ISO 6578:2025

Refrigerated hydrocarbon liquids - Static measurement - Calculation procedure

This document specifies the calculation procedure for converting the volume of liquefied petroleum gas (LPG) and liquefied natural gas (LNG) under the conditions at the time of measurement to the equivalent volume of liquid or vapour at the standard condition (i.e. 15 °C and 101,325 kPa absolute), or to the equivalent mass or energy (calorific content). This document applies to the quantities of refrigerated hydrocarbon liquids that are stored in or transferred to or from tanks and measured under static storage conditions. This document does not cover the calculation of pressurized gases.

Hydrocarbures liquides réfrigérés — Mesurage statique — Procédure de calcul

General Information

• Status: Published
• Publication Date: 23-Oct-2025
• ICS: 75.180.30 - Volumetric equipment and measurements
• Technical Committee: ISO/TC 28/SC 5 - Measurement of refrigerated hydrocarbon and non-petroleum based liquefied gaseous fuels
• Drafting Committee: ISO/TC 28/SC 5 - Measurement of refrigerated hydrocarbon and non-petroleum based liquefied gaseous fuels
• Current Stage: 6060 - International Standard published
• Start Date: 24-Oct-2025
• Due Date: 09-May-2026
• Completion Date: 24-Oct-2025

Relations

• Revises: ISO 6578:2017 - Refrigerated hydrocarbon liquids - Static measurement - Calculation procedure
• Effective Date: 13-May-2023

Overview

ISO 6578:2025 - "Refrigerated hydrocarbon liquids - Static measurement - Calculation procedure" specifies the validated calculation methods for converting measured volumes of refrigerated hydrocarbon liquids (LPG and LNG) under static storage conditions to standard conditions (15 °C and 101,325 kPa). The standard covers conversion to equivalent liquid or vapour volume at standard conditions, and to mass and energy (gross calorific content). It applies to fully refrigerated hydrocarbon liquids stored or transferred from tanks (including marine carriers) at near-atmospheric vapour pressures. It does not cover pressurized gases or predominantly ethane/ethylene mixtures or partially refrigerated liquids at substantially elevated pressures.

Key topics and technical requirements

• Volume-to-standard conversion: Procedures to convert liquid and vapour volumes measured in tanks to volumes at the standard reference condition (15 °C, 101,325 kPa).
• Mass calculation: Methods to determine liquid and total (liquid + vapour) mass from measured volumes, including corrections for vapour-phase content and vacuum/air corrections.
• Energy (calorific) content: Calculation of gross calorific value from composition and conversion of transferred quantity to energy units (MJ).
• Density from composition: Algorithms to calculate liquid density from chemical composition for LPG and LNG.
• Real vs ideal vapour bases: Guidance on using volumetric (ideal) and volumetric (real / compressible) bases, including compression (Z) factor use.
• Supporting data and annexes: Reference tables and informative annexes for molar volumes, correction factors, component calorific values, molar masses, compression factors, and worked examples.
• Normative references and alignment: Cross-references to ISO 91 (temperature and pressure volume correction factors) and alignment of component data with ISO 6976:2016 where applicable.

Applications and who uses this standard

ISO 6578:2025 is essential for organizations involved in custody transfer, inventory and energy accounting of refrigerated hydrocarbon liquids:

• Marine carrier operators and surveyors measuring LPG/LNG cargoes on board
• Terminal and tank farm operators performing static inventory calculations
• Custody transfer engineers and commercial energy auditors
• Measurement and metrology specialists in the petroleum and gas sectors
• Regulators and quality assurance professionals specifying accepted conversion procedures Use cases include cargo transfer on LNG/LPG carriers, shore tank inventory, mass reconciliation for trade, and conversion of measured quantities to standard mass/energy for billing and reporting.

Related standards

• ISO 91:2017 - Temperature and pressure volume correction factors (petroleum measurement tables)
• ISO 6976:2016 - Calculation of calorific values and related component data (aligned component tables)

ISO 6578:2025 provides the technical backbone for accurate static measurement and conversion of refrigerated hydrocarbon liquids (LPG, LNG), enabling consistent custody transfer, safety-compliant inventory control, and reliable energy accounting.

