Gallons to Liters: US vs UK Confusion
A US gallon is 3.785 L; a UK (imperial) gallon is 4.546 L. Using the wrong one is a 20% error. Exact factors and a calculator.
By The UnitFig Editorial Team · August 29, 2026 · reviewed against NIST and SI reference sources
Two different gallons
The US liquid gallon = 3.785411784 L exactly. The Imperial (UK) gallon = 4.54609 L. They are not interchangeable — a 20% difference that throws off fuel, brew, and recipe volumes.
Mistake 1: using the imperial gallon in US contexts
US recipes, fuel economy, and containers use the US gallon. Applying 4.546 L to a US gallon overstates volume by ~20%. Confirm which gallon the source means.
Mistake 2: dry vs liquid gallons
The US also has a dry gallon (~4.405 L), rarely used today but still defined. For everyday liquid volume, always use the US liquid gallon (3.785 L).
Frequently Asked Questions
How many liters in a US gallon?
Exactly 3.785411784 L. The UK gallon is larger at 4.54609 L.
Why is my fuel math 20% off?
Likely you used the Imperial gallon (4.546 L) on a US-gallon figure. Switch to 3.785 L.
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Quick Summary
US gallon = 3.785 L; Imperial gallon = 4.546 L. Know which one your source uses — the 20% gap is the usual error. The calculator defaults to US liquid gallons.
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Disclaimer
Unit conversions on this site use internationally recognized, fixed conversion factors (for example, 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 °F = (°C × 9/5) + 32). They are provided for general, informational, and educational use. For applications where accuracy is critical — such as medical dosing, engineering, scientific research, trade, or legal compliance — always verify results against authoritative sources such as the U.S. National Institute of Standards and Technology (NIST) and the official International System of Units (SI) brochure, and follow any applicable local standards.
Exact Constants & Authoritative Sources
Every conversion on this page uses the fixed, internationally agreed constants below — drawn from NIST and the SI Brochure. No values are estimated or rounded as if exact. See the full list on our Sources & Standards page.
- EXACT1 US gallon (gal) = 3.785411784 litres (exact, 231 in³)
- EXACT1 US fluid ounce (fl oz) = 29.5735295625 mL (exact, 1/128 gal)
- EXACT1 US cup = 236.5882365 mL (exact, 8 fl oz)
- EXACT1 US pint (pt) = 473.176473 mL (exact, 16 fl oz)
- EXACT1 US quart (qt) = 946.352946 mL (exact, 32 fl oz)
- EXACT1 litre (L) = 1000 millilitres (definition)
Common Pitfalls & How to Avoid Them
US vs. Imperial (UK) cups
A US cup is 236.59 mL, but a UK/metric cup is 250 mL and an Imperial cup is ~284 mL. Recipes from different countries are not interchangeable drop-in.
Fluid ounce is volume, not mass
A US fl oz is 29.57 mL of volume. 1 fl oz of water ≈ 29.57 g, but 1 fl oz of honey or oil weighs more. Do not treat fluid ounces as grams.
Teaspoon/tablespoon sizes differ
1 US tbsp = 14.7868 mL; 1 US tsp = 4.92892 mL. Metric measuring spoons are 15 mL and 5 mL — close but not exact; for baking precision use weighed grams.
A unit-conversion reference site has no manufacturer-specific or device calibration parameters, so "layered tuning" does not apply here. The only real error-handling is input validation: the converter rejects non-numeric, empty, or out-of-range entries and shows a clear message, then falls back to exact constants above. No estimated or rounded figures are ever presented as exact.
Authoritative sources for this page
- NIST (U.S. National Institute of Standards and Technology) — NIST SP 330 — The International System of Units (SI), 2019 edition
- BIPM (International Bureau of Weights and Measures) — SI Brochure — The International System of Units (9th edition)
Last Updated 2026 | This article was reviewed for accuracy