US gallons and liters
The US liquid gallon is defined as exactly 231 cubic inches, which is 3.785411784 liters. Gasoline, milk and paint are sold by the gallon in the US.
The liter is exactly one cubic decimeter, or 1/1000 of a cubic meter. Drinks, fuel, engine displacement and household liquids are sold by the liter in most countries.
Worked example
Convert 15 gal: 15 × 3.785411784 = 56.78117676, so 15 US gallons is 56.78117676 liters. Likewise 20 gal = 75.70823568 L.
Estimating in your head
Multiply by 3.8 for a quick figure; it is off by about 0.4%. For 15 gal that gives 57 L, against 56.7812 exactly.
Going the other way
To convert liters to US gallons, divide by 3.785411784 (or multiply by 0.264172052358). The swap button between the boxes opens that page.
How this works
L = gal × 3.785411784
1 gal = 0.003785411784 m³ and 1 L = 0.001 m³, both by definition, so the factor is their ratio: 3.785411784 exactly. Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many liters are in a US gallon?
1 US gallon = 3.785411784 liters, exactly. The value follows from the unit definitions, so it never changes.
How do I convert US gallons to liters?
Multiply by 3.785411784. For example, 20 gal × 3.785411784 = 75.70823568 L.
What is 100 gal in liters?
100 gal = 378.5411784 L. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- NIST Handbook 44 — Appendix C, General tables of units of measurement
- Wikipedia — United States customary units
Reviewed Oct 17, 2026 · test vectors fixtures/units/volume/gallons-to-liters/