How to calculate fuel cost
US: fuel cost = distance ÷ MPG × price per gallon. A 300-mile trip in a car that gets 28 MPG uses 10.7 gallons; at $3.20 a gallon that’s about $34.
Metric: fuel used = distance × (L/100 km) ÷ 100. A 500 km trip at 7.5 L/100 km uses 37.5 litres.
UK: UK miles per gallon use the larger imperial gallon (4.546 litres), while fuel is sold by the litre — the calculator converts automatically.
US vs UK miles per gallon
A US gallon is 3.785 litres and a UK (imperial) gallon is 4.546 litres, so the same car shows a higher MPG figure in the UK. 30 US MPG equals about 36 UK MPG or 7.8 L/100 km.
| US MPG | UK MPG | L/100 km |
|---|---|---|
| 20 | 24 | 11.8 |
| 25 | 30 | 9.4 |
| 30 | 36 | 7.8 |
| 35 | 42 | 6.7 |
| 40 | 48 | 5.9 |
| 50 | 60 | 4.7 |
Tips to save fuel on a trip
- Keep tires at the recommended pressure and remove roof boxes when not needed.
- Drive smoothly and stick near the speed limit — economy drops quickly above about 50–60 mph (80–100 km/h).
- Use cruise control on highways and avoid long idling.
- Compare prices with a fuel-price app; highway stations are often more expensive.
Use real-world fuel economy for the best estimate — it’s often 10–20% worse than the official rating, especially with luggage, hills or air conditioning. Thinking about an electric car? Compare charging costs with the electricity cost calculator.
Frequently asked questions
How do I calculate gas cost for a trip?
Divide the distance by your car’s MPG, then multiply by the price per gallon.
How much gas does a 500-mile trip take?
At 25 MPG, 20 gallons. At 35 MPG, about 14.3 gallons.
Why is UK MPG different from US MPG?
The UK uses the larger imperial gallon, so UK MPG figures are about 20% higher.
How do I split gas money with friends?
Enter the number of people — the calculator shows each person’s share.