Fuel Price Converter — litres, US gallons and imperial gallons

Fuel is sold by the litre in most of the world, by the US gallon in the United States, and by the imperial gallon in a handful of places that never fully went metric. Those three units are not interchangeable, and two of them share a name while differing by roughly a fifth. This fuel price converter moves a price between all three using the exact legal definitions, and compares two stations that quote in different units on a single common basis.

Arb Digital publishes this as part of a wider set of everyday conversion tools, because the fuel-price comparison is one of the few unit problems people meet regularly with real money attached. Getting it wrong once at a hire-car desk or on a cross-border drive is more expensive than any other unit slip in daily life.

What This Fuel Price Converter Does

Enter a price and say which unit it is quoted in, and the tool returns the equivalent per litre, per US gallon and per imperial gallon. Choose which of the three you want as the headline figure and it moves to the top. Enter a second price in any of the three units and it converts both to a common basis, tells you which is cheaper, by what percentage, and what the difference amounts to on a fill of the size you specify.

No exchange rate is applied. The currency field is a label only, so both prices must already be in the same currency for the comparison to mean anything.

How to Use It

1. Type the price exactly as advertised and select the unit it is quoted in. Leave the currency symbol out of the number field.
2. Pick the headline unit you think in. The three-way conversion is shown regardless, so nothing is hidden.
3. Add a second station price in whatever unit it is quoted in. Set it to zero if you only want the conversion.
4. Enter the size of a typical fill in litres to see the difference as money rather than as a fraction of a cent.
5. Set the currency symbol for display. It changes nothing in the arithmetic.

The Formula: The Exact Definitions

Both conversions rest on definitions that are exact by law rather than measured.

The US liquid gallon is defined as 231 cubic inches, and with the inch defined as exactly 25.4 millimetres this makes it exactly 3.785411784 litres. The National Institute of Standards and Technology publishes the conversion factor tables that carry this figure, and its Unit Conversion page directs readers to those tables in NIST Special Publication 811 Appendix B and NIST Handbook 44 Appendix C, with the standing instruction to use exact conversion factors whenever they are available.

The imperial gallon is defined in United Kingdom law as exactly 4.54609 cubic decimetres, which is 4.54609 litres. Schedule 1 of the Weights and Measures Act 1985 carries that definition. The imperial gallon is therefore about 20.095 per cent larger than the US gallon.

To convert a price, multiply or divide by the volume ratio in the direction opposite to the volume. A larger unit holds more fuel, so the price per unit is larger. Price per US gallon = price per litre × 3.785411784. Price per imperial gallon = price per litre × 4.54609. Going the other way, divide.

Worked through with the defaults: 1.45 per litre becomes 1.45 × 3.785411784 = 5.489 per US gallon, and 1.45 × 4.54609 = 6.592 per imperial gallon. The second station at 5.60 per US gallon converts back to 5.60 ÷ 3.785411784 = 1.4794 per litre, so the first station is cheaper by 0.0294 a litre, about 2.0 per cent, which on a 50 litre fill is 1.47 in whatever currency you are using.

Why the Two Gallons Are Different at All

They come from different definitions of the same historical word. The US gallon descends from the English wine gallon of 231 cubic inches, fixed in law in 1707 and carried into American practice at independence. Britain then replaced its patchwork of wine, ale and corn gallons in 1824 with a single imperial gallon defined by the volume of ten pounds of water at a specified temperature — a larger unit. The United States never adopted the new definition, so the two systems diverged and stayed diverged.

The practical consequence is a 20 per cent gap that hides in plain sight because both are called a gallon. A price of 6.00 a gallon means substantially different things in Detroit and in a country still quoting imperial. Fuel economy figures carry exactly the same trap: an imperial miles-per-gallon figure is about 20 per cent higher than the US figure for the same car, which is why cross-border economy comparisons so often look wrong.

Where Fuel Price Comparisons Actually Go Wrong

Unit confusion is the obvious failure, but it is not the most common one. These are:

Different fuel grades. A station advertising a low price is often advertising its lowest-octane grade, while the one next door leads with mid-grade. Compare like with like or the conversion is precise arithmetic on the wrong pair of numbers.

Tax and levy differences. Pump prices in most countries are heavily tax-inclusive, and the tax component can differ between neighbouring jurisdictions by more than the retail margin. That is real money and it is a legitimate part of the comparison, but it means the difference tells you little about the retailer.

Membership and card prices. A displayed price that requires a loyalty card, a specific payment method or a minimum spend is not the price you will pay unless you meet the condition. Use the price that applies to you.

Detour cost. Driving further to reach a cheaper station burns fuel and time. On a 50 litre fill, a two per cent saving is roughly one and a half units of currency — often less than the fuel used getting there. The fuel cost calculator prices the detour itself, which is the check that usually settles the argument. That tool works out what a journey costs from distance, economy and a price you already have in the right unit; this page is the step before it, turning a price quoted in one unit into the unit you need.

How Many Decimal Places You Actually Need

Fuel prices are quoted to an unusual precision. In the United States the pump price carries a fixed nine tenths of a cent on the end, a pricing convention that has survived for a century. In much of Europe the price per litre is displayed to three decimal places. Both practices mean a conversion rounded to two places throws away real money at the scale of a full tank.

This tool keeps four decimal places on every converted price for that reason. On a 50 litre fill, a difference in the fourth decimal place is worth only a fraction of a currency unit, but a difference in the third is worth several — and rounding to two places can move the third. If you are comparing two stations that are genuinely close, keep the extra places until the very end and round once, at the total, rather than at each step.

Litres, Gallons and Fuel Economy Are Separate Problems

A fuel price conversion and a fuel economy conversion look similar and behave differently. Price scales directly with volume: a bigger unit costs proportionally more. Economy in miles per gallon scales the same way, but economy in litres per 100 kilometres is an inverse measure, so it moves the opposite way and cannot be converted by simple multiplication. Mixing the two up produces answers that are wrong by a factor rather than by a percentage.

The fuel economy converter handles that side of it properly, moving between miles per US gallon, miles per imperial gallon, litres per 100 kilometres and kilometres per litre. For plain volume rather than price or economy, the litres to gallons converter and the volume converter do the direct arithmetic.

Comparing Fuel the Way You Compare Groceries

The underlying skill here is the same one that makes supermarket shelf-edge labels useful: reduce every option to a price per common unit before comparing. Fuel makes it harder only because the common unit differs by country and because the two gallons share a name. Once both prices are per litre, the comparison is trivial and the percentage is meaningful.

The same reduction works on anything sold in mismatched pack sizes, which is what the unit price comparison calculator is for. And if the question is not which station to use but how much a habit costs, the idling fuel cost calculator prices engine time rather than distance.

Common Mistakes to Avoid

• Treating a gallon as a gallon — the imperial gallon is about 20 per cent larger than the US one, and both are written the same way.
• Dividing when you should multiply — a larger unit means a larger price per unit, so converting litres to gallons multiplies the price.
• Comparing different fuel grades — an exact conversion of the wrong two numbers is still the wrong answer.
• Forgetting the currency — this tool applies no exchange rate, so two prices in different currencies cannot be compared here.
• Chasing a small per-litre saving across town — price the detour before you drive it.

Related Free Tools From Arb Digital

Price a journey with the fuel cost calculator, convert consumption with the fuel economy converter, handle plain volume with the litres to gallons converter or the volume converter, and compare mismatched pack sizes with the unit price comparison calculator. The free online tools hub has more.

Frequently Asked Questions