How to calculate fuel cost per mile and journey cost
A useful journey budget needs three inputs that belong together: the distance, a realistic MPG for that type of driving and the price you expect to pay for fuel.
The calculation
Worked example: 180 miles, 46 MPG, 145p/litre
180 ÷ 46 × 4.54609 = 17.79 litres
17.79 × £1.45 = £25.80
£25.80 ÷ 180 = 14.3p per mile for fuel
How sensitive is the result to MPG?
| UK MPG | Fuel for 180 miles | Cost at 145p/L |
|---|---|---|
| 30 | 27.28 L | £39.55 |
| 40 | 20.46 L | £29.66 |
| 50 | 16.37 L | £23.74 |
| 60 | 13.64 L | £19.78 |
This is why a towing, urban or winter journey should not automatically be costed using a best-ever motorway MPG figure.
Fuel cost is not the same as total motoring cost
Fuel-only calculations are useful for a trip decision or comparing two routes. For ownership decisions, depreciation, insurance, tax, servicing, tyres and finance can outweigh the fuel difference. Keep those questions separate so a simple journey tool does not imply a whole-car answer.
One-way, return and shared journeys
Check the distance definition before entering it. If a route planner shows 180 miles one way, a same-route return is roughly 360 miles before detours. For a cost split, calculate the whole vehicle journey first and divide only the costs you actually intend to share. A four-person split does not change fuel consumption; it changes who pays it.
For repeat commuting, convert the pattern into annual miles rather than multiplying a single day's cost by 365. A five-day work pattern has fewer commuting days after weekends, leave and home-working. The Commute Cost Calculator keeps that frequency visible.
Common budgeting mistakes
- Using a manufacturer's best-case MPG instead of your own route-specific economy.
- Entering pence per litre as pounds per litre, or vice versa.
- Forgetting the return leg or regular detour.
- Calling fuel cost “cost per mile” when insurance, depreciation and maintenance are intentionally excluded.