Your car’s real fuel economy is easy to measure and useful to know: it tells you what each mile actually costs, helps you budget for road trips, and can warn you of a maintenance problem when the number suddenly drops. This calculator works in US units (miles and gallons) or metric (kilometers and liters) and converts between them.
How to use the gas mileage calculator
- Choose Miles & gallons or Kilometers & liters.
- Fill your tank and reset the trip meter. At your next fill-up, enter the distance driven since then and the fuel used to refill the tank.
- Optionally enter the fuel price to see the cost per mile or kilometer.
- Optionally enter a planned trip distance to estimate the fuel and cost for the trip.
- Read your fuel economy on the tape in mpg, L/100 km, km/L and UK mpg.
Fuel economy formulas
The constant 235.215 comes from the exact definitions of a US gallon (3.785411784 liters) and a mile (1.609344 kilometers).
Worked examples
US units. 320 miles on 10.5 gallons: 320 ÷ 10.5 = 30.48 mpg = 7.72 L/100 km. At $3.45 a gallon, fuel costs $0.113 a mile and the fill-up cost $36.23. A 600-mile trip needs 600 ÷ 30.48 = 19.69 gallons, or about $67.92.
Metric. 500 km on 38 liters: 38 ÷ 500 × 100 = 7.60 L/100 km = 30.95 US mpg. At $1.85 a liter, a 300 km trip uses 22.8 liters and costs $42.18.
MPG and L/100 km compared
| MPG (US) | L/100 km | km/L | Gallons per 100 miles |
|---|---|---|---|
| 15 | 15.68 | 6.38 | 6.67 |
| 20 | 11.76 | 8.50 | 5.00 |
| 25 | 9.41 | 10.63 | 4.00 |
| 30 | 7.84 | 12.75 | 3.33 |
| 35 | 6.72 | 14.88 | 2.86 |
| 40 | 5.88 | 17.01 | 2.50 |
| 50 | 4.70 | 21.26 | 2.00 |
Why gallons per mile is more intuitive than mpg
Going from 15 to 20 mpg saves 1.67 gallons every 100 miles, while going from 40 to 50 mpg saves only 0.5 gallons. That’s why the EPA fuel economy label also shows gallons per 100 miles, and why replacing a very thirsty vehicle saves far more fuel than upgrading an already efficient one.
Getting an accurate number
- Measure over several tanks. Pump shut-off points vary, so one fill-up can be off by a few percent. Average three or more.
- Fill to the same point each time — the first automatic click — and don’t top off.
- Note the conditions. Highway trips, winter blends of gasoline and towing all change the figure.
- Watch for sudden drops. A 10% to 15% fall without a change in driving can point to low tire pressure, a dragging brake or an engine issue.
To share road-trip fuel costs with friends, use the split the bill calculator. For travel time at a given speed, see the speed, distance and time calculator.
Results are estimates. Actual fuel use depends on driving conditions, vehicle condition and fuel quality.
Frequently asked questions
How do I calculate my gas mileage?
Fill the tank and reset the trip odometer. At the next fill-up, divide the miles driven by the gallons it takes to refill the tank. 320 miles on 10.5 gallons is 30.5 miles per gallon.
How do I convert mpg to liters per 100 km?
Divide 235.215 by the US mpg figure. 30 mpg equals 235.215 ÷ 30 = 7.84 L/100 km. The same formula works in reverse: 235.215 ÷ L/100 km gives US mpg.
Why is the UK mpg figure higher?
An imperial gallon holds 4.546 liters, about 20% more than a US gallon's 3.785 liters, so the same car travels farther on an imperial gallon. 30 US mpg is about 36 UK mpg.
How much will gas cost for my trip?
Divide the trip distance by your mpg to get gallons needed, then multiply by the price per gallon. A 600-mile trip at 30.48 mpg needs 19.69 gallons, which costs $67.92 at $3.45 a gallon.
Why does my car get worse mileage than the EPA rating?
EPA ratings come from standardized laboratory tests. Real-world mileage drops with high speeds, short trips, cold weather, heavy loads, aggressive acceleration, idling and under-inflated tires.