How the MPG calculator works
MPG is miles per gallon. In the UK that means the imperial gallon of 4.54609 litres, which is about 20% bigger than a US gallon. Because fuel is sold in litres, you need to convert before you can work out your mpg.
Litres per 100km = litres ÷ (miles × 1.609344) × 100
Cost per mile = litres × price per litre ÷ miles
The calculator also gives km per litre, miles per US gallon (handy when comparing with American reviews) and what the fuel cost you per mile and per 1,000 miles.
Worked example: 350 miles on 40 litres
- Gallons used: 40 ÷ 4.54609 = 8.80 gallons
- MPG: 350 ÷ 8.80 = 39.8 mpg
- Distance in km: 350 × 1.609344 = 563.3 km
- L/100km: 40 ÷ 563.3 × 100 = 7.10 L/100km
- At 174.6p a litre, the 40 litres cost £69.84, which is 20.0p a mile
How to measure your real mpg
The trip computer in most cars gives an average mpg, but it's often a few percent optimistic. The brim-to-brim method is more reliable:
- Fill the tank until the pump clicks off, and reset your trip meter.
- Drive as normal until you need fuel again.
- Fill up at the same pump if you can, again until it clicks off.
- Enter the miles from the trip meter and the litres from your receipt.
Repeat over two or three tanks to smooth out differences in how full the tank gets.
MPG and L/100km conversion table
Higher mpg is better, while lower L/100km is better. They're inversely related, so a 10 mpg gain saves far more fuel at 30 mpg than at 60 mpg.
| MPG (UK) | L/100km | Petrol cost per mile at 174.6p |
|---|---|---|
| 25 | 11.3 | 31.7p |
| 35 | 8.1 | 22.7p |
| 45 | 6.3 | 17.6p |
| 55 | 5.1 | 14.4p |
| 65 | 4.3 | 12.2p |
To convert mpg to L/100km, divide 282.48 by the mpg figure. To go the other way, divide 282.48 by L/100km.
What's a good mpg?
It depends on the car. A small petrol hatchback might manage 45 to 55 mpg in mixed driving, a family diesel estate 50 to 60 mpg on the motorway, and a large SUV 30 mpg or less. Compare your result with the official WLTP figure for your model, bearing in mind real-world economy is usually lower. A sudden drop can point to a problem such as under-inflated tyres, a dragging brake or a sensor fault.
Diesel cars usually return higher mpg than similar petrol cars, but diesel costs more per litre. At the October 2026 averages of 199.3p for diesel and 174.6p for petrol, a diesel needs about 14% better mpg just to match a petrol car's cost per mile. That's why cost per mile is a fairer comparison than mpg alone.
Tips for better fuel economy
- Check tyre pressures monthly. Under-inflated tyres increase rolling resistance.
- Stick to the speed limit. Fuel use rises sharply above 60 mph.
- Take unnecessary weight out of the boot and remove roof boxes after use.
- Combine short trips, as a cold engine uses more fuel.
Once you know your mpg, use the fuel cost calculator to price up a journey, or compare it with an electric car using the EV charging cost calculator.
Frequently asked questions
How do I calculate mpg from litres?
Divide the litres by 4.54609 to get UK gallons, then divide the miles by that figure. For example, 350 miles on 40 litres is 350 ÷ 8.80 = 39.8 mpg.
How many litres are in a gallon in the UK?
A UK (imperial) gallon is 4.54609 litres. A US gallon is 3.785 litres, so the same car shows a lower mpg figure in the US.
What is 40 mpg in L/100km?
40 mpg (UK) is about 7.06 litres per 100km. Divide 282.48 by the mpg figure to convert.
How much does it cost per mile to run my car?
Multiply the price per litre by 4.54609 and divide by your mpg. At 174.6p a litre and 40 mpg, fuel costs 19.8p a mile.
Why is my real mpg lower than the official figure?
The official WLTP test is done in controlled conditions. Real-world driving with motorway speeds, cold starts, traffic, air conditioning and extra weight usually gives lower economy.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.