Convert Gallons to Liters

A gallon is not a gallon. The US gallon is 3.785411784 liters, and the imperial gallon, used in the UK and Commonwealth, is 4.54609 liters. That is a 20% difference. If you convert a 10-gallon fuel purchase with the wrong gallon, you are off by more than 7.5 liters. State which gallon you mean before you convert. It provides both, with exact factors, a reference chart, and the formulas that make the math repeatable.

The question has two answers, and you must pick one first. For the US gallon, the answer is 3.785411784 liters, a figure fixed by US law and published in NIST Handbook 44 Appendix C. For the imperial gallon, the answer is 4.54609 liters, set by the UK Weights and Measures Act 1824. The imperial gallon is larger because it was defined as the volume of 10 pounds of water, while the US gallon descends from an older wine gallon of 231 cubic inches. Memorize the two numbers: 3.785411784 and 4.54609.

The conversion is a single multiplication. For US gallons: liters = gallons × 3.785411784. For imperial gallons: liters = gallons × 4.54609. Write the unit into your work, because the gallon you pick changes the answer by one fifth.

Worked Examples

Example one: 5 gallons to liters. If those are US gallons, 5 × 3.785411784 = 18.92705892 liters. If they are imperial gallons, 5 × 4.54609 = 22.73045 liters. The same 5 gallons, two answers 3.8 liters apart. Example two: a 55 US gallon drum. 55 × 3.785411784 = 208.19764812 liters, usually rounded to 208.2 L. That is the nominal industrial barrel size, and it is not 200 L, a mistake that costs you 8.2 liters per drum. Example three: a UK driver filling 10 imperial gallons. 10 × 4.54609 = 45.4609 liters, not the 37.85 liters a US-gallon assumption would produce.

For a rough mental check, remember that one US gallon is just under 3.8 liters, and one imperial gallon is just over 4.5 liters. That is enough for a sanity check on a pump or a recipe. For any real work, use the exact factors. The error from rounding to 3.8 or 4.0 is too large for anything beyond a guess.

The table below gives the exact liter equivalents for both gallons across the ranges you are most likely to meet. The US column uses 3.785411784 L/gal, the imperial column uses 4.54609 L/gal. Use it to read off a value, or to confirm a calculation you just did by hand.

The distinction is not a technicality; it is the difference between a correct answer and a 20% error. The US gallon is 231 cubic inches exactly, and the imperial gallon is 4.54609 liters exactly. The imperial gallon is about 20% larger. That matters most at the fuel pump: a UK driver paying per imperial gallon is buying 4.54609 liters, while a US driver paying per US gallon is buying 3.785411784 liters. The price per liter is not comparable without adjusting for the gallon.

If you are converting a recipe, the rule is simple: US recipes use US gallons, UK and Australian recipes use imperial gallons. Note that Australian recipes usually measure in metric cups (250 mL) and liters, not gallons. If you are converting fuel economy, a car rated at 30 MPG (US) is rated at about 36 MPG (imperial), because the imperial gallon is larger. Never compare the two MPG figures directly.

When you work in the other direction, the value ranges change the feel of the numbers.97 imperial gallons. A 55 US gallon drum is 208.2 liters, not 200. A 5-liter jug is 1.32 US gallons or 1.10 imperial gallons. The ranges tell you which gallon the other party means without asking: if someone says a 'gallon' of milk in the US, it is 3.785 L; in the UK, it is 4.546 L. Keep a conversion factor at hand and write the gallon name into every calculation.

One fact that settles many arguments: the liter is exactly one cubic decimeter. That is the current definition, fixed in 1964. It is not the same as the old 'kilogram of water' definition. From 1901 to 1964, the liter was defined as the volume of 1 kg of water, but that was a specific measurement with a specific temperature and pressure, and it is obsolete. A liter is now exactly 1 dm³, no more, no less. The mass of 1 L of water at 20°C is about 0.998 kg, so the old claim that 'a liter is a kilogram of water' is close but not exact. For precise work, use the cubic decimeter definition and the density of water at the temperature you need.

The fluid ounce is where the US and imperial systems diverge in the opposite direction. The US fluid ounce is 1/128 of a US gallon, about 29.574 mL. The imperial fluid ounce is 1/160 of an imperial gallon, about 28.413 mL. So the imperial gallon is larger, but the imperial fluid ounce is smaller. A US pint is 473.176 mL, an imperial pint is 568.261 mL. If a UK recipe calls for a pint of milk, that is 568 mL, not 473 mL. The US FDA cup, used for nutrition labeling, is 240 mL, while the metric cup in Australia is 250 mL, and the US culinary cup is a derived unit from the US gallon. Three different 'cups', three different volumes. When a recipe says 'cup', know which standard it is using, or you will be off by 10 to 25 mL per cup.

If you need to turn gallons to liters inside a spreadsheet, the Microsoft CONVERT function handles it, but only if you use the right unit codes. For the US gallon, the code is "gal". For the imperial gallon, the code is "uk_gal". For the liter, the code is "l" or "L". A formula like =CONVERT(5,"gal","l") returns 18.92705892, while =CONVERT(5,"uk_gal","l") returns 22.73045. The function is stable, but the exact list of codes is in Microsoft's documentation, not here. If you mistype a code, the function returns a #N/A error, which is a clear signal to check your spelling. Do not assume "gal" means imperial; it does not. The US gallon is the default in that function, and that default is exactly the kind of assumption that causes a 20% error.

The 55 US gallon drum is a standard industrial container, but its metric equivalent is not 200 liters. It is 208.2 liters. The confusion comes from rounding: 55 gallons at 3.785 L/gal is 208.175 L, and someone rounded to 200 for convenience. That is an 8.175-liter error per drum. If you are ordering a drum of a chemical, a lubricant, or a food ingredient, ordering '200 L' instead of '208.175 L' means you are short by 3.9% of the drum's capacity.If a supplier quotes a drum in gallons, ask which gallon, and if they quote liters, check the arithmetic against the exact factor.

Fuel economy is the most common place the gallon ambiguity bites. A car rated at 30 MPG in the US is rated at about 36 MPG in the UK, not because the car changed, but because the imperial gallon is larger. The US MPG is miles per US gallon (3.785 L), and the imperial MPG is miles per imperial gallon (4.546 L).If you are reading a European review that quotes L/100km, convert that to US MPG using 235.215 divided by the L/100km value. The EPA publishes US ratings, and Natural Resources Canada publishes Canadian ratings, which use imperial gallons. Never quote one without the other.

This conversion is for anyone who must move between the US and imperial systems without losing a fifth of the volume. It fits the US home cook who has an imported recipe and needs to turn gallons to liters, then to cups and fluid ounces. It fits the UK or Commonwealth driver who tracks fuel in liters but thinks in miles per imperial gallon. It fits the traveller moving between the US and UK who must convert fuel prices from dollars per liter to pence per imperial gallon. It does not fit the professional engineer who already knows the exact definitions and needs a standards document, nor the person converting a single, unambiguous measurement like a US gallon of water in a recipe, where the US gallon is the only sensible choice. And it does not fit anyone doing currency conversion, because the price per liter or gallon changes daily and must be sourced to a live exchange rate. If the context is clear, a simple calculator is enough. If it is not, this is the place to get the number right.