Standards Content (Sample)

International
Standard
ISO 6578
Third edition
Refrigerated hydrocarbon liquids —
2025-10
Static measurement — Calculation
procedure
Hydrocarbures liquides réfrigérés — Mesurage statique —
Procédure de calcul
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions .1
3.2 Symbols .2
4 Outline of the calculation . 3
4.1 Liquefied petroleum gas .3
4.2 Liquefied natural gas .4
4.3 Data for calculation .5
5 Mass . 5
5.1 Mass of liquid phase .5
5.2 Correction for vapour phase .6
5.3 Mass in vacuum to mass in air .6
6 Energy content (calorific content) . 7
7 Inter-conversion of liquid mass and vapour volume at standard conditions . 8
8 Calculation of liquid density from composition . 8
8.1 General .8
8.2 Liquid petroleum gas .8
8.3 Liquid natural gas .9
9 Calculation of calorific value from composition . 9
9.1 Volumetric basis .9
9.2 Mass basis .9
Annex A (informative) Molar volume of individual component .10
Annex B (informative) Correction factors for volume reduction of LNG mixtures .11
Annex C (informative) Gross calorific values for individual components.12
Annex D (informative) Molar mass, compression factor and summation factor of individual
component .13
Annex E (informative) Calculation examples . 14
Bibliography . 19

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 28, Petroleum and related products, fuels and
lubricants from natural or synthetic sources, Subcommittee SC 5, Measurement of refrigerated hydrocarbon
and non-petroleum based liquefied gaseous fuels.
This third edition cancels and replaces the second edition (ISO 6578:2017), which has been technically
revised.
The main changes are as follows:
— Table D.1 has been aligned with ISO 6976:2016;
— calculation examples have been moved from the main body to Annex E.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Large quantities of refrigerated hydrocarbon liquids such as liquefied natural gas (LNG) and liquefied
petroleum gas (LPG), are transported by marine carriers especially designed for these applications. These
gases are traded based on the static measurement on board marine carriers rather than the measurement at
shore tanks or pipelines, mainly due to the nature of the tank operation.
The measurement on board involves determining the liquid/vapour interface, i.e. liquid level, average
temperatures of liquid and vapour, and the vapour pressure in the tanks of marine carriers. The volumetric
quantity of the liquid and gas is then computed with the tank capacity tables.
This document specifies calculations for the volume at standard conditions, the liquid density from chemical
composition and the mass and energy content of fully refrigerated hydrocarbon liquids at a vapour pressure
near to atmospheric pressure from the results of custody transfer measurement. This document also
specifies calculations for ascertaining the inventory in shore tanks. This document does not cover calculation
procedures for refrigerated hydrocarbon liquids consisting predominantly of ethane or ethylene or partially
refrigerated hydrocarbon liquids at pressures substantially above atmospheric. No recommendations are
given for the measurement of small parcels of refrigerated liquids, which are directly weighed.
Aspects of safety are not dealt with in this document. It is the responsibility of the user to ensure that the
procedure of measurement meets applicable safety regulations.
Basic data and source references used in the calculation procedures are given in Annexes A to E.

