas molar volume to the real-gas molar volume at the metering reference, yielding the volumetric GHV actually delivered to a burner or sales meter. The same quantity is reported in kWh/Sm³ (metric markets) and BTU/SCF (US markets) using standard unit conversions.

The difference between the real-gas and ideal-gas volumetric GHVs is typically ~0.3% for commercial LNG (reflecting Zmix ≈ 0.997) — small but significant over million-MMBTU cargoes. Both ideal-gas and real-gas volumetric GHVs are exposed in the intermediate values panel for audit and verification purposes.

Step 7 — Total Energy and MMBTU

Total energy content (ISO 6578:2017, Formula 4):

Q = m · Hm

The conversion chain to MMBTU:

• Q (MJ) = mass (kg) × Hm (MJ/kg)
• Q (MWh) = Q (MJ) ÷ 3,600
• Q (MMBTU) = Q (MWh) × 3.41214163513

The conversion coefficient (MMBTU/m³) = Q (MMBTU) ÷ V (m³) = ρₜ × Hm (converted to MMBTU units). This is a derived figure that expresses the energy density of the specific cargo per unit liquid volume — it depends on composition and temperature but not on volume.

Step 8 — Wobbe Index

The Wobbe Index is the primary measure of gas interchangeability — it determines whether the regasified LNG is compatible with the downstream pipeline network and burner equipment at the receiving terminal. Many Sale and Purchase Agreements specify contractual Wobbe Index limits that the cargo must satisfy.

The real-gas Wobbe Index is defined as the real-gas volumetric gross calorific value divided by the square root of the real-gas relative density (ISO 6976:2016, Formulas 17–20):

W = Hv_real / √G_real

Both the volumetric GHV and the relative density require correction from ideal-gas to real-gas behavior using the compression factor Z of the mixture. The compression factor is calculated from ISO 6976:2016, Formula (1):

Zmix = 1 − [Σ(xᵢ · sᵢ)]²

where sᵢ is the summation factor for component i from ISO 6976:2016, Table 2, at the selected metering reference temperature. The summation factors vary with metering temperature — they are largest at 0 °C and smallest at 20 °C. For a typical LNG mixture, Zmix ≈ 0.997, indicating a small (~0.3%) deviation from ideal-gas behavior.

The real-gas volumetric GHV and relative density are then:

Hv_real = Hc / (Vm · Zmix)

G_real = (Mmix / Mair) · (Zair / Zmix)

where Vm = R·T₂/P₂ is the ideal molar volume at the metering reference conditions (T₂ is the metering reference temperature in Kelvin, P₂ = 101.325 kPa), Mair = 28.96546 kg/kmol is the molar mass of dry air, and Zair is the compression factor of dry air at the metering reference conditions from ISO 6976:2016, Annex A, Table A.4 (0.999419 at 0 °C, 0.999595 at 15 °C, 0.999601 at 15.55 °C, 0.999645 at 20 °C).

The Wobbe Index calculation involves two separate reference temperatures: the combustion reference temperature (which determines the GHV values used in the numerator) and the metering reference temperature (which determines the gas volume basis, the summation factors for Z, and the Zair value). These are independent selections — a common combination is 25 °C combustion with 0 °C metering (expressing the result in kWh/Nm³).

A higher Wobbe Index means more energy is delivered through a given orifice at a given pressure — two gases with the same Wobbe Index will deliver the same thermal output to a burner. Typical Wobbe Index ranges for LNG (at 0 °C metering, 101.325 kPa): 14.1–15.6 kWh/Nm³.

Step 9 — Gas Volume Equivalent & Expansion Ratio

When regasified, one cubic metre of LNG produces several hundred cubic metres of natural gas at atmospheric conditions. Two related figures capture this:

The total gas volume equivalent is the quantity of gas the entire cargo yields when fully regasified to the metering reference conditions:

V_gas = (m / Mmix) · V_m

where m is cargo mass (kg), Mmix is the mixture molar mass (kg/kmol), and V_m = R·T/P is the ideal-gas molar volume at the metering reference (m³/kmol). The result is expressed in m³ at the selected reference (Sm³ at 15 °C, Nm³ at 0 °C, or SCF at 60 °F in GPA mode).

The LNG-to-gas expansion ratio is the dimensionless multiplier between liquid and gas volume:

Expansion = V_gas / V_liquid = (ρ / Mmix) · V_m

For typical LNG, the expansion ratio falls in the range 590–615, reflecting the ~600× volume increase from liquid to gaseous state at atmospheric pressure. The ratio depends on both the LNG density (which depends on composition and temperature) and the molar mass of the mixture. Leaner cargoes (high methane, low heavies) yield slightly higher expansion ratios because their lower molar mass produces more moles per unit mass.

