ملخص
The fuel cost calculator compares the cost of a journey by petrol, diesel, or electric vehicle. It uses the standard UK formula: convert miles to litres (via MPG and the imperial gallon), multiply by the fuel price, and optionally estimate CO2 emissions using DEFRA conversion factors.
For electric vehicles, the calculation is simpler: divide miles by efficiency (mi/kWh) to get energy needed, then multiply by the electricity price.
كيف يعمل
Petrol and diesel
UK fuel economy is measured in miles per gallon (MPG) using the imperial gallon (4.54609 litres). To find the fuel cost of a journey:
- Divide the distance by MPG to get gallons used
- Multiply by 4.54609 to convert to litres
- Multiply by the price per litre
Electric vehicles
EV efficiency is measured in miles per kWh. To find the electricity cost:
- Divide the distance by miles per kWh to get kWh needed
- Multiply by the electricity price per kWh
Home charging uses the domestic electricity rate (set by the Ofgem price cap). Public rapid charging is typically 2–3× more expensive.
CO2 emissions
The calculator uses DEFRA’s greenhouse gas conversion factors for tailpipe emissions:
- Petrol: 2.31 kg CO2e per litre
- Diesel: 2.68 kg CO2e per litre
- Electric: 0 kg (tailpipe — grid emissions not included)
الصيغة
Where
Where
For regular trips, the annual cost is simply:
Where
مثال محلول
100-mile journey: petrol at 40 MPG, 131.6p/litre
Gallons used
= 2.5 gal
Convert to litres
= 11.37 L
Fuel cost
= £14.96
CO2 emissions
= 26.3 kg
Result
Total cost: £14.96 · CO2: 26.3 kg
Same journey by EV (home charging): 4.0 mi/kWh, 27.7p/kWh
Energy needed
= 25 kWh
Electricity cost
= £6.93
Result
Total cost: £6.93 · CO2: 0 kg (tailpipe) · Saving vs petrol: £8.03
Default values
| Parameter | Default | Source |
|---|---|---|
| Petrol price | 131.6p/litre | RAC Fuel Watch (Feb 2026) |
| Diesel price | 140.8p/litre | RAC Fuel Watch (Feb 2026) |
| Home electricity | 27.7p/kWh | Ofgem Q1 2026 price cap (27.69p rounded) |
| Public charging | 76p/kWh | Zapmap weighted average (Jan 2026) |
| Petrol MPG | 40 | NimbleFins UK average is 36; 40 is a common round default |
| Diesel MPG | 45 | NimbleFins UK average is 43; 45 is a common round default |
| EV efficiency | 4.0 mi/kWh | Typical modern EV (range 3.5–4.5) |
| CO2 petrol | 2.31 kg/L | DEFRA 2024 |
| CO2 diesel | 2.68 kg/L | DEFRA 2024 |
الافتراضات والقيود
- UK imperial gallons only. The calculator uses 1 gallon = 4.54609 litres. US gallons are smaller (3.785 litres) and would give incorrect results.
- Tailpipe CO2 only. EV emissions are shown as zero because we measure at the exhaust. In reality, grid electricity has a carbon intensity of ~150–200 g CO2/kWh (UK average), so EVs are not truly zero-carbon — but they are significantly lower than petrol or diesel.
- Fixed fuel prices. The calculator uses a single price per litre. In practice, motorway services charge 15–40p/litre more than supermarkets.
- No depreciation or running costs. This calculator compares fuel cost only, not total cost of ownership (insurance, road tax, servicing, depreciation). See the Car Running Cost calculator for a fuller picture.
- Real-world MPG varies. Official WLTP figures are typically 10–20% higher than real-world driving. Our defaults reflect real-world averages.
- Public charging prices vary widely. The 76p/kWh default is a weighted PAYG average across major networks. Some networks charge 50p/kWh, others 89p/kWh.
التحقق
| Test case | Input | Expected cost | Source |
|---|---|---|---|
| Petrol 100 mi | 40 MPG, 131.6p/L | £14.96 | Manual calculation, cross-checked vs PetrolPrices |
| Diesel 250 mi | 45 MPG, 140.8p/L | £35.56 | Manual calculation |
| EV home 100 mi | 4.0 mi/kWh, 27.7p/kWh | £6.93 | Manual calculation |
| EV public 100 mi | 4.0 mi/kWh, 76p/kWh | £19.00 | Manual calculation |
| PetrolPrices cross-check | 100 mi, 40 MPG, 131.9p/L | £14.99 | PetrolPrices.co.uk worked example |
Sources
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