v
International Standard ISO 6578:2025(en)
Refrigerated hydrocarbon liquids — Static measurement —
Calculation procedure
1 Scope
This document specifies the calculation procedure for converting the volume of liquefied petroleum gas
(LPG) and liquefied natural gas (LNG) under the conditions at the time of measurement to the equivalent
volume of liquid or vapour at the standard condition (i.e. 15 °C and 101,325 kPa absolute), or to the equivalent
mass or energy (calorific content). This document applies to the quantities of refrigerated hydrocarbon
liquids that are stored in or transferred to or from tanks and measured under static storage conditions. This
document does not cover the calculation of pressurized gases.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 91:2017, Petroleum and related products — Temperature and pressure volume correction factors (petroleum
measurement tables) and standard reference conditions
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1.1
compression factor
actual (real) volume of a given amount of gas at a specified pressure and temperature divided by its volume,
under the same conditions as calculated from the ideal gas law
[SOURCE: ISO 6976:2016, 3.10]
3.1.2
gross calorific value
amount of heat that would be released by the complete combustion with oxygen of a specified quantity of
gas, in such a way that the pressure, p , at which the reaction takes place remains constant, and all the
products of combustion are returned to the same specified temperature, T , as that of the reactants, all of
these products being in the gaseous state except for water, which is condensed to the liquid state at T
Note 1 to entry: T and p are the combustion reference temperature and combustion reference pressure, respectively.
1 1
[SOURCE: ISO 6976:2016, 3.1, modified — Note 1 to entry has been replaced; t has been replaced by T .]
1 1
3.1.3
liquefied natural gas
LNG
liquid composed predominantly of methane
3.1.4
liquefied petroleum gas
LPG
liquid composed predominantly of any of the following hydrocarbons or mixtures thereof: propane, propene,
butanes and butene
3.1.5
refrigerated hydrocarbon liquid
liquid composed predominantly of hydrocarbons, which are stored in a fully refrigerated condition at
pressures near atmospheric
3.1.6
volumetric basis (ideal)
volume calculated on the basis that the vapour behaves like an ideal gas
3.1.7
volumetric basis (real)
volume calculated on the basis that the vapour behaves like a super-compressible gas
3.2 Symbols
For the purpose of this document, the following symbols apply. In addition, some symbols are given more
restricted meanings when used in certain formulae. The restricted meaning is given after the formulae.
H gross (superior) calorific value on a mass basis, in megajoules per kilogram, of component i (see
s,m,i
Table C.1)
H gross (superior) calorific value on a mass basis, in megajoules per kilogram, of the liquid
s,m
H gross (superior) calorific value on a volumetric basis (ideal), in megajoules per cubic metre, of com-
s,V,i
ponent i (see Table C.1)
H gross (superior) calorific value on a volumetric basis, in megajoules per cubic metre, of the vapour
s,vol
at standard conditions
m mass, in kilograms, of product transferred, i.e. liquid plus vapour
m mass, in kilograms, of liquid
liq
M molar mass, in kilograms per kilomole, of component i (see Table D.1)
i
M relative molar mass, in kilograms per kilomole, of the vapour mixture
mix
P standard reference pressure, i.e. 101,325 kPa absolute
s
P pressure, in kilopascals absolute, of the vapour in the container
vap
Q net energy, in megajoules, transferred, based on gross calorific value
Q energy (calorific) content, in megajoules, of the liquid
liq
−1 −1
R molar gas constant, 8,314 462 1 J·mol ·K (see ISO 6976:2016, A.1)
T temperature, in degrees Celsius or Kelvin
T standard reference temperature, i.e. 288,15 K (15 °C)
s
T temperature, in kelvins, of the vapour in the container
vap
V molar volume, in cubic metres per kilomole, of component i, as a liquid at T °C
i
V volume, in cubic metres, of the liquid at T °C
liq
V ideal gaseous molar volume, in cubic metres per kilomole, at standard conditions:
m
3 −1
i.e. V = (R × T )/P = 23,644 8 m ∙kmol at 15 °C and 101,325 kPa absolute
m s s
V vapour volume, in cubic metres, in the container
vap
V vapour volume at standard condition
vap,s
x ; x mole fractions of the components i and j, respectively
i j
x mole fraction of methane in the LNG
x mole fraction of nitrogen in the LNG
Z compression factor for component i at the required pressure and temperature
i
Z compression factor for the vapour mixture under known conditions of temperature and pressure
mix
ρ density, in kilograms per cubic metre, of the liquid at T
s s
ρ density, in kilograms per cubic metre, of the liquid at T °C
(T)
NOTE Additional subscripts F and I indicate, respectively, the final and initial measurements or product
properties in either of the two containers used for a transfer.
4 Outline of the calculation
4.1 Liquefied petroleum gas
Figure 1 outlines the calculation of the mass of liquefied petroleum gas (LPG) from the liquid volume.
a)  Density by hydrometer b)  Density from composition
Figure 1 — Calculation flow of LPG

