Fuel cost calculator
At 140p a litre, a car doing 50 mpg costs 12.7p a mile. An electric car doing 3.5 miles per kWh at 28p costs 8.0p. Over 10,000 miles that is a difference of £473 — on fuel alone.
How this is worked out
Petrol and diesel are priced per litre while economy is quoted in miles per imperial gallon, so the price of a gallon is the price per litre multiplied by 4.54609, and the cost of a mile is that divided by the mpg. Electricity needs no conversion, because it is priced per kWh and efficiency is quoted in miles per kWh — the cost of a mile is simply one divided by the other. Every other figure on the page is that per-mile cost multiplied by a distance, which is why the two halves of the comparison are directly comparable rather than merely adjacent.
A car doing 50 mpg with petrol at 140p a litre, against an electric car doing 3.5 miles per kWh at 28p
- A gallon of petrol costs 140 × 4.54609 = 636.45p.
- At 50 mpg that gallon covers 50 miles, so a mile costs 12.7p.
- For the electric car, 28p per kWh divided by 3.5 miles per kWh gives 8.0p a mile.
- Over 10,000 miles that is about £1,273 against £800 — a difference of roughly £473 on fuel alone.
The imperial gallon is 4.54609 litres and the US gallon is 3.785. A test pins the constant, because using the wrong one is both easy to do and invisible in the output — it simply makes petrol look a sixth cheaper than it is.
Why there is nothing to cite here
Fuel and electricity prices change weekly and vary by forecourt, tariff and time of day, so this asks for the ones you actually pay rather than publishing a figure that would be wrong by the time you read it. The only fixed number is the size of a gallon.
What catches people out
- UK mpg is quoted against the imperial gallon, which is 4.54609 litres — not the US gallon of 3.785. Using the American one understates the cost of a mile by about a sixth, and it is the most common error in fuel calculators written outside the UK.
- Where you charge changes the answer more than which car you drive. Home electricity is a fraction of the price of public rapid charging, and an off-peak overnight tariff is cheaper again. One electricity figure cannot represent both, so enter the one you would actually use most.
- Official mpg and range figures are laboratory numbers. Real consumption is worse, and both petrol economy and electric efficiency fall considerably in cold weather — an electric car in winter can lose a fifth of its range to heating alone. Figures from your own car's trip computer are worth far more than any published ones.
- Fuel is only part of running a car, and rarely the largest part. Insurance, tax, servicing, tyres and above all depreciation usually cost more over a year. This compares fuel alone, which is one input to a decision rather than the decision itself.
Questions
How do I convert mpg to pence per mile?
Multiply the price per litre by 4.54609 to get the price of a gallon, then divide by the mpg. At 140p a litre a gallon costs £6.36, and at 50 mpg that is 12.7p a mile.
For an electric car there is no conversion at all — divide the price per kWh by the miles per kWh. At 28p and 3.5 miles per kWh that is 8p a mile.
Is an electric car actually cheaper to run?
On fuel alone, usually yes if you charge at home — often by a factor of two or three. On public rapid chargers the gap narrows sharply and can close entirely, because rapid charging is priced well above domestic electricity.
Fuel is not the whole picture, though. Purchase price, depreciation, insurance and servicing all differ, and this page deliberately compares only the part that is simple arithmetic.
What is a realistic miles per kWh?
Most electric cars sit between about 3 and 4.5 miles per kWh in mixed use, with small cars at the upper end and large SUVs below 3. Cold weather, motorway speeds and heating all reduce it noticeably.
Your car will tell you what you actually get. Use that figure rather than a brochure one.
Does this include the cost of charging losses?
No. Charging an electric car is not perfectly efficient — some energy is lost as heat, typically a few per cent on a home charger and more in cold weather — so the electricity that goes through your meter is a little more than the battery receives.
If you want to account for it, enter your miles per kWh as measured from the meter rather than from the car.