Gallons to Liters Converter — US & UK (Imperial) Gallons with a Draggable Live Tank

How many liters are in a gallon?

There are exactly 3.785411784 liters in one US liquid gallon and exactly 4.54609 liters in one UK (Imperial) gallon. Both figures are legal definitions, not rounded approximations, and both are used every day — which is why a converter that only knows one of them is actively misleading.

The US gallon is defined as 231 cubic inches; the UK gallon is defined as 4.54609 cubic decimetres. That makes the Imperial gallon about 20% larger, and the gap between them is around 0.76 liters per gallon — enough to matter in fuel economy, brewing, aquariums and imported product specs.

Which gallon do you have? US vs UK vs dry

The word “gallon” is ambiguous, and the ambiguity is not academic. If a US recipe says 5 gallons and you convert with the UK factor, you will add nearly 4 extra liters. If a UK fuel figure says 40 miles per gallon and you convert it with the US factor, your liters-per-100 km result will be wrong by 20%.

💡 Practical rule: if the source is American (a US car spec, a US recipe, a US product label), use 3.785411784. If it is British or Commonwealth and predates metrication — or is a UK MPG figure — use 4.54609. If you are unsure, this calculator shows both at once so you can see the size of the difference before committing.

Why are there two different gallons?

The divergence is a historical accident, not a mathematical one. England historically used several different gallons: a wine gallon, an ale gallon, and a corn gallon, each with its own size. The wine gallon of 231 cubic inches was standardised during the reign of Queen Anne in 1706 and travelled with the American colonists.[reference:2]

In 1824, Britain replaced its old wine and ale gallons with a single new Imperial gallon, defined as the volume of 10 pounds of distilled water at 62°F. That works out to about 277.42 cubic inches — roughly 20% larger than the wine gallon the Americans had already adopted. Britain abandoned the wine gallon in 1826; the United States kept it.

The result is that the two countries have been using different gallons ever since. Canada, Australia, New Zealand and Ireland used the Imperial gallon historically and have since metricated, but Imperial gallons still appear in older documents, beer keg sizes and fuel-economy discussions.

Is the Imperial gallon still used in the UK today?

Partly, and the situation is more nuanced than “the UK went metric”. Since 1995 the gallon has not been a primary legal unit of trade in the UK — petrol is sold by the litre at every pump, and most goods are priced per litre or per kilogram. But several Imperial usages survive:

• Fuel economy. Miles per Imperial gallon is still the default way British drivers and motoring journalists discuss efficiency, even though the fuel itself is bought in litres. That is why converting UK MPG to L/100 km uses 282.481, not 235.215.
• Beer and cider. Draught beer is still legally sold by the Imperial pint (568.26125 mL), and kegs are commonly described in Imperial gallons — a standard UK keg is 11 Imperial gallons, which is nominally 50 litres.
• Milk in returnable bottles. Still sold by the pint in some outlets.
• Informal speech. “A gallon of beer”, “a gallon of paint” and similar phrases persist in everyday British English long after the unit left official trade.

So if you see a UK figure in gallons today, it is almost always either a fuel-economy figure or a beer/keg reference — and in both cases it means the Imperial gallon of 4.54609 litres.

The conversion formulas

Every one of these is a single multiplication or division. There is no rounding hidden inside the calculator: it multiplies by the exact defined factor and only rounds the displayed number. The live matrix in the tool uses these same six relationships to produce all fifteen output units, so the figures are always internally consistent.

How to use this converter

1. Choose your input unit. US gallon, UK / Imperial gallon, or Liter for reverse conversion. The three results for the other units update immediately, so you can always see both gallon definitions side by side.
2. Type a value, drag the tank, or move the slider. All three controls are two-way: typing changes the tank and slider, dragging changes the input and slider, and sliding changes the input and tank. The formula strip below shows the exact arithmetic with your numbers substituted in.
3. Read the comparison panels. The dual tanks show one US gallon and one UK gallon at the same scale, so the 20% difference is visible rather than abstract. The volume ladder breaks your value into whole gallons, quarts, pints, cups and fluid ounces.
4. Use a preset. Fuel-tank and container presets set both the unit and the value — a US half-barrel keg, a 55-gallon drum, a UK 11-gallon keg, a 20-litre fuel can, a 10-gallon aquarium, and more.
5. Batch-convert a list. Paste values separated by commas, spaces, semicolons or newlines into the batch panel, and get a full table with all gallon types plus a CSV download.
6. Compare fuel. Enter a US price per gallon and a UK price per litre (plus an exchange rate if the currencies differ) to see which is cheaper per litre and per US gallon. The MPG panel shows the same MPG figure interpreted as US and as UK gallons.
7. Copy, share or export. Click any result card or matrix row to copy it, share a link that reproduces your exact unit and value, or download the tank as a PNG image.

Worked examples

5 US gallons of fuel

5 × 3.785411784 = 18.92705892 liters. If the same 5 gallons were Imperial gallons, it would be 5 × 4.54609 = 22.73045 liters — a difference of 3.8 liters, roughly a whole extra US gallon.

A 10-gallon aquarium

US: 10 × 3.785411784 = 37.85411784 L. A 10-gallon US aquarium is therefore about 37.9 liters, which is the figure most European aquarium products use. If the tank were specified in Imperial gallons it would be 45.46 L — a 20% larger tank with the same nominal number.

A 15.5-gallon US keg

15.5 × 3.785411784 = 58.67388265 L. This is the standard US half-barrel keg, roughly 124 US pints.

An 11-gallon UK keg

11 × 4.54609 = 50.00699 L. The UK standard keg is nominally 50 liters, which is why the Imperial figure is 11 gallons rather than 10.

