Gallons to Liters Converter

US gallons to liters — quick reference

The formula, worked line by line

A US gallon is exactly 3.785411784 liters, so US gallons into liters is a single multiplication by that constant. It is an exact figure rather than a measurement, and it comes from two definitions stacked on top of each other.

The US gallon is legally 231 cubic inches, and the inch has been exactly 25.4 mm since the international yard and pound agreement of 1959. Multiply 231 by 2.54 cubed and you get 3,785.411784 cubic centimetres exactly, with nothing left over to round.

The imperial gallon needs a different constant, 4.54609, exact since the Weights and Measures Act 1985. Because both are exact, the difference between them is exact too: 0.760678216 liters per gallon, or 20.09 percent of the US figure. That is the whole arithmetic of the mistake, and it scales linearly, so the error on a 50-gallon figure is fifty times the error on a 1-gallon one.

liters = US gallons × 3.785411784
liters = imperial gallons × 4.54609
15 × 3.785411784 = 56.78 L
error from the wrong gallon = gallons × 0.760678216 L

The rules of thumb and their error budgets. Multiplying by four gives 60 liters for 15 gallons against a true 56.78, so it overstates by 5.67 percent; the fix is to take four times the gallons and then subtract about five percent, or more precisely multiply by four and then by 0.9464. The overstatement is the safe direction for choosing a container and the unsafe direction for ordering liquid to fill one.

What this conversion will not do

Two limits worth stating plainly. This is a volume conversion only — it will not turn gallons into kilograms, because that needs the density of the specific liquid, and water is the only substance where the numbers happen to line up conveniently. And the smaller US units divide cleanly out of the gallon if you need them.

How to use the gallons to liters converter

Type a US gallon figure and the liter equivalent appears as you type, to two decimal places; Swap reverses the direction without retyping. This is the outward-facing conversion — you have an American number and the equipment, the audience or the supplier is metric.

The four common cases are scaling a US recipe or a brewing batch for metric vessels, quoting a tank or appliance rating to someone abroad, matching a US capacity against imported equipment sold in liters, and working out how much fuel a US-rated tank will take at a foreign pump.

The gallon this tool uses is the US gallon, defined as exactly 231 cubic inches and therefore exactly 3.785411784 liters. The imperial gallon still used in parts of the UK and in Canada is a different size, exactly 4.54609 liters, and it shares the name without qualification.

Brewing and scaled recipes are where the numbers get reused most. A 5-gallon batch is 18.93 liters, a 1-gallon jug is 3.79 liters, and a 10-gallon system is 37.85 liters — worth converting once and marking on the vessel, because a fermenter, stockpot or carboy graduated in liters will never show you a gallon line. The awkward part is that the round American number becomes an untidy metric one, and the temptation is to round it to something neat.

Tank and equipment specs are the other driver, and here rounding in the safe direction actually matters. A 15-gallon aquarium is 56.78 liters, a 40-gallon water heater is 151.42 liters, and a 15-gallon fuel tank is about 56.8 liters at a metric pump. Because filters, heaters, stands and pumps abroad are all sized in liters, the liter figure is the one that matches what you can buy.

The old shortcut of multiplying by four runs high — 15 gallons becomes 60 liters against a true 56.78 — which is the harmless direction when you are checking that a container is big enough, and the wrong direction when you are calculating how much liquid you need to fill it, since you would over-order by about 5.7 percent.

Which gallon? The 20 percent question

One further trap deserves its own section, because it is the single most expensive mistake on this page: reading a gallon figure with the wrong gallon in mind moves the answer by a fifth.

The reference table above prices the mistake at every common size, and the practical rule is simple: a figure from a US source, a US window sticker or a US appliance label is a US gallon, while UK, Canadian, older marine and older aviation documents may well mean the imperial one.

Every US gallons value, worked out

59 common US gallons figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.

Questions people ask