Volume conversions.

Four definitions, and everything else

Volume conversion looks like a table of arbitrary numbers. It is not. Every factor on this site descends from four exact definitions, and once you have those, each conversion is a short chain of arithmetic with no measurement uncertainty anywhere in it.

Notice what is not in that list: no measured constants, no rounded ratios, no temperature or pressure. A gallon-to-litre conversion is exact at any temperature, because both units are geometric. What changes with temperature is how much substance occupies the volume — a separate calculation, and the one where real-world errors live.

Which factors terminate, and which don't

This is the practical difference between a conversion you can write down completely and one where every printed factor is already a rounding. When the decimal repeats, divide by the exact factor rather than multiplying by a truncated reciprocal.

The three mistakes worth guarding against

• The wrong gallon. US and imperial gallons differ by about 20 %. On UK, Irish and older Canadian documentation, trust the litre figure over the gallon figure.
• Drums read as barrels. A petroleum barrel is 42 US gallons; a steel drum is 55. Reading drums as barrels overstates volume by 31 %.
• Rounding early. Round once, at the end. Converting with a rounded factor and then converting back doubles the error, and round-tripping through three units triples it.

Sources and standards

These are the documents the numbers come from — national metrology institutes, treaty organisations and primary legislation. Nothing here is sourced from another converter site.

1. Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cThe legal-metrology reference for U.S. customary units. Fixes the U.S. liquid gallon at exactly 231 cubic inches and tabulates the full liquid-volume chain (minim → fluid dram → fluid ounce → gill → pint → quart → gallon).
2. Special Publication 811, Appendix B.8 — Factors for Units Listed AlphabeticallyNational Institute of Standards and Technologyhttps://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8NIST's master table of conversions into SI. Factors printed in boldface are exact by definition rather than measured or rounded — which is how you tell a defined unit from an approximated one.
3. Unit Conversion — Office of Weights and MeasuresNational Institute of Standards and Technologyhttps://www.nist.gov/pml/owm/metric-si/unit-conversionNIST's index page explaining which of its publications governs which kind of conversion: SP 811 for exact factors, Handbook 44 Appendix C for trade and commerce, Handbook 133 Appendix E for packaged goods.
4. Refinement of Values for the Yard and the Pound — Federal Register Doc. 59-5442 (24 FR 5348, 1 July 1959)National Bureau of Standards; scan hosted by NOAA's National Geodetic Surveyhttps://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdfThe International Yard and Pound Agreement, signed by the United States, United Kingdom, Canada, Australia, New Zealand and South Africa. Fixes the yard at exactly 0.9144 m, from which the inch follows as exactly 25.4 mm. Every inch-to-metric volume factor on this site descends from this one page.
5. The International System of Units (SI), 9th edition (SI Brochure)Bureau International des Poids et Mesures (BIPM)https://www.bipm.org/en/si-brochure-9The defining document for the SI, maintained by the treaty organisation that owns the definitions. The litre is exactly one cubic decimetre — 10⁻³ m³ — and is a unit accepted for use with the SI rather than an SI unit itself.
6. Weights and Measures Act 1985, Schedule 1, Part IV — Measurement of CapacityUK Parliament; published by The National Archiveshttps://www.legislation.gov.uk/ukpga/1985/72/schedule/1Primary UK statute. Defines the imperial gallon as 4.54609 cubic decimetres — a flat legal definition, not a measurement, which is why the imperial gallon converts to litres exactly and to U.S. gallons only approximately.
7. 43 CFR Part 3170, Subpart 3174 — Measurement of Oil (§ 3174.2, Definitions)U.S. Bureau of Land Management, via the electronic Code of Federal Regulationshttps://www.ecfr.gov/current/title-43/subtitle-B/chapter-II/subchapter-C/part-3170/subpart-3174Binding federal regulation for oil measurement on federal and Indian leases. Defines a barrel as 42 standard U.S. gallons and sets the base conditions those gallons are referred to: 60 °F and 14.696 psia.
8. Energy Glossary — “Barrel”U.S. Energy Information Administrationhttps://www.eia.gov/tools/glossary/index.php?id=bThe U.S. federal energy statistics agency defines the barrel as a unit of volume equal to 42 U.S. gallons. All EIA production, refining and stock series are published on this basis.
9. Manual of Petroleum Measurement Standards (MPMS) — Ch. 1 “Vocabulary”, Ch. 11.1 “Temperature and Pressure Correction Factors”, Ch. 12.1 “Calculation of Static Petroleum Quantities”American Petroleum Institute (with ASTM D1250 / ISO 91 volume correction tables)Paid standard — cited by designation; obtain from the publisher.The industry standard for turning a gauged tank volume into a traded quantity: observed volume, gross standard volume at 60 °F, and net standard volume after sediment and water. A unit conversion is only the first step of that chain.
10. ISO 7507-1 — Petroleum and liquid petroleum products: Calibration of vertical cylindrical tanks, Part 1: Manual strapping methodInternational Organization for StandardizationPaid standard — cited by designation; obtain from the publisher.Defines how a tank's capacity table (its “strapping table”) is produced from physical measurement, including corrections for plate thickness, shell expansion and tank tilt. It is the reason a real tank's chart rarely matches naive geometry to the last gallon.

Every linked source above was retrieved and checked on 2026-09-02. Factors quoted as exact are exact by definition in the cited document, not rounded by us.