MPG calculator
Work out your car's real fuel economy from a fill-up: miles per gallon, litres per 100 km and the cost a mile.
UK gallons of 4.54609 litres, from the Weights and Measures Act. Price filled in from DESNZ for the week beginning 9 November 2026.
Result
400 miles on 40 litres is 45.5 miles per gallon.
miles per UK gallon
| Litres per 100 km | 6.21 |
|---|---|
| Miles per litre | 10 |
| Fuel cost a mile | 17.1p |
Show the workings
| Step | Sum | Answer |
|---|---|---|
| Gallons used | 40 litres ÷ 4.54609 litres a gallon | 8.799 gallons |
| Miles per gallon | 400 miles ÷ 8.799 gallons | 45.461 mpg |
| Litres per 100 km | 40 litres ÷ 643.7 km × 100 | 6.214 litres |
| Fuel cost a mile | 40 litres × 170.71p ÷ 400 miles | 17.1p |
What this leaves out or assumes
- Fill to full both times, so the litres you add are the fuel you used. UK gallons, which are bigger than US gallons.
Where the figures come from
- One UK gallon, in litres: 4.54609 litres. Source: Weights and Measures Act 1985, Schedule 1. Checked 13 November 2026.
- Kilometres in one mile: 1.609344 km. Source: Weights and Measures Act 1985, Schedule 1. Checked 13 November 2026.
How to measure your mpg
- Fill the tank to full and reset the trip counter.
- Drive as normal until you need fuel again.
- Fill to full again, and note the litres you put in and the miles on the trip counter.
The litres you put in are the fuel you used, so miles divided by gallons is your mpg. A longer gap between fill-ups gives a more accurate answer.
Miles per gallon and litres per 100 km
| Miles per UK gallon | Litres per 100 km |
|---|---|
| 30 | 9.4 |
| 40 | 7.1 |
| 45 | 6.3 |
| 50 | 5.6 |
| 60 | 4.7 |
| 70 | 4 |
A mile is 1.609344 km and a UK gallon 4.54609 litres. Lower litres per 100 km means better economy; higher mpg means better economy.