Step 10 — Equilibrium Vapor Pressure (EVP) & Boiling Point

The EVP is the bubble point pressure of the LNG mixture at the observed cargo temperature. It represents the pressure at which the liquid is in thermodynamic equilibrium with its vapor — essentially the pressure the cargo tank would settle to if left undisturbed.

The calculation uses modified Raoult's Law for an ideal liquid mixture:

P_bubble = Σ(xᵢ · Pᵢ_sat(T))

where Pᵢ_sat(T) is the saturation (vapor) pressure of pure component i at temperature T, calculated using the Antoine equation:

log₁₀(P / mmHg) = A − B / (C + T)

where T is in °C and A, B, C are empirically determined constants for each substance, sourced from the NIST Chemistry WebBook and Yaws' Handbook of Antoine Coefficients.

At typical LNG temperatures (−155 to −167 °C), the dominant contributors to vapor pressure are methane (Psat near atmospheric, ~100 kPa) and nitrogen (Psat ~1,500 kPa, but limited by its low mole fraction). Ethane and heavier components are deeply subcooled with negligible vapor pressures at these temperatures.

EVP and Tank Pressure Relationship

The relationship between the EVP and the actual tank pressure is fundamental to understanding cargo behavior:

• Tank pressure < EVP — The liquid is above its bubble point for the prevailing pressure. It will boil, and the evaporation absorbs latent heat from the liquid, cooling the cargo down until the EVP drops to match the tank pressure.
• Tank pressure > EVP — The liquid is subcooled relative to the tank pressure. Boiling is suppressed, and heat ingress through the tank insulation gradually warms the cargo up, increasing the EVP until it approaches the tank pressure.
• Tank pressure = EVP — Thermodynamic equilibrium. The liquid is at its bubble point for the prevailing pressure. Any heat ingress causes boiling (boil-off gas generation) without a net change in liquid temperature.

In practice, LNG carriers maintain tank pressure slightly above the EVP (typically 100–250 mbar gauge). This means the cargo is subcooled, which is the normal and desired operating condition. The EVP is strongly influenced by composition — small changes in nitrogen content have a disproportionate effect because nitrogen's saturation pressure at LNG temperatures is roughly 15 times that of methane.

The normal boiling point (BP at 1 atm) reported alongside the EVP is the temperature at which the mixture's bubble-point pressure equals 101.325 kPa (standard atmospheric pressure). It is obtained by numerically inverting the bubble-point equation — solving Σ(xᵢ · Pᵢ_sat(T)) = 101.325 kPa for T. For pure methane this would be −161.5 °C; for typical commercial LNG it sits between −160 and −163 °C depending on composition. A cargo warmer than its atmospheric BP would flash-boil if exposed to 1 atm; a cargo cooler than its atmospheric BP is subcooled at 1 atm and can absorb heat ingress before boil-off begins.

Applicability & Limitations

• Valid for LNG mixtures with average molar mass ≤ 20 kg/kmol
• Nitrogen < 5 mol%, butanes < 5 mol%, pentanes+ < 1 mol%
• Temperature range: −167.15 to −155.15 °C (extrapolated outside)
• Fully refrigerated LNG at vapour pressures near atmospheric — typical LNG carrier membrane tanks operate at 100–250 mbar gauge (~111–126 kPaA), well within scope. Not valid for pressurized or semi-refrigerated systems.
• EVP is approximated by modified Raoult's Law (ideal liquid mixing, P = Σ xᵢ·Pᵢ_sat). For typical LNG (≥95 mol% CH₄, <0.5% N₂), deviations from experimental bubble-point are ≤2–3% (a few mbar absolute), well below tank-pressure gauge precision. Nitrogen-rich cargoes (>2 mol% N₂) or ethane-rich cargoes (>5 mol% C₂) can see 5–10% deviations; for those, a cubic equation of state (Peng-Robinson with appropriate kᵢⱼ) would be more accurate but is not standard for custody-transfer applications.
• C6+ is approximated as a linear extrapolation of the n-C₅ orthobaric molar volume (Vi ≈ 2·V(n-C₅) − V(n-C₄)). Actual C6+ often contains aromatics and cycloalkanes, but at the <0.01 mol% levels typical of commercial LNG the impact on density is <0.001 kg/m³ — negligible.
• Antoine vapor-pressure correlation is used for Psat,i(T) in the EVP calculation, with constants from NIST/Yaws. These correlations are most accurate in the 80–150% of normal boiling point range. For methane below about −160 °C (very close to its normal boiling point of −161.5 °C) the Antoine form loses accuracy vs. the full Wagner/REFPROP reference equations; impact on LNG bubble-point remains within a few mbar.
• CO₂ content above ~0.01 mol% (100 ppm) may exceed typical LNG solubility; cargoes pretreated at the liquefaction plant are normally below 50 ppm. The calculator will flag compositions above this limit because solid-phase CO₂ may be present, invalidating the homogeneous-liquid assumption used in the density correlation.
• Wobbe Index and volumetric GHV use the real-gas compression factor Z, with summation factors from ISO 6976:2016 Table 2 (ISO mode, at the selected metering reference temperature) or GPA Midstream 2145-16 at 60 °F (GPA mode). The simplified form Z = 1 − [Σ(xᵢ·sᵢ)]² is the exact ISO form evaluated at the reference pressure P₀ where the sᵢ values are tabulated — no approximation at this pressure.

