Fuel cost calculator UK
Fuel cost is miles divided by mpg, times 4.54609 litres per gallon, times the price per litre. With petrol at 174.1p a litre (UK average, week of 26 October 2026), 100 miles at 45 mpg costs £17.59. Diesel is 199.2p. Work out a journey, your real mpg or your fleet's fuel bill below.
Prices: DESNZ weekly road fuel prices, week commencing 26 October 2026. Updated every Tuesday.
Fuel cost and MPG calculator
Drive the motorway part at 60 instead of 70 mph and you use less fuel on every one of those miles.
Get a quoteUK gallons (4.54609 litres). Prices are UK averages; enter what you pay. The speed figures use the Department for Transport's fuel curves for cars and vans and assume the share of motorway miles you set.
How much does 100 miles cost in fuel?
At this week's UK average pump prices, by fuel economy. Multiply by your mileage, or divide by 100 for the cost per mile.
| MPG (UK) | Petrol, 100 miles | Petrol, per mile | Diesel, 100 miles | Diesel, per mile |
|---|---|---|---|---|
| 25 mpg | £31.65 | 31.7p | £36.23 | 36.2p |
| 30 mpg | £26.38 | 26.4p | £30.19 | 30.2p |
| 35 mpg | £22.61 | 22.6p | £25.88 | 25.9p |
| 40 mpg | £19.78 | 19.8p | £22.64 | 22.6p |
| 45 mpg | £17.59 | 17.6p | £20.13 | 20.1p |
| 50 mpg | £15.83 | 15.8p | £18.11 | 18.1p |
| 55 mpg | £14.39 | 14.4p | £16.47 | 16.5p |
| 60 mpg | £13.19 | 13.2p | £15.09 | 15.1p |
MPG to litres per 100 km
To convert UK mpg to litres per 100 km, divide 282.48 by the mpg. US gallons are smaller (3.785 litres against 4.546), so the same car shows a lower mpg in US figures.
| MPG (UK) | Litres per 100 km | Km per litre | MPG (US) |
|---|---|---|---|
| 20 | 14.12 | 7.08 | 16.7 |
| 25 | 11.30 | 8.85 | 20.8 |
| 30 | 9.42 | 10.62 | 25.0 |
| 35 | 8.07 | 12.39 | 29.1 |
| 40 | 7.06 | 14.16 | 33.3 |
| 45 | 6.28 | 15.93 | 37.5 |
| 50 | 5.65 | 17.70 | 41.6 |
| 55 | 5.14 | 19.47 | 45.8 |
| 60 | 4.71 | 21.24 | 50.0 |
| 70 | 4.04 | 24.78 | 58.3 |
Why your fuel cost depends on how fast you drive
The same journey costs more at higher speed, because air resistance rises with the square of speed. The Department for Transport's fuel curves show a diesel car uses 9.6% less fuel per mile at 60 mph than at 70, and a diesel van 14.5% less at 60 than at 68. For a business running vans on the motorway every day, that difference is the largest fuel cost you can control. See fuel use by speed for every vehicle type.
Guides
Questions people ask
How do I calculate the fuel cost of a journey?
Divide the miles by your car's mpg to get gallons, multiply by 4.54609 to get litres, then multiply by the price per litre. At 174.1p a litre (UK average petrol, week of 26 October 2026), 100 miles at 45 mpg uses 10.1 litres and costs £17.59.
How do I work out my mpg?
Fill the tank, reset the trip meter, drive normally, then fill up again. Divide the miles on the trip meter by the litres it took to refill, and multiply by 4.54609. That is your real mpg, which is usually lower than the figure on the window sticker.
How do I convert mpg to litres per 100 km?
Divide 282.48 by the UK mpg. So 40 mpg is 7.06 litres per 100 km, and 50 mpg is 5.65. UK gallons are 4.54609 litres; a US gallon is 3.785 litres, so a US mpg figure is about 17% lower than the UK figure for the same car.
What does fuel cost per mile?
In the week of 26 October 2026, a petrol car at 45 mpg cost 17.6p a mile in fuel and a diesel van at 36 mpg cost 25.2p, using the DESNZ UK average pump prices. The Fleet Cost Index updates these every week.
Does driving speed change the fuel cost?
Yes, a lot. On the Department for Transport's fuel curves a diesel car uses 9.6% less fuel per mile at 60 mph than at 70, and a diesel van 14.5% less at 60 than at 68. On a long motorway trip that is often the biggest single difference to what you pay.
Sources and method
- Department for Energy Security and Net Zero, Weekly road fuel prices. Prices pre-filled from the week of 26 October 2026: petrol 174.07p, diesel 199.22p a litre
- DfT TAG data book v2.03 (May 2026), Table A1.3.8. Fuel use against speed, for the speed comparison
- DESNZ Greenhouse gas reporting: conversion factors 2026. Petrol 2.075 and diesel 2.58354 kg CO2e a litre
Work it out in the calculator above.
DfT's fuel curves show a diesel van at 60 mph uses 14.5% less fuel per mile than at 68 mph.
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