A 55-gallon drum

55 × 3.785411784 = 208.19764812 L. The international standard drum is 200 litres, so the two are close but not identical — a point that matters when a drum is described as “55 gallon” in the US and “200 litre” elsewhere.

Converting 20 liters back to gallons

20 ÷ 3.785411784 = 5.28344 US gallons. 20 ÷ 4.54609 = 4.39938 UK gallons. A 20-liter jerrycan is therefore about 5.28 US gallons or 4.40 Imperial gallons.

Where this conversion matters

• Fuel economy. US miles per gallon and UK miles per gallon are not comparable until you know which gallon each uses. Converting US MPG to liters per 100 km uses 235.215; converting UK MPG uses 282.481. The difference comes entirely from 3.785411784 vs 4.54609.
• Fuel pricing. The US quotes petrol per gallon; the UK quotes it per litre. Converting a UK pump price to an equivalent US gallon price means multiplying by 4.54609 (if you want the Imperial gallon) or by 3.785411784 (if you want the US gallon equivalent) before converting currency.
• Brewing and beer kegs. A US homebrew batch is usually 5 US gallons (18.9 L); a metric homebrew batch is usually 20 L. A US half-barrel keg is 15.5 US gallons (58.7 L); a UK keg is 11 Imperial gallons (50.0 L).
• Aquariums. Tank sizes are quoted in US gallons in North America and in litres almost everywhere else. A “55-gallon” tank is 208 L in US gallons; if it were Imperial it would be 250 L, which is why the US figure is the one used internationally for that standard tank.
• Cooking and baking. Large-batch recipes in US cookbooks use US gallons; older British recipes may use Imperial. Scaling a recipe across the two without converting the gallon type produces a 20% error.
• Pools and water storage. Pool volume, water butts, rain barrels and chemical dosing are all quoted in gallons or litres depending on the country. Dosing rates assume the correct gallon type.
• Shipping and import. Product labels from the US give US gallons; labels from the UK or Commonwealth may give Imperial gallons or litres. Customs and logistics paperwork often require the litre figure.

Gallons to liters conversion chart

The table below covers the values people ask about most often. The last column shows how much larger the Imperial result is, which is the size of the error you would make by using the wrong factor.

Quarts, pints, cups and fluid ounces

Gallons are built from smaller units, and those smaller units also differ between the US and UK systems. This is why a US pint of beer is smaller than a UK pint even though both are called a pint.

MPG, litres per 100 km and the gallon trap

Fuel economy is where the US/UK gallon difference causes the most confusion, because both countries still use “MPG” while measuring the gallon differently. The conversion constants are not arbitrary — they are just 3.785411784 and 4.54609 folded into a single divisor:

So a car that returns 30 US MPG is doing 235.215 ÷ 30 = 7.84 L/100 km. The same number, 30, read as 30 UK MPG, is 282.481 ÷ 30 = 9.42 L/100 km. That is a 20% difference in apparent efficiency from the same digits — exactly the 20% between the two gallons.

The relationship is its own inverse, which is why the same constant converts both ways. It is also non-linear: doubling MPG does not double efficiency in L/100 km terms, it halves the fuel used per distance — which is why comparing MPG figures directly can be misleading even within one gallon system.

Common mistakes

• Using the US factor for UK gallons. The single biggest error. A UK gallon is 20% larger, so using 3.785411784 instead of 4.54609 understates the litre figure by about 17%.
• Using the UK factor for US gallons. The reverse error overstates the litre figure by about 20%.
• Assuming “gallon” means the same thing everywhere. If a source does not say which country it comes from, check. Product labels, recipes and car specs almost always originate in one system or the other.
• Converting fuel economy with the wrong factor. US MPG to L/100 km uses 235.215; UK MPG to L/100 km uses 282.481. Using the wrong one makes a car look 20% more or less efficient than it is.
• Confusing the dry gallon with the liquid gallon. The US dry gallon is 4.40488377086 L. It is for grain and other dry commodities, never for fuel, milk or water.[reference:11]
• Rounding too early. Rounding 3.785411784 to 3.8 at the start of a large calculation introduces a 0.4% error. Multiply by the exact factor and round only the final display.
• Confusing fluid ounces with ounces by weight. A US fluid ounce is a volume; an ounce is a mass. They are not interchangeable, and the conversion between them depends on the density of the liquid.
• Mixing US and UK sub-units. “4 quarts in a gallon” is true in both systems, but a US quart (0.946 L) is not the same as a UK quart (1.137 L). Multiplying a US quart figure by 4 to get UK gallons gives the wrong answer.

Privacy

Every conversion, tank drag, slider move, batch calculation, fuel comparison and export runs in your browser with plain JavaScript. Nothing you type is uploaded, logged or sent to a server. There is no account and no network request after the page loads. Recent conversions are optionally kept in your browser's localStorage so you can reload them, and clearing them removes them permanently from your device.

Limitations

• The tank is a visual scale, not a specification of any real container. Its capacity adjusts automatically or can be fixed with the capacity selector.
• Batch mode processes up to 100 values at a time. Negative values are rejected; zero is accepted.
• The fuel-price comparison assumes you supply the exchange rate yourself. It does not fetch live rates, because rates change constantly and a hard-coded value would quickly be wrong.
• Displayed values are rounded for readability; the underlying calculation uses the exact defined factors. For regulatory or scientific work, use the factors printed in the formula section directly.
• The converter covers US liquid gallons, UK / Imperial gallons, liters and US dry gallons. It does not cover historical gallons such as the ale gallon or the Winchester corn gallon.

Frequently asked questions

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