UK Fuel Cost Calculator

How does the fuel cost calculator work?

The trip calculator works out the cost of a single journey from your distance, fuel economy, and local fuel price. The annual calculator does the same for a full year of driving at your entered mileage, and also shows a side-by-side comparison against typical petrol, diesel, and electric costs so you can see where your own numbers sit against the average.

For petrol and diesel, the calculator converts your MPG into litres used (1 UK gallon = 4.546 litres) and multiplies by the price per litre. For electric, it works directly from miles per kWh and your electricity price per kWh — switch the fuel type dropdown and the input labels update automatically.

3 worked examples

Example 1 — Weekend trip, petrol car

A 150-mile round trip in a petrol car averaging 35 MPG, at the May 2026 average price.

Example 2 — Annual diesel commuter

A diesel estate covering 12,000 miles a year at 48 MPG, diesel at 170p/litre.

Example 3 — Annual EV, home charging

An electric car doing 8,000 miles a year at 3.5 mi/kWh, home electricity at 27p/kWh.

Petrol vs diesel vs electric: which is cheapest per mile in 2026?

At current UK average prices (May 2026), home-charging electric vehicles cost approximately 7–10p per mile, compared to around 20–25p per mile for a petrol car averaging 35 MPG and 17–20p per mile for diesel. Public charging narrows this gap significantly — rapid charging at motorway services can cost as much as 75p/kWh, making EVs comparable to diesel per mile.

For the full breakdown across every MPG range and charging tariff, see our detailed guide: Fuel Cost Per Mile UK 2026.

How to measure your real-world MPG

To measure your car's actual MPG accurately: brim your fuel tank, reset your trip counter, drive normally for a week or two, then brim the tank again. Divide the miles driven by the litres used to refill, then multiply by 4.546 to convert to UK MPG. Most modern cars also display live average fuel economy on the dashboard, which is a reasonable shortcut once you trust it against a manual brim-to-brim check.

Hidden costs most estimates miss

1. Weekly fuel price swings

Fuel prices genuinely move week to week, sometimes by several pence per litre, tracking wholesale oil prices and exchange rates. A calculation done in January can be noticeably out of date by June. Check the current price at RAC Fuel Watch before relying on an old estimate for a big decision.

2. Cold weather EV efficiency drop

EV range and efficiency typically drop 15–30% in cold weather, due to battery chemistry and cabin heating draw. A car that returns 3.5 mi/kWh in summer might only manage 2.6–3.0 mi/kWh in winter — worth adjusting for if you're calculating annual costs rather than a single trip.

3. Roof boxes, racks, and load

Aerodynamic drag from a roof box or roof rack can increase fuel consumption by 10–15% at motorway speeds, even when empty. Heavier loads add further consumption — every extra 50kg typically adds 1–2%.

4. Mixed urban vs motorway driving

MPG varies significantly by driving conditions — stop-start urban driving typically returns worse MPG than steady motorway cruising for petrol and diesel cars, while the reverse is often true for EVs (regenerative braking helps more in stop-start traffic). A single MPG figure is always an average across your actual mix of driving.

Common calculation mistakes

Using the manufacturer's claimed MPG instead of real-world MPG

As covered above, official figures typically overstate real-world economy by 10–20%. Always adjust downward, or use a verified real-world figure, for an estimate you can actually budget against.

Forgetting that diesel and petrol prices differ

It's an easy slip to compare a diesel car's MPG against the petrol price, or vice versa — since diesel usually costs more per litre, this understates diesel running costs and can make diesel look artificially cheaper than it is. Always match the fuel price to the actual fuel type.

Not adjusting the EV comparison for charging location

The "7–10p per mile" EV figure assumes home charging. Public and rapid charging cost significantly more — see the EV vs Petrol Calculator for a full comparison that accounts for your actual home/public charging mix.

Ignoring that annual mileage estimates are often wrong

Drivers frequently underestimate their actual annual mileage, particularly if it includes occasional long trips beyond the regular commute. Check your last MOT certificate or service record for your actual mileage over the past year rather than guessing.

Tips to reduce your fuel costs

• Keep tyres inflated — under-inflation increases rolling resistance and fuel consumption by up to 4%
• Remove roof boxes and racks when not in use — aerodynamic drag significantly increases fuel use at motorway speeds
• Accelerate smoothly — harsh acceleration uses significantly more fuel than gradual acceleration
• Use cruise control on motorways to maintain a steady speed rather than fluctuating
• Shop around for fuel — use apps like PetrolPrices.com to find the cheapest nearby forecourt; supermarket forecourts are typically 3–5p/litre cheaper than motorway services
• Consider cashback credit cards that offer rewards on fuel purchases

How we calculate

Petrol/diesel cost = (Distance or mileage ÷ MPG) × 4.546 × fuel price per litre

Electric cost = (Distance or mileage ÷ mi/kWh) × electricity price per kWh

The trip calculator applies this once for your entered distance. The annual calculator applies it to your entered annual mileage, and the comparison bar chart applies the same formula three times using fixed reference efficiencies (35 MPG petrol, 45 MPG diesel, 3.5 mi/kWh electric) and May 2026 average prices, so you can see where your own fuel type and efficiency sit relative to the other two options.

Frequently asked questions

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