Fuel Economy Converter
US mpg, imperial mpg, kilometres per litre, litres per 100 km and gallons per 100 miles. Half of those measure distance per volume and half measure volume per distance, which makes the conversion a reciprocal rather than a factor — and makes equal mpg steps very unequal savings.
Convert a fuel economy figure
The figure you have, in the unit chosen below.
Check whether an mpg figure is US or imperial before you trust it — they differ by about 20%.
The unit you want the answer in. Every other unit is listed underneath the result anyway.
These two units measure opposite things, so this is a reciprocal: 30 mpg (US) is 12.7543 km/L, and the answer is one divided by that, not a factor times it.
- Fuel over 10,000 miles
- 333.333 US gallons
- Fuel over 10,000 km
- 784.049 litres
- Which way is better
- A lower L/100 km figure is the better car.
- Definition
- Litres burned every 100 kilometres — the standard across Europe. Because it is a volume per distance, it scales with what you actually pay.
| Unit | Symbol | Value | Better when |
|---|---|---|---|
| US miles per gallon | mpg (US) | 30 | Higher |
| Imperial miles per gallon | mpg (imp) | 36.0285 | Higher |
| Kilometres per litre | km/L | 12.7543 | Higher |
| Litres per 100 kilometres | L/100 km | 7.84049 | Lower |
| US gallons per 100 miles | gal/100 mi | 3.33333 | Lower |
Two families of unit, and why that changes the arithmetic
Nearly every unit converter is a factor table: store how many base units each unit is worth, multiply on the way in, divide on the way out. Fuel economy breaks that, because the five units in common use are not all measuring the same thing.
- Distance per volume — US mpg, imperial mpg, km/L. Higher is better.
- Volume per distance — L/100 km, gal/100 mi. Lower is better.
Those two families are reciprocals of one another. Every value here is routed through kilometres per litre, and the direction of travel depends on which family the unit belongs to:
distance/volume: km/L = value × f • volume/distance: km/L = f ÷ value- f
- The km/L a value of exactly 1 in that unit represents — 0.4251 for US mpg, 100 for L/100 km
- km/L
- The base every conversion passes through
- result
- The same two lines run backwards, into the target unit
The two gallon sizes are exact definitions, not measurements. A US liquid gallon is exactly 231 cubic inches, and since the inch has been exactly 2.54 cm since 1959, that is exactly 3.785411784 litres. An imperial gallon has been exactly 4.54609 litres since the UK Weights and Measures Act 1985. A statute mile is exactly 1.609344 km. Combining them gives the two constants worth memorising:
- 235.215 ÷ US mpg = L/100 km, and the same division run backwards.
- 282.481 ÷ imperial mpg = L/100 km — a different constant, for a gallon 20% larger.
The MPG illusion, over 10,000 miles
Because mpg is a reciprocal scale, the same numerical improvement means wildly different amounts of fuel depending on where you start. Fuel used over 10,000 miles is simply 10,000 divided by the mpg figure:
| Economy | Gallons per 10,000 miles | L/100 km |
|---|---|---|
| 10 US mpg | 1,000 | 23.52 |
| 15 US mpg | 667 | 15.68 |
| 20 US mpg | 500 | 11.76 |
| 30 US mpg | 333 | 7.84 |
| 40 US mpg | 250 | 5.88 |
| 50 US mpg | 200 | 4.70 |
Read the first two rows: improving from 10 to 15 mpg — five miles per gallon — saves 333 gallons over 10,000 miles. Now read the last three: improving from 30 to 40 mpg saves 83 gallons, and 40 to 50 saves another 50. Ten miles per gallon at the top of the table is worth a quarter of five miles per gallon at the bottom.
The sharpest version of this, and the one Richard Larrick and Jack Soll used in Science in 2008, is a straight comparison: replacing a 10 mpg vehicle with a 20 mpg one saves 500 gallons per 10,000 miles. Replacing a 20 mpg vehicle with a 50 mpg one — a far more impressive-sounding upgrade — saves 300. If a household runs two cars and can only change one, the arithmetic says change the thirsty one.
None of this is visible on the mpg scale, and all of it is obvious in the L/100 km column, where the differences are the savings: 23.52 − 15.68 = 7.84 L per 100 km for the first step, and 7.84 − 5.88 = 1.96 for the second. Exactly four times less, stated plainly. That linearity is why Europe measures fuel this way, and why the US EPA added a gallons-per-100-miles figure to the redesigned window sticker for 2013 model-year cars.