The procedure for converting the volume of refrigerated LPG to its equivalent volume at a standard
temperature and corresponding equilibrium pressure includes the following aspects.
a) It can be necessary to apply very large factors for the correction of observed density to density
at standard temperature, e.g. a correction for the effect of a temperature difference of 60 °C can be
necessary for refrigerated propane. Provided that the LPG does not contain more than 20 % of
unsaturated hydrocarbons, the correction tables introduced in ISO 91 shall be used for volume
corrections. The mass of LPG is calculated by multiplying its volume at standard temperature by its
density at standard temperature.
b) The equivalent liquid content in the vapour space of a container holding refrigerated LPG is significantly
less than the liquid in the container if the tank and contents are at ambient temperature. Therefore, any
error in accounting for the equivalent liquid content in the vapour space will be of lesser significance.
EXAMPLE The following examples illustrate the magnitude of errors that can be introduced by using the tables
referred to in ISO 91.
a) Pure butene or propene: the maximum error will be approximately 2 % for a correction from −60 °C to +20 °C.
b) Mixtures containing approximately 20 % of unsaturated hydrocarbons: a typical error will be approximately
0,1 % for a temperature difference of 20 °C.
A condition in which a liquid has a vapour pressure significantly higher than atmospheric pressure at a
standard temperature of 15 °C can only be considered as a pseudo-condition. The volume of the liquid in
this condition may be used only when convenient in a procedure for obtaining the density at refrigerated
temperatures by means of pressure hydrometer measurement at ambient conditions (see ISO 3993).
Refer to Annex E for LPG calculation examples.
4.2 Liquefied natural gas
Figure 2 outlines the calculation of the energy content of liquefied natural gas (LNG) from the liquid volume
at a temperature.
Figure 2 — Calculation flow of LNG
The energy content of LNG is the product of its volume at the observed temperature, the density at that
temperature and the calorific value per unit mass. This calculation does not involve the conversion of volume
at the observed temperature to the equivalent volume at standard temperature.
Refer to Annex E for the LNG calculation examples.

4.3 Data for calculation
The physical properties of components of refrigerated hydrocarbon liquids, constants and factors are given
in Annexes A, B, C and D.
The decimal places of these values may be rounded to a specific decimal place, which may be decided by the
user of this document according to the level of required accuracy.
5 Mass
5.1 Mass of liquid phase
5.1.1 The mass of liquid (m ), in kilograms, is calculated from Formula (1):
liq
mV= ρ (1)
liqliq
where V and ρ shall be at the same temperature.
liq
5.1.2 The density of refrigerated LPG may be determined at the standard temperature of 15 °C by use
of the pressure hydrometer method (see ISO 3993) or a suitable densimeter. The liquid sample drawn into
a suitable container is allowed to approach ambient temperature under pressure, without loss of vapour,
before it is introduced into the hydrometer cylinder.
The density of liquid may also be calculated from a composition analysis (see Clause 8).
5.1.3 If the actual temperature T , at which the density is measured, does not differ by more than 5 °C from
the temperature T of the main bulk of liquid in the container, then the observed density may be corrected to
the required bulk temperature by using Formula (2). The density at T shall be measured or calculated if the
difference of the temperatures exceeds 5 °C.
ρρ=+FT()−T (2)
()TT() 21
where
ρ and ρ are the densities at temperatures T and T , respectively;
(T1) (T2) 1 2
F is the density correction factor applicable to the particular liquid that is shown in Table 1.
The units of F shall be compatible with the units of ρ, e.g. when ρ is expressed in kilograms
−3 −1
per cubic metre, F is expressed in kg ∙m ∙°C .
Table 1 — Density correction factor
F
Product
−3 −1
kg ∙m ∙°C
a
LNG (80 % or more by mass fraction in methane content) 1,4
b
Liquid propanes (60 % or more by mass fraction in propane content) 1,2
c
Liquid butanes (60 % or more by mass fraction in butane content) 1,1
a [5]
Source: NBS, Table 13.
b [6]
Source: NBS, Table 20.
c [7]
Sourc
...