The Klosek-McKinley method was developed at the US National Bureau of Standards (NIST) and validated against experimental LNG density data with accuracy of approximately ±0.1%.

Mass & Molar

• kg/kmol — kilograms per kilomole. Molar mass unit. Numerically equal to g/mol.
• kJ/mol — kilojoules per mole. Molar gross calorific value.
• kg — kilogram. 1 tonne = 1,000 kg.

Volume

• m³ — cubic metre. Volume at observed cargo temperature (not corrected).
• m³/kmol — molar volume unit (Table B.2).
• Nm³ — normal cubic metre. Gas at 0 °C, 101.325 kPa.
• Sm³ — standard cubic metre. Gas at 15 °C, 101.325 kPa.
• Sm³(60°F) — standard cubic metre at 60 °F (15.55 °C), 14.696 psia. Used in GPA mode.
• SCF — standard cubic foot. Gas at 60 °F, 14.696 psi. 1 Sm³(60°F) = 35.31467 SCF.
• MSm³ — thousand standard cubic metres (10³ Sm³).
• MMSCM — million standard cubic metres (10⁶ Sm³). Used for total cargo gas volume equivalents.
• MMSCF — million standard cubic feet (10⁶ SCF). US-customary equivalent of MMSCM.

Density

• kg/m³ — LNG density at cargo temperature. Typical: 420–480 kg/m³.
• t/m³ — tonnes per cubic metre = kg/m³ ÷ 1,000.

Energy

• MJ — megajoule (10⁶ J). SI energy unit.
• TJ — terajoule (10¹² J). 1 TJ = 1,000,000 MJ.
• kWh — kilowatt-hour. 1 kWh = 3.6 MJ (exact).
• MWh — megawatt-hour. 1 MWh = 3,600 MJ.
• BTU — British Thermal Unit (IT). 1 BTU = 1.05505585 kJ.
• MMBTU — one million BTU. Primary LNG trading unit. 1 MWh = 3.41214 MMBTU.
• GJ/tonne — energy per metric tonne. Numerically equal to MJ/kg. Typical LNG: 54–55 GJ/tonne.
• MMBTU/tonne — energy density on mass basis. Typical LNG: 51–53 MMBTU/tonne. Widely quoted in LNG commercial contracts.

Calorific Value & Wobbe Index

• MJ/kg — gross calorific value, mass basis.
• kWh/kg — same quantity. MJ/kg ÷ 3.6.
• Btu/lb — mass-basis GHV in US customary units. 1 MJ/kg = 429.923 Btu/lb.
• kWh/Nm³ — volumetric GHV at normal conditions (0 °C, 101.325 kPa).
• MJ/Sm³ — volumetric GHV at the metering reference (15 °C or 60 °F depending on mode).
• BTU/SCF — volumetric GHV at 60 °F, 14.696 psi. Native unit of GPA mode.
• Wobbe Index — volumetric GHV ÷ √(relative density). Key gas interchangeability parameter. Expressed in the same units as volumetric GHV (e.g., kWh/Nm³, MJ/Sm³, or BTU/SCF). Typical LNG range: 14.1–15.6 kWh/Nm³ ≈ 1,320–1,420 BTU/SCF.

Dimensionless

• Relative density (G) — mixture molar mass ÷ molar mass of dry air (28.96546 g/mol). Also called specific gravity. For natural gas, typically 0.55–0.70.
• Compression factor (Z) — real-gas correction, Z = 1 − [Σ(xᵢsᵢ)]². For LNG vapour, Z ≈ 0.997 at metering reference conditions.
• Expansion ratio — gas volume ÷ liquid volume when LNG is regasified to reference conditions. Typical: 590–615. Dimensionless; expressed as "X : 1".

