The most accurate way to measure fuel economy is the fill-to-fill method: fill the tank, reset the trip meter, drive normally, then fill up again and note the litres or gallons added. Dashboard figures are often 3–5% optimistic.
How to use the MPG Calculator
- Distance driven
- Fuel used
- Fuel price
- Every result field is editable: type into Fuel economy and Fuel used plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the distance since your last fill-up and the fuel you added, plus the fuel price. Type a target mpg into the result to see how much fuel that distance would take.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Distance driven | Units: mi, km, m, nmi |
| Fuel used | Units: US gal, L, UK gal |
| Fuel price | Units: /US gal, /L, /UK gal |
What each result means
- Fuel economy = [Distance driven]÷[Fuel used]
- Fuel consumption = 100000×[Fuel used]×1000÷[Distance driven]
- Cost of the fuel = [Fuel used]×[Fuel price]
- Cost per distance = [Fuel price]÷[Fuel economy]
Example values
How the main result changes when Fuel used changes and everything else stays at the example values above:
| Fuel used (US gal) | Fuel economy (mpg (US)) |
|---|---|
| 6 | 58.33 |
| 9 | 38.89 |
| 12 | 29.17 |
| 15 | 23.33 |
| 18 | 19.44 |
| 24 | 14.58 |
Formula
MPG = distance ÷ fuel usedL/100 km = 100 × litres ÷ kilometresCost per mile = fuel price ÷ MPGWorked example
Inputs — Distance driven: 350 mi; Fuel used: 12 US gal; Fuel price: $3.5 /US gal.
Results — Fuel economy = 29.17 mpg (US); Fuel consumption = 8.06 L/100 km; Cost of the fuel = $42.00; Cost per distance = $0.120 /mi.
Frequently asked questions
How do I calculate MPG?
Divide miles driven by gallons used: 350 miles ÷ 12 gallons = 29.2 mpg.
What is the difference between US and UK mpg?
A UK (imperial) gallon is 4.546 L versus 3.785 L, so UK mpg figures are about 20% higher for the same car.
How do I convert mpg to L/100 km?
235.21 ÷ US mpg (or 282.48 ÷ UK mpg).
What formula does the MPG Calculator use?
MPG = distance ÷ fuel used.