Fuel Economy Converter

Convert fuel economy between miles per gallon (US and UK), litres per 100 km, kilometres per litre and miles per litre, with CO₂ per km for petrol.

How to use the fuel Economy Converter

  1. Enter the value.
  2. Choose the from from the list.
  3. Choose the to from the list.
  4. The result updates instantly as you type. There is no button to press.
  5. Use Copy result to copy the figures, or Copy link to share a link that reopens the fuel Economy Converter with the same inputs.

How it works

Fuel economy is measured in two opposite ways. Distance per fuel units, such as miles per gallon (mpg) and kilometres per litre (km/L), tell you how far you go on a fixed amount of fuel, so higher is better. Fuel per distance, as in litres per 100 kilometres (L/100 km), used across Europe, Canada, Australia and much of Asia, tells you how much fuel a fixed distance needs, so lower is better.

Because they are reciprocals, you can't convert between them with a single multiplication. The calculator goes through kilometres per litre, so 30 mpg (US) becomes 12.75 km/L, and then 100 ÷ 12.75 = 7.84 L/100 km. One consequence is that mpg figures exaggerate improvements at the high end: going from 10 to 20 mpg saves far more fuel over a given distance than going from 40 to 50 mpg.

Watch out for US vs UK gallons. A UK gallon is about 20% bigger, so the same car shows a higher mpg figure in Britain than in the US: 30 US mpg is 36 UK mpg. The converter also estimates tailpipe CO₂ for a petrol car, since burning a litre of petrol releases about 2.31 kg of CO₂.

Formula

L/100 km = 235.215 / mpg (US) L/100 km = 282.481 / mpg (UK) km/L = 100 / (L/100 km) mpg (UK) = mpg (US) × 1.20095

Example

A car rated at 30 mpg (US) uses 235.215 ÷ 30 = 7.84 L/100 km, which is 12.75 km/L or 36.0 UK mpg, and emits roughly 181 g of CO₂ per km on petrol. A European car using 5 L/100 km gets 47.0 US mpg or 56.5 UK mpg.

Frequently asked questions

Is a lower L/100 km better?

Yes. L/100 km measures fuel used per distance, so lower means more efficient. With mpg and km/L, higher is better.

Why is UK mpg higher than US mpg for the same car?

Because a UK gallon (4.546 L) is larger than a US gallon (3.785 L). A car goes further on the bigger gallon, so the mpg number is about 20% higher.

How do I convert mpg to L/100 km quickly?

Divide 235 by the US mpg figure, or 282 by the UK mpg figure. 25 US mpg ≈ 9.4 L/100 km.