Fuel Economy Calculator
Calculate fuel economy from distance traveled and fuel used. Get mpg US, mpg UK, km per liter, and L/100 km to compare vehicles across regions.
Fuel Economy Calculator
Result will appear here...
What the fuel economy calculator does
Say you are comparing two cars, and one is sold with a figure in miles per gallon while the other quotes liters per 100 km. They are describing the same thing in different languages, and your head should not have to do the translation. That is what this tool is for.
You give it a distance and the fuel that distance used, and it hands back your fuel economy in all four common units at once: miles per gallon US, miles per gallon UK, kilometers per liter, and liters per 100 km. One measurement, every unit, so you can line up cars and figures from any region side by side.
How to use it
Just two things to enter.
- Distance. How far the vehicle travelled, in miles or kilometers, whichever you choose.
- Fuel Used. How much fuel that distance took. You can enter it in US gallons, UK (imperial) gallons, or litres, so it does not matter how your pump measured it.
Press Calculate and all four economy figures appear together. Press Reset to start again.
How the four figures are worked out
The tool first puts everything into one common pair of units, kilometers and liters, no matter what you entered. Then the core figure is a single division:
Kilometers per liter = distance in km ÷ fuel in liters
Once it has km per liter, the other three fall straight out of it. Liters per 100 km is just 100 divided by the km per liter. Miles per gallon US is the km per liter multiplied by about 2.352, and miles per gallon UK is the km per liter multiplied by about 2.825. Those two multipliers are the only reason the US and UK numbers come out different, which is the next section.
Why US mpg and UK mpg are not the same number
This catches a lot of people, so it is the most useful thing on this page. A US gallon and a UK gallon are not the same size. A US gallon is about 3.785 liters. A UK, or imperial, gallon is bigger, about 4.546 liters.
Because the UK gallon holds more fuel, the same car goes further on it, so its miles-per-gallon number comes out higher. As a rule, a UK mpg figure is roughly 20 percent higher than the US figure for the very same vehicle. A car rated at 32 mpg in the US is about 38 mpg in the UK. Nothing about the car changed, only the size of the gallon. So when you see a glowing mpg number on a British review and compare it to an American one, check which gallon they mean before you get excited.
An example with real numbers
Say a car covered 400 miles on 12.5 US gallons. Here is what the tool reports.
- Miles per gallon US = 400 ÷ 12.5 = 32
- Kilometers per liter works out to about 13.6
- Miles per gallon UK = about 38.4
- Liters per 100 km = about 7.35
Look at that 32 and that 38.4. Same car, same drive, same fuel, and the only difference between those two mpg numbers is which country's gallon you measured with. That is exactly the trap this calculator saves you from.
The four units at a glance
Here is what each one means and which direction is good.
| Unit | Measures | Better when |
|---|---|---|
| Miles per gallon (US) | Distance per US gallon | Higher |
| Miles per gallon (UK) | Distance per imperial gallon | Higher |
| Kilometers per liter | Distance per liter | Higher |
| Liters per 100 km | Fuel for a fixed 100 km | Lower |
The first three count distance on a set amount of fuel, so up is good. The last one counts fuel over a set distance, so down is good.
What counts as good economy
It varies by vehicle, but as a rough yardstick, a modern petrol sedan in the 30 to 40 mpg US range, or roughly 6 to 8 liters per 100 km, is doing well. Crossovers and small SUVs sit a bit lower, full size SUVs and trucks lower still, often under 25 mpg US. Hybrids land much higher, frequently 45 to 55 mpg US. Anything under about 20 mpg US for an ordinary passenger car is on the thirsty side. Compare like with like, a truck to a truck, not a truck to a hybrid hatchback.
Questions people ask
Why does this one not ask for fuel price?
Because economy and cost are two different questions. This tool measures how efficient the vehicle is, full stop. If you want the money side, the fuel cost and fuel consumption calculators take a price and give you cost per mile or per trip.
Is a 40 mpg car the same everywhere?
No. A UK gallon is larger than a US gallon, so 40 mpg measured in the UK is a less efficient car than 40 mpg measured in the US. Roughly, UK mpg runs about 20 percent higher than US mpg for the same vehicle. Always check which gallon a figure uses.
Can I use this just to convert between economy units?
Yes. Enter any real distance and the matching fuel amount, and you get the same economy expressed in all four units, which is effectively a conversion. For a pure unit conversion with no distance involved, the dedicated converters do that directly.
Which unit should I actually pay attention to?
The one your country uses for buying fuel and rating cars, so your comparisons stay consistent. Many people find liters per 100 km the easiest for thinking about cost and emissions, because it scales directly with the fuel you burn.
References
- National Institute of Standards and Technology (NIST), Special Publication 811: Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
- U.S. Department of Energy and U.S. Environmental Protection Agency, FuelEconomy.gov. https://www.fueleconomy.gov/
Skanda Aryal is a full stack engineer focused on accessible web experiences, with personal interests in time zones, travel, hiking, and geography. His enjoys playing with utilities tied to movement, schedules, places, and time based coordination. At Eon Tools, he reviews geography, transportation, times now, and date and time tools.