Everyday Life⏱ 5 min read
How to Calculate Fuel Cost for Any Journey
Whether you're budgeting a road trip or comparing petrol vs electric costs, the maths is simple once you know the right formula. Here's how to calculate fuel cost per mile and per journey.
Knowing exactly what a journey costs in fuel helps with budgeting, route planning, and comparing driving against other transport options. The calculation is simple — here's how to do it for petrol, diesel, and electric vehicles.
Petrol and Diesel: The Core Formula
Fuel cost = (Distance ÷ MPG) × Fuel price per litre × 4.546
Why 4.546? There are 4.546 litres in a UK gallon.
MPG = Miles Per Gallon (UK gallon).
Alternatively, in litres per 100km:
Fuel cost = (Distance km ÷ 100) × L/100km × Fuel price per litre
MPG to L/100km Conversion
L/100km = 282.5 ÷ MPG (UK)
MPG (UK) = 282.5 ÷ L/100km
40 MPG = 282.5 ÷ 40 = 7.06 L/100km
6.5 L/100km = 282.5 ÷ 6.5 = 43.5 MPG
Worked Example: London to Edinburgh
Distance: 415 miles
Car: 45 MPG (UK)
Fuel price: £1.52/litre
Fuel used = 415 ÷ 45 = 9.22 gallons
= 9.22 × 4.546 = 41.9 litres
Cost = 41.9 × £1.52 = £63.73
Cost per mile: £63.73 ÷ 415 = 15.4p per mile
Real-World vs Official MPG
Manufacturer MPG figures are measured under laboratory conditions. Real-world fuel economy is typically 15–30% lower than official figures. For budgeting, use the "real MPG" figure from sites like Honest John or Fuelly, or divide the official figure by 1.2 as a rough adjustment.
Real-world estimate = Official MPG ÷ 1.15 to 1.25
Official 55 MPG → real-world ~44–48 MPG
Electric Vehicles: Cost per Mile
EV cost = (Distance ÷ Miles per kWh) × Electricity price per kWh
Or using efficiency in miles per kWh:
A typical EV does 3.5–4.5 miles per kWh
Example: 415 miles, 4 miles/kWh, 24p/kWh (home rate)
kWh needed = 415 ÷ 4 = 103.75 kWh
Cost = 103.75 × £0.24 = £24.90
vs petrol car: £63.73
Saving: £38.83 per journey (62% less fuel cost)
Public Charging vs Home Charging
Charging TypeTypical Rate (2025)Cost per 100 miles (4mi/kWh)
Home (off-peak tariff)7–12p/kWh£1.75–£3.00
Home (standard tariff)20–28p/kWh£5.00–£7.00
Public AC charger30–50p/kWh£7.50–£12.50
Public rapid DC charger55–85p/kWh£13.75–£21.25
Rapid public charging at 75p/kWh makes EVs significantly more expensive per mile than petrol cars for the same journey. The economics of EVs depend heavily on home charging access — for drivers without it, the cost advantage largely disappears.
Cost Per Mile: Quick Comparison
VehicleTypical EfficiencyCost per Mile (approx)
Small petrol car45–55 MPG12–15p/mile
Family petrol car35–45 MPG15–19p/mile
SUV / large petrol25–35 MPG19–26p/mile
EV (home charging)3.5–4.5 mi/kWh5–7p/mile
EV (public rapid)3.5–4.5 mi/kWh15–22p/mile