MPG Calculator
Calculate your car's real fuel economy from miles driven and gallons used — or from two odometer readings at fill-up — in US mpg, UK mpg, L/100 km and km/L.
MPG Calculator: with the default inputs, miles per gallon (us) is 30.
Used in 'distance driven' mode.
Fill the tank fully both times; the pump reading is the fuel you used.
- Miles per gallon (UK)
- 36
- Liters per 100 km
- 7.84
- Kilometers per liter
- 12.75
- Distance
- 300 mi
- Gallons per 100 miles
- 3.33The EPA's preferred 'fuel consumption' figure — lower is better and it adds linearly.
Assumptions
- Both fill-ups filled the tank to the same level.
- 'Gallon' means the US gallon unless the UK figure is shown.
| mpg (US) | mpg (UK) | L/100 km | km/L |
|---|---|---|---|
| 15 | 18 | 15.68 | 6.38 |
| 20 | 24 | 11.76 | 8.5 |
| 25 | 30 | 9.41 | 10.63 |
| 30 | 36 | 7.84 | 12.75 |
| 35 | 42 | 6.72 | 14.88 |
| 40 | 48 | 5.88 | 17.01 |
| 45 | 54 | 5.23 | 19.13 |
| 50 | 60 | 4.7 | 21.26 |
| 60 | 72.1 | 3.92 | 25.51 |
How this is worked out
The formula
mpg = miles driven ÷ US gallons used mpg (UK) = miles ÷ imperial gallons = mpg (US) × 1.201 L/100 km = liters used ÷ km driven × 100 = 235.215 ÷ mpg (US) km/L = km driven ÷ liters used
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- How do you want to enter the trip?
- Choose one of 2 options.Distance driven · Odometer readings (fill-up to fill-up)
- Distance driven
- Used in 'distance driven' mode.in mi, km · 0 or more · defaults to 300
- Odometer at last fill-up
- A number.in mi, km · 0 or more · defaults to 45210
- Odometer at this fill-up
- A number.in mi, km · 0 or more · defaults to 45510
- Fuel used
- Fill the tank fully both times; the pump reading is the fuel you used.in gal, L · 0.001 or more · defaults to 10
What you get back
- Miles per gallon (US)main answer
- Miles per gallon (UK)
- Liters per 100 km
- Kilometers per liter
- Distance
- Gallons per 100 miles
- The EPA's preferred 'fuel consumption' figure — lower is better and it adds linearly.
What this assumes
- Both fill-ups filled the tank to the same level.
- 'Gallon' means the US gallon unless the UK figure is shown.
About this calculator
The only reliable way to know a car's fuel economy is to measure it: fill the tank, drive, fill it again, and divide the distance by the fuel it took to refill. This calculator does the division and converts the result into every common unit at once.
The fill-up method
- Fill the tank until the pump clicks off. Note the odometer (or reset the trip meter).
- Drive normally until you need fuel.
- Fill up again to the first click. The pump shows the gallons or liters used; the odometer difference is the distance.
Choose Odometer readings to enter the two readings directly, or Distance driven if you reset a trip meter. Do this over several tanks and average them — a single tank can be off by 5–10% depending on how the pump cut off and whether the car was on a slope.
Reading the results
- mpg (US) is the North American figure. mpg (UK) uses the larger imperial gallon (4.546 L vs 3.785 L), so it's always about 20% higher for the same car.
- L/100 km is the metric standard: lower is better, and 7.84 L/100 km equals 30 US mpg.
- Gallons per 100 miles is the US EPA's "fuel consumption" figure and is the honest way to compare cars: going from 10 to 20 mpg saves 5 gal per 100 miles, while going from 40 to 50 mpg saves only 0.5.
Why your number differs from the sticker
EPA and WLTP ratings come from standardised lab cycles. Cold weather (winter fuel blends and a cold engine can cost 15–25% on short trips), speed above 60 mph, roof racks, under-inflated tyres, ethanol content and short trips all push real economy below the label. A consistent 10–15% gap is normal; a sudden drop on the same commute is worth investigating (tyre pressure, a stuck brake, an oxygen sensor).
Trip planning
Once you know your real mpg, the fuel cost calculator turns it into dollars for any trip, and the range for a tank is simply tank size × mpg.
Frequently asked questions
▸How do I calculate MPG?
Divide the miles driven by the gallons used. Fill the tank, drive, fill it again and use the pump reading as the gallons: 300 miles ÷ 10 gallons = 30 mpg.
▸How do I convert MPG to L/100 km?
Divide 235.215 by the US mpg figure. 30 mpg is 7.84 L/100 km; 40 mpg is 5.88; 50 mpg is 4.70. For UK mpg divide 282.481 instead.
▸Why is UK MPG higher than US MPG?
An imperial gallon is 4.546 liters and a US gallon 3.785 liters — 20.1% larger — so the same car travels 20% more miles per UK gallon. 30 US mpg is 36 UK mpg.
▸Is the trip computer's MPG accurate?
Usually within a few percent, but many read 3–8% optimistic. The fill-up method over several tanks is the ground truth.
▸What is a good MPG?
In the US the fleet average for new cars is around 25–27 mpg combined; efficient hybrids do 45–55, full-size trucks 15–22. Compare against cars of the same class rather than a single number.
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