Pressure

• kPa — kilopascal. 1 atm = 101.325 kPa.
• kPaA — kilopascals absolute (vs gauge).
• mbar — millibar. 1 mbar = 0.1 kPa. LNG tank gauge pressure typically 100–250 mbar.
• bar — 1 bar = 100 kPa.
• psi — pounds per square inch. 14.696 psi = 101.325 kPa.
• mmHg — millimetres of mercury. 760 mmHg = 101.325 kPa. Unit used in Antoine equation.

Temperature

• °C — Celsius. Typical LNG: −155 to −167 °C.
• K — kelvin. K = °C + 273.15.
• °F — Fahrenheit. 60 °F = 15.555… °C.

Conversion Factors

• 1 MWh = 3.41214163513 MMBTU
• 1 kWh = 3.6 MJ (exact)
• 1 BTU = 1,055.05585 J
• 1 bar = 100 kPa = 14.5038 psi
• 1 atm = 101.325 kPa = 1013.25 mbar = 760 mmHg
• 1 m³ = 35.31467 ft³ · 1 Sm³(60°F) = 35.31467 SCF
• 1 MJ/kg = 429.923 Btu/lb = 0.947817 MMBTU/tonne
• 1 GJ/tonne = 1 MJ/kg (numerically identical)
• 1 Btu/ft³ at 60 °F, 14.696 psia ≈ 0.882681 kJ/mol (via molar volume V_m = 0.023690 m³/mol)

Combustion Reference Temperature

The combustion reference temperature is the notional temperature at which the fuel is burned and to which all products of combustion are returned. It determines the gross calorific value (GHV) used in energy calculations. Different values of GHV apply at different combustion reference temperatures because the enthalpy of condensation of water vapour (a combustion product) varies with temperature.

• 0 °C (273.15 K) — Used alongside Normal cubic metres (Nm³) in continental European gas markets, particularly Germany, the Netherlands, Belgium, Austria, and Switzerland. Often paired with 0 °C metering reference. Common in European pipeline gas transmission and distribution contracts.
• 15 °C (288.15 K) — The ISO 13443 international standard reference condition. Widely used in international LNG trade, particularly in contracts governed by European or international standards. Common in Japan, South Korea, and many Asian import terminals. Often paired with 15 °C metering reference.
• 15.55 °C (60 °F, 288.706 K) — The US customary standard, defined by GPA 2145, ASTM, and API standards. Used throughout the United States, United Kingdom, and countries following Anglo-American measurement practices. US LNG export terminals (Sabine Pass, Corpus Christi, Cameron, Freeport, Cove Point, Elba Island) typically use this reference. Often paired with 60 °F metering reference and BTU/SCF units.
• 20 °C (293.15 K) — The GOST R 31369 standard used in Russia and CIS countries (Kazakhstan, Uzbekistan, Turkmenistan). Also used in China under GB/T 11062. Common in pipeline gas contracts in Central Asia and Eastern Europe.
• 25 °C (298.15 K) — The IUPAC thermochemical standard state temperature. Used in some Middle Eastern LNG Sale and Purchase Agreements and by certain trading entities. This is the base temperature from which ISO 6976:2016 derives all other reference temperature values, as it corresponds to the standard conditions for thermodynamic reference data. Also common in academic and research contexts.

The combustion reference temperature is a contractual specification, not a geographic rule. The same terminal may issue calculations at different reference temperatures depending on the Sale and Purchase Agreement with each buyer. Always verify which reference temperature is specified in the applicable contract or Certificate of Quality.

Metering Reference Temperature

The metering reference temperature is the notional temperature at which the volume of gas is determined. It defines the "standard volume" unit used for volumetric properties such as the Wobbe Index and GHV on a volumetric basis. It also determines the summation factors for the compression factor Z and the air compression factor Zair used in the real-gas correction.

The metering reference temperature is independent of the combustion reference temperature — these are two separate selections that can be combined freely depending on contractual requirements (see ISO 6976:2016, Figure 1).

• 0 °C (273.15 K) — Defines Normal cubic metres (Nm³). Widely used in continental European gas quality specifications, Middle Eastern terminal calculation documents, and many international LNG contracts. The Wobbe Index is expressed in kWh/Nm³. This is the most common metering reference in European gas regulation (e.g., EASEE-gas Common Business Practice).
• 15 °C (288.15 K) — Defines Standard cubic metres (Sm³). The ISO 13443 international standard reference. Used in international LNG trade and by many Asian import terminals. The Wobbe Index is expressed in kWh/Sm³.
• 15.55 °C (60 °F, 288.706 K) — Defines Standard cubic feet (SCF). The US customary standard per GPA 2145, ASTM, and API. Used throughout the United States and United Kingdom. The GHV is expressed in BTU/SCF. US LNG export terminal Certificates of Quality report heating values in BTU/SCF at this metering reference.
• 20 °C (293.15 K) — Used in GOST R 31369 (Russia, CIS countries) and GB/T 11062 (China). Common in pipeline gas contracts in the post-Soviet space and Chinese domestic gas standards.

