Fuel Economy Calculator
Convert fuel economy between US mpg, UK mpg, litres per 100 km, and km per litre — or work it out from the distance driven and fuel used.
How to use this calculator
- 1Choose whether you are converting a known figure or working it out from a drive.
- 2To convert, enter the number and pick its unit.
- 3To measure, enter the distance you drove and the fuel it took, and the calculator finds your economy.
How it works
Fuel-economy conversion
km/L = 100 ÷ (L/100km) US mpg = km/L × 3.785 ÷ 1.609 UK mpg = km/L × 4.546 ÷ 1.609 L/100km and mpg are reciprocals: efficiency ↔ consumption
Fuel economy is measured two opposite ways. Miles or kilometres per unit of fuel is a measure of efficiency, where bigger is better. Litres per 100 km is a measure of consumption, where smaller is better. Because they are reciprocals, they do not scale evenly: the fuel saved by improving from 20 to 25 mpg is far greater than from 45 to 50 mpg, even though both are a 5-mpg gain — which is exactly why the litres-per-100 km measure is often considered the more honest one.
Worked example
30 US mpg converts to a consumption of 7.84 litres per 100 km, or 12.75 km per litre. In UK terms it is 36 mpg, because the UK gallon is larger. Enter your own figure above and read it in all four units at once.
Fuel Economy Calculator: the complete guide
Two opposite ways to measure the same thing
There are two philosophies for expressing fuel economy, and they point in opposite directions. The English-speaking tradition uses miles per gallon — distance covered per unit of fuel — so higher is better. Most of the rest of the world uses litres per 100 kilometres — fuel burned per distance — so lower is better. A car cannot have a high mpg and a high L/100 km; they are inverses of each other.
This matters for intuition. Because they are reciprocals, equal steps in one are unequal in the other. Going from 15 to 20 mpg cuts consumption far more than going from 40 to 45 mpg, even though both look like a 5 mpg improvement. The L/100 km measure makes this visible, which is one reason regulators increasingly prefer it.
The mpg illusion
Studies have shown that people systematically misjudge fuel savings when thinking in mpg, an effect known as the mpg illusion. Ask which change saves more fuel over the same distance — a car going from 10 to 20 mpg, or one going from 25 to 50 mpg — and most people pick the second because the mpg numbers rise more. The first actually saves twice as much fuel.
The reason is the reciprocal again: doubling a low mpg halves a large consumption, while doubling an already-high mpg halves an already-small one. This is why replacing the least efficient vehicle in a fleet delivers the biggest savings, and why converting to L/100 km — where the saving is simply the difference — removes the illusion entirely.
Measuring your own economy
The most reliable economy figure is the one you measure. Fill the tank completely, reset the trip odometer, drive normally for a few hundred miles, then fill up again and note how much fuel it took. Distance divided by fuel is your real economy, and it captures your actual driving, roads, and climate in a way no lab rating can.
Do this over several tanks and average the results, because a single tank is noisy — one long highway trip or a week of short cold-start commutes will skew it. The 'measure' mode above does the arithmetic; your job is just the odometer reading and the pump total. The number you get is the one to trust for budgeting a trip.
Frequently asked questions
How do I convert mpg to L/100km?
It is a reciprocal conversion, not a simple ratio. Divide 235.215 by the US mpg figure to get litres per 100 km — so 30 US mpg is 235.215 ÷ 30 ≈ 7.84 L/100 km. The calculator handles all four units, including UK mpg, automatically.
Why is UK mpg higher than US mpg for the same car?
Because a UK gallon is about 20% larger than a US gallon, so the same car travels further on one UK gallon. 30 US mpg is about 36 UK mpg. Neither is 'better' — they measure the same efficiency against different-sized gallons.
Which saves more fuel, improving a low or a high mpg?
Improving a low one, for the same step. Going from 15 to 20 mpg saves far more fuel over a given distance than 40 to 45 mpg, because fuel economy is reciprocal. This counterintuitive fact is called the mpg illusion, and converting to L/100 km makes it obvious.
How do I measure my car's real fuel economy?
Fill the tank, reset the trip meter, drive normally, then refill and record the fuel added and distance covered. Distance ÷ fuel is your economy. Average several tanks for accuracy — a single tank is easily skewed by one unusual trip.