Economy units and consumption units are opposites
Fuel efficiency is quoted two different ways around the world, and the two are reciprocals of each other. Economy units put distance on top: miles per gallon, kilometres per litre. A bigger number is better. Consumption units put fuel on top: litres per 100 km, US gallons per 100 miles. A smaller number is better.
That flip is the whole reason this conversion trips people up. You cannot convert between them by multiplying — you have to divide into a constant. Going from 30 US MPG to litres per 100 km is not 30 × k, it is 235.214583 ÷ 30. Get the direction wrong and you produce a number that looks plausible and is off by a factor of fifty.
There is a third complication: there are two gallons. The US liquid gallon is 231 cubic inches, which is 3.785411784 litres exactly. The imperial gallon used in the UK and historically across the Commonwealth is 4.54609 litres exactly. An imperial gallon is about 20% bigger, so the same car rates roughly 20% higher in imperial MPG than in US MPG with no change in efficiency at all. A British brochure quoting 48 MPG and an American window sticker quoting 40 MPG can describe identical vehicles. If you are also converting the tank capacity, the US to imperial gallons calculator handles that leg on its own.
The units themselves rest on definitions that have been fixed since 1959, when the international yard and pound agreement made the inch exactly 25.4 mm and therefore the mile exactly 1.609344 km. NIST Special Publication 811 restates those definitions. Because the inch, the litre and the gallon are all exact, the conversion constants below are exact too — they are not measurements with uncertainty.
Where 235.214583 comes from
Build the constant from the definitions rather than memorising it. A US gallon is 231 cubic inches, and an inch is 2.54 cm exactly, so a cubic inch is 2.54³ = 16.387064 cm³. Therefore one US gallon is 231 × 16.387064 = 3785.411784 cm³, which is 3.785411784 litres.
Now take a car that covers M miles on one US gallon. In 100 km it covers 100 ÷ 1.609344 = 62.13712 miles. Those miles need 62.13712 ÷ M gallons, which is 62.13712 × 3.785411784 ÷ M litres. Multiply the two constants and you get 235.2145833 ÷ M. That single number carries both the mile-to-kilometre conversion and the gallon-to-litre conversion, which is why it looks arbitrary until you factor it.
The imperial version is the same derivation with 4.54609 litres in place of 3.785411784: 100 × 4.54609 ÷ 1.609344 = 282.4809363. And kilometres per litre needs no constant at all, because litres per 100 km and kilometres per litre are pure reciprocals scaled by 100: km/L = 100 ÷ (L/100 km).
The ratio between the two MPG scales falls out of the two gallon sizes: 4.54609 ÷ 3.785411784 = 1.2009499. So imperial MPG is always US MPG multiplied by 1.2009499, whatever the vehicle. If you need the underlying distance and volume conversions separately, see the miles to kilometres calculator and the gallons to litres calculator.
Worked example: a 32 MPG (US) sedan
A window sticker reads 32 MPG combined. You want that in the units a European brochure would use, and you want the annual fuel bill at 12,000 miles a year and $3.50 a US gallon.
- Litres per 100 km. 235.2145833 ÷ 32 = 7.3505 L/100 km, which rounds to 7.35.
- Imperial MPG. 32 × 1.2009499 = 38.43 MPG (imp). The same car, the same fuel, a bigger gallon.
- Kilometres per litre. 100 ÷ 7.3505 = 13.60 km/L. Cross-check: 32 × 0.425143707 = 13.6046, which agrees.
- US gallons per 100 miles. 100 ÷ 32 = 3.125 gal/100 mi.
- Annual distance in kilometres. 12,000 × 1.609344 = 19,312.1 km.
- Fuel burned. 7.3505 × 19,312.1 ÷ 100 = 1,419.5 litres. Cross-check in US units: 12,000 ÷ 32 = 375 gallons, and 375 × 3.785411784 = 1,419.5 litres. The two routes agree, which is the check worth doing.
- Fuel price per litre. $3.50 ÷ 3.785411784 = $0.92460 per litre.
- Annual cost. 1,419.5 × 0.92460 = $1,312.50. Cross-check: 375 gallons × $3.50 = $1,312.50 exactly.
Every intermediate above can be reproduced on a phone calculator, and the two independent routes to the fuel bill landing on the same figure is what tells you no unit was dropped.
How to read the result, and the MPG trap
The single most useful thing this conversion exposes is that equal steps in MPG are not equal steps in fuel burned. Because the relationship is a reciprocal, a 5 MPG improvement saves a different amount of fuel depending on where you start.
Work it in gallons per 100 miles, where the arithmetic is linear. Going from 20 to 25 MPG changes consumption from 100 ÷ 20 = 5.00 to 100 ÷ 25 = 4.00 gallons per 100 miles: a saving of 1.00 gallon. Going from 40 to 50 MPG — twice the MPG improvement — changes it from 2.50 to 2.00 gallons per 100 miles: a saving of 0.50 gallons. The bigger MPG jump saves half as much fuel. In litres per 100 km the same pair reads 11.76 → 9.41 (2.35 L saved) against 5.88 → 4.70 (1.18 L saved), and you can read both pairs straight off the reference table below.
