Gallons to Liters Formula Explained

4 min read•June 1, 2025•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

The formula for converting US gallons to liters is L = gal × 3.78541. One US gallon is defined as exactly 231 cubic inches, which equals 3.785411784 liters. This is not the same as the UK Imperial gallon (4.54609 liters), which is about 20% larger.

This conversion arises when comparing fuel prices between countries, buying bulk water or chemicals, or understanding vehicle fuel efficiency. Mental shortcut: multiply by 3.8 — accurate within 0.4%.

The Formula

L = gal × 3.78541  (US gallon)
L = gal × 4.54609  (Imperial/UK gallon)

Worked examples (US gallons):
   1 gal × 3.78541 =   3.785 L
   5 gal × 3.78541 =  18.927 L  (standard water jug)
  10 gal × 3.78541 =  37.854 L
  15 gal × 3.78541 =  56.781 L  (car fuel tank)
  20 gal × 3.78541 =  75.708 L

Mental shortcut (within 0.4%):
  gal × 3.8 ≈ L
  15 gal × 3.8 = 57 L  (exact: 56.78 L)

US Gallon vs. Imperial Gallon

The US gallon (3.785 L) is defined as exactly 231 cubic inches. The UK/Imperial gallon (4.546 L) is defined as 10 pounds of water at a specific temperature — about 20% larger. Canada, Australia, and most Commonwealth countries abandoned the Imperial gallon when they adopted metric.

Fuel economy comparisons between US and UK vehicles require using the correct gallon. A UK car rated at 40 mpg (Imperial) achieves about 33 mpg US — the same car, but different gallon. When comparing international fuel economy figures, always verify which gallon is being used.

Quick Tips

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Multiply by 3.8 for a quick estimate — within 0.4% of the exact 3.785.
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5 gallons = 18.93 L — the standard 5-gallon water jug holds just under 19 liters.
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1 gallon ≈ 3.8 L — the most important fuel tank conversion anchor.
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UK vs US: a UK gallon (4.546 L) is 20% larger than a US gallon (3.785 L).
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For swimming pools or large containers, volume is often given in gallons in the US and liters elsewhere.