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Fuel cost calculator

Miles, MPG and the pump price — what the journey actually costs, with the gallon conversion done properly.

Fuel used11.37 litres
Journey cost£16.48
Cost per mile16.5p
There and back£32.96

100 mi ÷ 40 MPG × 4.54609 L/gal = 11.37 L × £1.45

The sum, shown

Britain measures distance in miles, economy in miles per imperial gallon, and sells fuel in litres — so the one conversion that matters is 1 UK gallon = 4.54609 litres. Litres used = miles ÷ MPG × 4.54609, and cost = litres × pence per litre. Using the US gallon (3.785 litres) — which many online calculators quietly do — understates every journey by about 17%.

Use your real MPG, not the brochure's. Official WLTP economy figures come from laboratory cycles; most drivers see 10–20% less, and short cold-engine trips less still. Your trip computer's long-term average, or miles between two fill-ups ÷ litres × 4.54609, is the honest number — and the costs here are only as honest as that input.
Nothing you type is sent anywhere.

The calculator runs entirely in your browser and works offline. No cookies, no third-party scripts, no record of your journeys on any server.

Common questions

What does a daily commute cost a month?

Multiply the there-and-back figure by your working days. A 15-mile each-way commute at 40 MPG and £1.45/litre costs about £4.94 a day — roughly £108 over 22 working days. Seeing the monthly number is usually what makes lift-sharing or one work-from-home day feel worth arranging.

Does speed really change the cost?

Above about 55–60 mph, drag rises steeply: motorway driving at 80 mph can use around 10–15% more fuel than at 70. Under-inflated tyres, roof boxes and a bootful of weight all push the real MPG down too — which is why this page asks for yours rather than assuming one.

Electric or hybrid?

The same structure works with different units: for an EV, cost = miles ÷ efficiency (miles per kWh) × pence per kWh. A car doing 3.5 mi/kWh charged at 25p/kWh costs about 7p a mile — this page's per-mile row makes the comparison direct.

The imperial gallon has been 4.54609 litres by definition since 1985 and does not go out of date; pump prices change daily, which is why you enter today's. Calculations verified by automated known-answer tests (e.g. 100 mi · 40 MPG · £1.45/L → £16.48). Page last reviewed: 14 August 2026.

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