This is why the EPA prints gallons per 100 miles beside MPG on US fuel-economy labels, and why the rest of the world quotes consumption rather than economy in the first place. When you are comparing two cars you might buy, or costing a fleet decision, do the comparison in a consumption unit. Differences in consumption units add up and scale with distance; differences in MPG do neither.
For orientation on the numbers themselves: consumption around 5 L/100 km (47 US MPG) is typical territory for a small hybrid, around 8 L/100 km (29 US MPG) for a mid-size petrol sedan, and above 14 L/100 km (17 US MPG) for a large petrol pickup or SUV. Treat those as rough bands rather than published figures — the number that matters is your own, and the way to get it is to fill the tank, drive, refill, and divide. The fuel economy (MPG) calculator does that tank-to-tank arithmetic.
US MPG converted to every other fuel-economy unit
| US MPG | L/100 km | Imperial MPG | km/L | US gal/100 mi |
|---|---|---|---|---|
| 15 | 15.68 | 18.01 | 6.38 | 6.667 |
| 20 | 11.76 | 24.02 | 8.50 | 5.000 |
| 25 | 9.41 | 30.02 | 10.63 | 4.000 |
| 30 | 7.84 | 36.03 | 12.75 | 3.333 |
| 35 | 6.72 | 42.03 | 14.88 | 2.857 |
| 40 | 5.88 | 48.04 | 17.01 | 2.500 |
| 45 | 5.23 | 54.04 | 19.13 | 2.222 |
| 50 | 4.70 | 60.05 | 21.26 | 2.000 |
| 60 | 3.92 | 72.06 | 25.51 | 1.667 |
Read down the L/100 km column: the gap between rows shrinks as MPG rises, which is the reciprocal relationship made visible.
MPGe is not miles per gallon
An electric vehicle rated at 110 MPGe does not convert to a litres-per-100-km figure, because it burns no litres. MPGe is an energy-equivalence rating: the EPA treats 33.7 kWh of electricity as equivalent to one gallon of gasoline on an energy basis, and reports how far the car travels on that much electricity. Feeding an MPGe number into this calculator produces an arithmetically correct answer to a question nobody asked.
The comparable European figure for an EV is kWh per 100 km, which is a genuine consumption unit and behaves the same way L/100 km does. Convert MPGe to it by dividing 33.7 kWh by the MPGe rating and scaling to 100 km, not by using the constants on this page.
Mistakes that produce a wrong conversion
- Multiplying when you should divide. The reciprocal structure means the constant sits in the numerator, not as a multiplier. If your answer for a normal car is not somewhere between 3 and 25 L/100 km, you have inverted the operation.
- Mixing the two gallons. A UK figure fed in as US MPG comes out 20% too optimistic. Check the source: if the brochure is British, Irish or from an older Commonwealth market, it is almost certainly imperial.
- Comparing test cycles rather than cars. The EPA cycle, the European WLTP cycle and the older NEDC cycle produce different numbers for the same vehicle. Converting the units does not convert the test protocol, and NEDC figures in particular were widely optimistic relative to WLTP.
- Averaging MPG figures. The arithmetic mean of two MPG numbers is not the MPG of the two trips combined. Convert to gallons per 100 miles, average those, then convert back — or better, divide total fuel by total distance.
- Assuming km/L and L/100 km scale together. They are reciprocals scaled by 100. Doubling km/L halves L/100 km; it does not double it.
- Costing fuel in the wrong volume unit. A price quoted per imperial gallon is about 20% higher than the same fuel priced per US gallon. The price per unit comparison calculator is the general tool for that class of check.
Key terms
- US liquid gallon
- Exactly 231 cubic inches, which is 3.785411784 litres. Used for fuel throughout the United States.
- Imperial gallon
- Exactly 4.54609 litres, defined in UK law. About 20.095% larger than the US gallon, which is why imperial MPG figures look better.
- L/100 km
- Litres of fuel consumed per 100 kilometres travelled. The standard consumption unit across the EU, Canada, Australia and much of Asia.
- MPGe
- Miles per gallon of gasoline-equivalent. An EPA energy-equivalence rating for electric and plug-in vehicles, based on 33.7 kWh per gallon-equivalent.
- WLTP
- Worldwide Harmonised Light Vehicles Test Procedure, the laboratory cycle behind European fuel-economy figures since 2017, replacing NEDC.
When to reach for a different tool
This calculator converts a figure you already have. It does not measure one. If you want your car's real-world economy rather than its rated figure, record the odometer and the litres or gallons at each fill, then divide distance by fuel over several tanks — a single tank is dominated by how full the pump cut off.
If you are comparing running costs between two vehicles, convert both to the same consumption unit first, then multiply by your own annual distance and your own local fuel price. Rated economy differences of one or two MPG rarely survive contact with real driving, but a difference of five or more litres per 100 km will show up on every statement.
If you are converting speeds from the same foreign brochure, the mph to km/h calculator uses the same exact 1.609344 factor. And if you are converting engine or tank figures rather than economy, note that the volume conversions are ordinary multiplications — only economy and consumption units carry the reciprocal trap described here.
One last practical note: fuel economy figures depend on fuel type as well as vehicle. Diesel contains roughly 10% more energy per litre than gasoline, so a diesel and a petrol car with the same L/100 km figure are not equally efficient in energy terms, and their fuel prices differ. When you are comparing across fuel types, compare cost per distance, which is the number this calculator's annual figure gives you directly.