How to Calculate Fuel Cost Per Mile for Petrol, Diesel, Hybrids and EVs

Most drivers know roughly what they pay at the pump. Far fewer know what one mile of driving actually costs in fuel or electricity.

The calculation is straightforward, but the formula changes slightly depending on the vehicle type: petrol, diesel, full hybrid, plug-in hybrid (PHEV) or fully electric (EV). This guide explains each formula, shows a worked example using simple numbers, and includes a comparison table.

Petrol and diesel: the same formula, different inputs

This formula works for both petrol and diesel. The 4.546 is the rounded number of litres in a UK imperial gallon, which is what converts UK mpg and pence per litre into pence per mile. Do not use 3.785. That is the US gallon and it will give you the wrong answer.

Use your real-world imperial mpg, ideally from your trip computer over normal driving. If you want a more exact long-run figure, a brim-to-brim fuel log is better. Either way, avoid using the WLTP figure as your main input.

You can then pair that mpg with the actual pump price at the stations you use.

Worked example: petrol

mpg = 40 (illustrative real-world figure for a petrol family hatchback)

Price per litre = 145p (illustrative)

Pence per mile = (4.546 ÷ 40) × 145 = 0.1137 × 145 = 16.5p per mile

At 16.5p per mile, driving 10,000 miles per year costs about £1,650 in fuel alone. That is pence per mile ÷ 100 × annual mileage.

Worked example: diesel

mpg = 50 (illustrative real-world figure for a diesel family hatchback)

Price per litre = 150p (illustrative)

Pence per mile = (4.546 ÷ 50) × 150 = 0.0909 × 150 = 13.6p per mile

Diesel only comes out cheaper per mile if the mpg gain is large enough to outweigh any pump price premium. On long runs that often happens. On smaller cars and lower mileages, it may not.

Full hybrid (HEV): same petrol formula, usually better mpg

A full hybrid such as a Toyota Yaris Hybrid or Honda Jazz e:HEV charges its own battery through regenerative braking and the engine. There is no external electricity cost to add. The benefit shows up in the mpg figure, particularly in urban and mixed driving.

Worked example

mpg = 55 (illustrative real-world figure for a full hybrid hatchback)

Price per litre = 145p (illustrative)

Pence per mile = (4.546 ÷ 55) × 145 = 0.0827 × 145 = 12.0p per mile

Do not confuse a full hybrid with a plug-in hybrid. A PHEV has a larger battery, can be charged externally and needs a blended formula.

Plug-in hybrid (PHEV): a blended petrol and electricity cost

E is the proportion of total miles driven on electricity, from 0 to 1. Miles per kWh is your real-world electric efficiency. Petrol mpg is your real-world petrol-mode efficiency. This matters more than the badge on the boot. A PHEV that is plugged in regularly can look cheap per mile. One that is never charged usually does not.

Worked examples

Case A: charged daily

80% electric at 24.5p/kWh and 3.5 miles/kWh. 20% petrol at 45 mpg and 145p/litre.

Electric term: 0.8 × (1 ÷ 3.5) × 24.5 = 5.6p per mile

Petrol term: 0.2 × (4.546 ÷ 45) × 145 = 2.9p per mile

Combined: 8.5p per mile

Case B: charged occasionally

30% electric and 70% petrol, using the same inputs.

Electric term: 2.1p per mile. Petrol term: 10.3p per mile.

Combined: 12.4p per mile

Case C: never charged

Formula collapses to: (4.546 ÷ 45) × 145 = 14.6p per mile

At that point you are paying petrol-car running costs in a heavier vehicle. That is why two drivers in the same PHEV can end up with very different real costs.

EV: electricity cost per mile

If your car reports kWh per 100 miles instead, divide that figure by 100 to get kWh per mile, then multiply by the electricity rate. The answer is the same.

Miles per kWh should be your real-world average from the trip computer. The electricity rate needs to reflect where you actually charge. That is the biggest variable in EV running cost.

For the most accurate result, use an efficiency figure that reflects energy bought from the charger, or allow a small uplift for charging losses.

Worked example: home charging

Miles per kWh = 3.5 (illustrative real-world figure)

Home electricity rate = 24.5p/kWh (illustrative example rate)

Pence per mile = (1 ÷ 3.5) × 24.5 = 7.0p per mile

Worked example: public rapid charging

Same vehicle efficiency: 3.5 miles/kWh

Public rapid rate = 79p/kWh (illustrative example rate)

Pence per mile = (1 ÷ 3.5) × 79 = 22.6p per mile

That makes the EV more expensive per mile than the petrol example above. At home-charging rates, the same EV costs less than half as much per mile. That is why blanket claims about EV running costs are rarely useful.

Real-world EV efficiency usually drops in cold weather and at motorway speeds, and your true electricity cost can also be affected by charging losses. Use your seasonal average if you have it. Where possible, base your calculation on the energy you actually buy rather than the ideal figure shown in marketing material.

Side-by-side comparison table

All figures below are illustrative examples, not live market averages. Replace them with your own real-world numbers.

What changes the result most

For petrol and diesel, pump price and real-world mpg are the big levers. A 10p per litre change in petrol price shifts the cost for a 40 mpg car by about 1.1p per mile.

For EVs, the electricity rate dominates. Switching from mostly home charging to mostly public rapid charging can roughly triple the cost per mile.

For PHEVs, charging behaviour matters most. The spread between charged daily and never charged in the examples above is large enough to change the whole ranking.

Three common misconceptions

EVs are not always cheaper per mile than petrol. They often are on home charging. They may not be on public rapid charging.

WLTP figures are not the best input for this calculation. They are useful for comparing cars, but they are test figures, not your real running average.

Diesel is not always cheaper per mile than petrol. It depends on the pump-price gap and the mpg difference in the cars you are actually comparing.

Use your own numbers

Find your real-world mpg or miles per kWh from your vehicle's trip computer. Reset it after a representative run if needed.

Use the pump price at the forecourts you actually use. If you charge at home, use your true home tariff. If you depend mostly on public charging, use the public rate you pay most often. All prices used in the examples above are illustrative only and should not be read as current national averages.

Then substitute your figures into the relevant formula. The method stays the same. Only the inputs change.