How to Calculate Fuel Cost for a Journey

Fuel cost for a journey comes down to three numbers:

• Distance — how far you’re driving (in miles)
• Fuel economy — your car’s efficiency (in miles per gallon, mpg)
• Fuel price — the current price per litre (in pence)

The 4.546 converts gallons to litres (1 imperial gallon = 4.546 litres). The ÷ 100 converts pence to pounds.

Scenario: You’re driving 200 miles in a car that does 40 mpg, and petrol costs 140p per litre.

1. Fuel needed: 200 ÷ 40 = 5 gallons
2. Convert to litres: 5 × 4.546 = 22.73 litres
3. Total cost: 22.73 × £1.40 = £31.82

That’s about 15.9p per mile in fuel alone. For a return trip, double it: £63.64.

This table shows approximate fuel cost per mile at 140p per litre for different fuel economies:

These are estimates based on real-world driving. Official manufacturer figures are typically 10–20% more optimistic than what you’ll achieve on the road.

Approximate one-way fuel costs at 140p/litre for a car averaging 40 mpg:

For the daily commute, that’s roughly £16 per week or £760 per year in fuel alone (48 working weeks).

The most accurate way to know your car’s fuel economy:

1. Fill your tank completely and note the mileage (or reset the trip counter).
2. Drive normally until you need to refuel.
3. Fill up again and note how many litres you put in.
4. Calculate: Miles driven ÷ (litres used ÷ 4.546) = your real-world MPG.

Most modern cars also show an average fuel consumption on the dashboard, usually in litres per 100km (l/100km) or mpg. The trip computer is usually within 5–10% of actual consumption. To convert l/100km to mpg: divide 282.5 by the l/100km figure.

Diesel is typically 5–8p per litre more expensive than petrol, but diesel engines are more fuel-efficient (often 15–20% better mpg). For longer journeys and motorway driving, the better fuel economy often makes diesel cheaper per mile despite the higher price per litre.

For a detailed comparison, see our petrol vs diesel guide →

If you use your own car for business travel, HMRC’s approved mileage allowance payment (AMAP) rates are:

• First 10,000 business miles: 45p per mile
• Above 10,000 miles: 25p per mile

These rates cover fuel plus wear, insurance, and depreciation. If your employer pays less than the AMAP rate, you can claim tax relief on the difference through your self-assessment or by contacting HMRC.

Your actual fuel cost per mile (see table above) is likely between 7–18p — well below the 45p AMAP rate, which is why claiming mileage is usually more tax-efficient than claiming actual fuel receipts.

You can reduce fuel costs per mile two ways:

• Pay less per litre. Use PumpCheck to find the cheapest station near your route. Even 5p per litre less saves £2.50 per 50-litre tank.
• Use less fuel per mile. Drive smoothly, keep tyres inflated, and remove unnecessary weight. See our fuel saving tips →

Combined, these can realistically save £300–£500 per year for a typical UK driver.