What this converter assumes
- An mpg figure is either US or imperial, and you know which. The tool cannot tell them apart — 40 is a plausible number in both — and the difference is 20%. UK and Irish figures are imperial; American and Canadian-marketed US figures are not.
- The gallon and mile sizes are exact definitions rather than measurements, so the conversion introduces no error of its own. The uncertainty in your answer is entirely the uncertainty in the figure you typed.
- Economy figures are test results, not promises. EPA city/highway numbers, WLTP figures and a hand-calculated tankful all measure different things, and none of them predicts your commute. Use this to compare like with like.
- Nothing here costs anything out. Fuel prices are local and volatile, and a converter that quoted them would be wrong within a week.
- Electric consumption is out of scope. kWh per 100 km is energy per distance, a fourth quantity, and bridging to it requires an efficiency assumption rather than a conversion factor.
Fuel economy conversion FAQ
How do I convert mpg to litres per 100 km?
Divide 235.215 by the US mpg figure. 30 US mpg is 235.215 ÷ 30 = 7.84 L/100 km. If the figure is imperial mpg — a UK number — the constant is 282.481 instead, because an imperial gallon is larger. Both constants come from the same arithmetic: 100 × litres-per-gallon ÷ kilometres-per-mile.
Why is the conversion a division rather than a multiplication?
Because mpg and L/100 km are not measuring the same quantity. Miles per gallon is distance divided by volume; litres per 100 km is volume divided by distance. They are reciprocals of one another, so converting between them flips the fraction. A converter built as a plain factor table gets a plausible-looking wrong answer here — 30 US mpg is 12.75 km/L and 7.84 L/100 km, and only one of those is a multiplication.
Why does the same car get better mpg in Britain than in America?
It does not — the gallon is different. A US liquid gallon is exactly 231 cubic inches, which works out to 3.785411784 litres. An imperial gallon has been exactly 4.54609 litres since the UK Weights and Measures Act 1985. The imperial gallon is about 20.1% larger, so any imperial mpg figure is about 20% higher than the US figure for the identical car. Multiply an imperial mpg by 0.8327 to get the US figure.
Does going from 30 to 40 mpg save as much fuel as 10 to 15 mpg?
No, and it is not close. Over 10,000 miles, 10 mpg burns 1,000 gallons and 15 mpg burns 667 — a saving of 333 gallons for a 5 mpg improvement. Over the same 10,000 miles, 30 mpg burns 333 gallons and 40 mpg burns 250 — a saving of 83 gallons for a 10 mpg improvement. Twice the mpg gain, a quarter of the fuel saved. That is what a reciprocal scale does, and it is why replacing the worst vehicle in a household saves more than upgrading the best one.
What is gallons per 100 miles, and why is it on new window stickers?
It is the American answer to the problem above. The US EPA added a gallons-per-100-miles figure to the redesigned fuel economy label for 2013 model-year vehicles, because a volume-per-distance number is linear in fuel: the difference between two cars' figures is the fuel you actually save, with no reciprocal to reason through. Europe has always used litres per 100 km, which has the same property.
Can I use this for an electric car?
No. Electric consumption is measured in kWh per 100 km or miles per kWh — energy per distance, not volume per distance. Converting between the two requires assuming how much energy a litre of fuel delivers to the wheels, which depends on the engine, and that assumption is the whole answer rather than a unit conversion. MPGe, the EPA's electric equivalent, is built on exactly such an assumption (33.7 kWh to a gallon of gasoline) and is not a measurement of anything the car does.
Sources and review notes
- US EPA and Department of Energy — fueleconomy.gov, the official source of US fuel economy figures and the gallons-per-100-miles label
- NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), for the exact gallon, litre and mile definitions used here
- US EPA — fuel economy and greenhouse gas labels for vehicles, including the 2013 label redesign
The conversion factors on this page are defined values rather than measured ones, so they do not go stale: the US gallon has been 231 cubic inches for two centuries, the imperial gallon exactly 4.54609 L since 1985, and the mile exactly 1.609344 km since 1959. What changes over time is the test procedure behind any published economy figure, not the arithmetic that converts it.