Gallon Conversion Calculator

Article — Gallon Conversion Calculator

Gallon conversion: a unit with two sizes and seven cousins

One US gallon equals 3.785411784 liters exactly, while the UK (Imperial) gallon is 4.54609 liters — about 20% larger. The US gallon also splits into 4 quarts, 8 pints, 16 cups, or 128 fluid ounces. This converter handles all seven targets with a single toggle.

Both gallon sizes are defined exactly, not measured. The US figure comes from the gallon being 231 cubic inches, with the inch fixed at 25.4 mm by international treaty in 1959. The UK figure comes from a 1976 metric redefinition that fixed the Imperial gallon at 4.54609 L — a clean tidy-up of the older "10 pounds of water at 62°F" definition from 1824.

US gallon vs. UK gallon

The 20% gap between the two gallons is the single biggest source of conversion confusion in volume units. It is large enough to matter in commerce, fuel economy and recipe scaling. A US fuel station selling at $3.50 a gallon is selling at a different per-liter rate than a UK forecourt selling at £1.50 a liter — but you cannot compare them directly without specifying which gallon.

The split traces back to colonial history. Both gallons descend from medieval English measures, but the British Parliament reformed its system in 1824 with the Weights and Measures Act, while the new United States kept the older "Queen Anne's wine gallon" (231 cubic inches). The two paths diverged, and the gap is now permanent. 1 UK gallon = 1.20095 US gallons; equivalently, 1 US gallon = 0.832674 UK gallons.

Convert gallons to liters

Multiply US gallons by 3.78541 to get liters. To go the other way, divide liters by 3.78541, or multiply by 0.26417. For UK gallons, the factor is 4.54609 — and the same reverse: divide by it, or multiply by 0.21997.

Both factors are exact. 1 US gallon = 3.785411784 L precisely, and 1 UK gallon = 4.54609 L precisely. Any rounding in the result comes from the input, not from the factor. CAD and chemistry software stores these constants to double precision because they are constants, not measurements.

Gallon to quart, pint, cup, fluid ounce

Inside the US system, every volume below the gallon is a power-of-two division. 1 gallon = 4 quarts. 1 quart = 2 pints. 1 pint = 2 cups. 1 cup = 8 fluid ounces. Multiply through: 1 gal = 128 fl oz exactly. The whole hierarchy uses round integers — a deliberate convenience for cooking and home commerce.

The UK side splits differently in cups but uses the same quart/pint structure. A UK Imperial pint is 568 mL, noticeably larger than the US 473 mL — a fact that British beer drinkers know intimately when ordering "a pint" in American bars. The cup is not a legal UK measure; recipes there give volumes in fluid ounces or grams instead.

The gallon in fuel pricing

Fuel pricing is the most public clash between gallon systems. US prices are posted in dollars per US gallon; UK forecourts post pounds per liter. To compare, convert both to a common unit and currency. A US price of $3.50/gal works out to about $0.925/L; at GBP 1.27 to the dollar, that is approximately £0.73/L — well below typical UK pump prices, which average around £1.45–1.60/L due to higher fuel taxes.

Inside Europe, all member states price fuel per liter, eliminating the gallon confusion within the EU bloc. Canada moved from Imperial gallons to liters in the 1970s. Mexico, most of Latin America and Asia all use liters at the pump. The US, Liberia and a handful of Caribbean nations are the holdouts.

The US dry gallon

The US dry gallon is a parallel unit, equal to 4.40488 L — about 16.4% larger than the liquid gallon. It survives in agricultural commerce for grain, berries and produce. A bushel is 8 dry gallons; a peck is 2 dry gallons. Both units appear in state weights-and-measures statutes covering farmers' market sales.

The dry gallon and liquid gallon are not interchangeable in any context. Mixing them silently is a common source of error in conversions between volume-based produce sales and weight-based shipping data. The UK does not use a separate dry gallon; British dry goods are measured by volume in liters or by weight in kilograms.

Gallon conversion shortcuts

Mental math: US gal × 4 = liters, minus 5%. So 10 gal × 4 = 40 L, minus 5% = 38 L (true: 37.85). To go from liters to US gallons, divide by 4 and add 5%: 20 L ÷ 4 = 5, plus 5% = 5.25 gal (true: 5.28). The 0.05% error margin in these shortcuts is small enough for everyday conversation.

For UK gallons, multiply by 4.5 and that is your answer to within 0.1%. So 10 UK gal ≈ 45 L (true: 45.46). The Imperial gallon happens to be very close to 4.5 L, which is one reason metric conversion in the UK has been gradual: the rounded mental shortcut works almost exactly.

• 1 US gal = 3.785411784 L exactly (NIST)
• 1 UK gal = 4.54609 L exactly
• 1 US gal = 0.832674 UK gal
• 1 US gal = 4 qt = 8 pt = 16 cups = 128 fl oz
• 1 US dry gal = 4.40488 L
• 1 bushel = 8 US dry gal = 35.239 L
• 1 US fl oz = 29.5735 mL
• 1 UK fl oz = 28.4131 mL (smaller than US)

Common gallon-conversion mistakes

The single most common mistake is treating US and UK gallons as interchangeable. The 20% gap means a recipe scaled from a UK source will be 20% over-volume if "gallon" is read as US, and vice versa. International cookbooks, fuel-economy comparisons and chemical-trade documents must specify the system.

The second is rounding 3.78541 to 3.8 or worse, 4. The 0.4% error from 3.78 → 3.8 is invisible in a 1-gallon recipe but becomes a measurable inventory variance in a fuel terminal handling 1 million gallons a day. Always store the full factor in software, then round only the output.

The third is the dry-vs-liquid gallon swap inside the US. Berries, grains and produce sold by volume use the dry gallon; everything else uses the liquid gallon. Mixing the two in farmers'-market accounting can shift quantities by 16% — small for a single sale, but a real problem for wholesale reporting.