The metering reference temperature does not affect density, mass, GHV on a mass basis (MJ/kg), total energy (MMBTU), conversion coefficient (MMBTU/m³), or EVP. It only affects the Wobbe Index, GHV on a volumetric basis, and the compression factor used in their real-gas correction.

ISO 6578:2017 Formulas

• Eq. 1 — Mass: m = V·ρ
• Eq. 4 — Energy: Q = m·Hs,m
• Eq. 9 — Density (Klosek-McKinley): ρₜ = Σ(xᵢMᵢ)/[Σ(xᵢVᵢ)−Vc]
• Eq. 10 — Volume correction: Vc = [k₁+(k₂−k₁)·x₂/0.0425]·x₁
• Eq. 12 — GHV mass basis: Hs,m = Σ[Hs,m,i·xᵢMᵢ/Σ(xᵢMᵢ)]

ISO 6976:2016 Formulas (ISO mode)

• Eq. 1 — Compression factor: Z = 1 − [Σ(xᵢ·sᵢ)]²
• Eq. 10 — Real-gas GHV volumetric: Hv = Hc / (Vm·Z)
• Eq. 17 — Real-gas relative density: G = (Mmix/Mair)·(Zair/Z)
• Eq. 20 — Real-gas Wobbe Index: W = Hv / √G

GPA Midstream 2145-16 (GPA mode)

• Same mathematical form as ISO 6976, but evaluated at fixed 60 °F (15.55 °C), 14.696 psia.
• Molar GHV from GPA 2145-16 "Fuel as Ideal gas (Btu/ft³)" column, converted to kJ/mol via Hv(kJ/mol) = Hv(Btu/ft³)·V_m(60°F)·1.055056.
• Summation factors sᵢ from GPA 2145-16 unitless-form column (Z = 1 − (P/P₀)·b² with P₀ = 14.696 psia).
• Zair = 0.999601 at 60 °F, 14.696 psia (GPA 2145-16 Table of Dry Air Properties).
• Cryogenic liquid-phase density uses ISO 6578 Table B.2 orthobaric molar volumes — GPA 2145-16 does not publish liquid data at LNG temperatures.

Gas Volume, Expansion & Mass-Basis GHV

• Molar volume at reference: V_m = R·T/P (ideal gas at metering reference)
• Total gas volume equivalent: V_gas = (m/Mmix)·V_m
• LNG-to-gas expansion ratio: r = V_gas/V_liq = (ρ/Mmix)·V_m
• Mass-basis GHV (Hm) unit conversions: Hm(MMBTU/tonne) = Hm(MJ/kg)·0.947817; Hm(kWh/kg) = Hm(MJ/kg)/3.6; Hm(Btu/lb) = Hm(MJ/kg)·429.923
• SCF conversion: V_gas(SCF) = V_gas(m³)·35.31467

Vapor Pressure

• Antoine: log₁₀(P/mmHg) = A − B/(C+T°C)
• Bubble point pressure: P = Σ(xᵢ·Psat,i(T)) (modified Raoult's Law)
• Boiling point: solve Σ(xᵢ·Psat,i(Tbp)) = 101.325 kPa by bisection

Data Sources

• ISO 6578 Table B.2 — Orthobaric molar volumes (linear interpolation). Used under both standards.
• ISO 6578 Tables C.1/C.2 — Klosek-McKinley k₁, k₂ factors (bilinear interpolation). Used under both standards.
• ISO 6976 Table 1 — Molar masses (IUPAC 2007 atomic weights). Used under both standards; identical to the GPA 2145-16 molar masses.
• ISO 6976 Table 2 — Summation factors sᵢ at four metering reference temperatures. Used when Calculation Standard = ISO.
• ISO 6976 Table 3 — Molar GHV at five combustion reference temperatures. Used when Calculation Standard = ISO.
• ISO 6976 Annex A — Gas constant R, molar mass of dry air, Zair values. Used under both standards.
• GPA Midstream 2145-16 — Ideal-gas GHV in Btu/ft³ and summation factors sᵢ at 60 °F, 14.696 psia. Used when Calculation Standard = GPA.
• NIST / Yaws — Antoine equation constants for vapor pressure (standard-independent).