MPG Calculator
Real fuel economy from your own fill-ups.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Fuel economy
- 30 mpg
- Litres per 100 km
- 7.84
- Kilometres per litre
- 12.75
- Imperial mpg
- 36.03
Working
- 1MPG = 342 ÷ 11.4 = 30
Quick answer
Miles per gallon (MPG) is the distance driven divided by the fuel used to cover it. Fill the tank, reset the trip meter, drive, then fill up again: divide the trip miles by the gallons the pump took. 342 miles on 11.4 gallons is 30 mpg, which equals 7.84 L/100 km or 36.0 mpg in UK (imperial) gallons.
What the MPG calculator does
This calculator gives your real-world fuel economy from two numbers you can read at the pump: miles driven since the last fill-up and gallons needed to refill. It converts the result to liters per 100 km, kilometers per liter and imperial mpg so you can compare it with figures from other countries.
Measuring it yourself is more reliable than the dashboard readout, which is an estimate and is often a little optimistic. Tracking mpg over several tanks also helps you spot problems early: a sudden drop with no change in driving can point to underinflated tires, a dragging brake or an engine fault.
Once you know your mpg, the fuel cost calculator turns it into a cost per trip or per month, and the road trip cost calculator estimates fuel for a planned route.
The formula
MPG = miles driven ÷ gallons used L/100 km = 235.215 ÷ MPG (US) Imperial MPG = US MPG × 1.20095
| Symbol | Meaning | Unit |
|---|---|---|
| miles driven | Trip-meter distance between two full fill-ups | miles |
| gallons used | Fuel needed to refill the tank to full at the second fill-up | US gallons |
| 235.215 | Converts US mpg to L/100 km (100 × 3.78541 L ÷ 1.609344 km) | — |
| 1.20095 | Ratio of the imperial gallon (4.54609 L) to the US gallon (3.78541 L) | — |
This is the full-tank ("tank-to-tank") method recommended by the US Department of Energy's fueleconomy.gov. It assumes both fill-ups stop at the same point, ideally the pump's first automatic click-off. The gallons field means US gallons; if your receipt is in liters, divide by 3.785 first. For UK-style imperial gallons, read the imperial mpg output instead.
Worked examples
Typical sedan, mixed driving
A commuter drives 342 miles between fill-ups and the pump takes 11.4 gallons.
Inputs
- Miles driven342 miles
- Gallons to refill11.4 gal
Result
- Fuel economy30 mpg
- Litres per 100 km7.84
- Kilometres per litre12.75
- Imperial mpg36.03
30 mpg, or 7.84 L/100 km. Compare this with the EPA combined rating on the window sticker; real-world results within about 10% either side are normal.
Hybrid on a highway trip
A hybrid covers 412 miles and needs 9.8 gallons to refill.
Inputs
- Miles driven412 miles
- Gallons to refill9.8 gal
Result
- Fuel economy42.04 mpg
- Litres per 100 km5.59
- Kilometres per litre17.87
- Imperial mpg50.49
About 42 mpg. Hybrids often do relatively worse on the highway than in town because there is little braking energy to recover at steady speed.
Pickup towing a trailer
A pickup towing a camper drives 268 miles and takes 17.2 gallons.
Inputs
- Miles driven268 miles
- Gallons to refill17.2 gal
Result
- Fuel economy15.58 mpg
- Litres per 100 km15.1
- Kilometres per litre6.62
- Imperial mpg18.71
About 15.6 mpg, or 15.1 L/100 km. Towing raises aerodynamic drag and weight, so plan fuel stops using this figure, not the unladen rating.
How to measure mpg accurately
- Fill to the first click both times, at the same pump if you can. Topping off adds an unpredictable amount.
- Use several tanks. One tank can be skewed by a long idle, a traffic jam or a slightly different fill level. Add up miles and gallons across three or more fill-ups and divide the totals.
- Check the odometer. Larger or smaller tires than stock change the distance the odometer records; the tire size calculator shows by how much.
- Don't average mpg figures directly. Averaging 20 mpg and 40 mpg tanks gives 30, but if the miles differed, the true figure is total miles ÷ total gallons.
Why your mpg is lower than the sticker
EPA ratings come from standardized lab cycles, adjusted to reflect typical driving. Your result can differ for reasons that have nothing to do with a fault.
| Cause | Why it costs fuel |
|---|---|
| Cold weather | Engines take longer to warm up, winter fuel blends contain less energy, and cold air is denser |
| High highway speeds | Aerodynamic drag rises steeply with speed, so economy usually falls above about 50–60 mph |
| Short trips | The engine spends much of the trip below its efficient operating temperature |
| Underinflated tires | More rolling resistance; check pressure monthly against the door-jamb placard |
| Roof racks and cargo boxes | Extra drag, most noticeable at highway speed |
| Aggressive acceleration and braking | Energy used to accelerate is lost as heat in the brakes |
US mpg, UK mpg and L/100 km
The US gallon is 3.785 liters and the UK imperial gallon is 4.546 liters, so the same car shows about 20% higher mpg in UK figures. Most of the world uses liters per 100 km, where a lower number is better. To convert a figure you already have, use the MPG to L/100km calculator.
If you are comparing a gasoline car with an electric one, the EV vs gas cost calculator compares running costs per mile directly.
How to use the MPG Calculator
- 1Fill the tank until the pump clicks off and reset your trip meter (or note the odometer).
- 2Drive normally until you next need fuel, ideally using at least half a tank.
- 3Fill up again to the first click and note the gallons on the pump and the trip miles.
- 4Enter the miles driven and gallons used. If your receipt shows liters, divide by 3.785 to get US gallons.
- 5Read mpg, L/100 km, km/L and imperial mpg; repeat over several tanks for a reliable average.
Frequently asked questions
How do I calculate my gas mileage?
Divide the miles you drove by the gallons it took to refill the tank. For example, 300 miles on 10 gallons is 30 mpg.
Is the mpg on my dashboard accurate?
It is an estimate based on fuel-injector data and is often slightly optimistic. Checking it against a few tank-to-tank calculations shows how far off your car's display is.
What is good gas mileage?
It depends on the vehicle type. Many compact gasoline cars achieve 30–40 mpg combined, hybrids often exceed 45 mpg, and full-size pickups commonly sit around 15–25 mpg.
Why is my mpg worse in winter?
Cold engines take longer to reach efficient temperature, winter-blend gasoline holds slightly less energy, and heater, defroster and denser air add load. Short trips in cold weather are hit hardest.
How do I convert mpg to L/100 km?
Divide 235.215 by the US mpg figure. 30 mpg is 235.215 ÷ 30 = 7.84 L/100 km.
References
- Fuel Economy Guide and Gas Mileage Tips — US Department of Energy and US Environmental Protection Agency (fueleconomy.gov)
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI) — National Institute of Standards and Technology
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
- Report an error