Gas Mileage Calculator
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31.3 mpg (US): 350 mi on 11.2 US gal.
Fill the tank completely both times: the fuel it takes to refill is the fuel used for the distance driven. One tank gives a rough figure; the fill-up log averages several.
How to use the gas mileage calculator
- Fill the tank to the top and note the odometer (or reset the trip meter).
- Drive as normal until you need fuel, then fill the tank completely again and note the fuel added and the odometer.
- For one tank, enter the two readings (or the trip distance) and the fuel. Add the price to see the cost per mile or km.
- For a longer view, switch to the fill-up log and add each fill-up in order. The log lives in the page link, so bookmark it to add more later.
Fuel economy formulas
L/100 km = liters used ÷ km driven × 100 · km/L = km driven ÷ liters used
Average over a log = total distance ÷ total fuel (first fill-up excluded)
Cost per mile = fuel cost ÷ miles driven
This is the full-tank method. Because the tank is full at both ends, the fuel needed to refill it equals the fuel burned over the distance. The calculator converts between units with the exact definitions (1 mile = 1.609344 km, US gallon = 3.785411784 L, UK gallon = 4.54609 L), so a figure in mpg (US) is always about 17% lower than the same economy in mpg (UK).
Worked examples
The odometer read 12,000 miles at the last full tank and 12,350 now, and the car took 11.2 US gallons to refill: 350 ÷ 11.2 = 31.25 mpg (US). That is 37.5 mpg in UK gallons, 7.53 L/100 km or 13.3 km/L. At $3.50 a gallon the fuel cost 11.2 cents a mile.
Over a log of three more tanks (320, 335 and 325 miles on 10.5, 11.0 and 10.2 gallons), the average is 980 ÷ 31.7 = 30.9 mpg. The tanks individually gave 30.5, 30.5, 31.9 mpg; their simple mean, 30.93, is close here but can drift when tank sizes differ, which is why the log divides totals. With the prices paid, fuel cost 11 cents a mile.
mpg from miles and gallons
| Miles driven | 8 gal | 10 gal | 12 gal | 14 gal | 16 gal |
|---|---|---|---|---|---|
| 250 | 31.2 | 25 | 20.8 | 17.9 | 15.6 |
| 300 | 37.5 | 30 | 25 | 21.4 | 18.8 |
| 350 | 43.8 | 35 | 29.2 | 25 | 21.9 |
| 400 | 50 | 40 | 33.3 | 28.6 | 25 |
| 450 | 56.3 | 45 | 37.5 | 32.1 | 28.1 |
L/100 km to mpg and km/L
| L/100 km | mpg (US) | mpg (UK) | km/L |
|---|---|---|---|
| 3 | 78.4 | 94.2 | 33.3 |
| 4 | 58.8 | 70.6 | 25 |
| 5 | 47 | 56.5 | 20 |
| 6 | 39.2 | 47.1 | 16.7 |
| 7 | 33.6 | 40.4 | 14.3 |
| 8 | 29.4 | 35.3 | 12.5 |
| 9 | 26.1 | 31.4 | 11.1 |
| 10 | 23.5 | 28.2 | 10 |
| 12 | 19.6 | 23.5 | 8.3 |
| 15 | 15.7 | 18.8 | 6.7 |
Tips for accurate mileage
- Fill the same way each time. Let the pump click off once and stop. Topping up past the click makes one tank look worse and the next better.
- Average several tanks. One tank of mostly city driving or a cold week can differ a lot from the next. Three or more fill-ups give a steadier figure.
- Missed a fill-up? If you didn't fill to full or forgot to log one, the interval is unreliable. Start a new log from the next full tank.
- Trip computers are estimates. The dashboard figure is calculated by the car and can differ from what you measure at the pump. The pump and odometer method is the check.
Plan the next journey with the fuel cost calculator, claim business driving with the mileage reimbursement calculator, or check how new tires affect your odometer with the tire size calculator.
Frequently asked questions
How do I calculate my gas mileage?
Fill the tank, note the odometer, drive, then fill up again and divide the miles driven by the gallons it took to refill. 350 miles on 11.2 US gallons is 31.25 mpg, which is 7.53 L/100 km.
How do I calculate L/100 km?
Divide the liters used by the kilometers driven and multiply by 100. A car that uses 36.4 liters to cover 520 km averages 7.0 L/100 km, or about 14.3 km per liter. Lower L/100 km figures mean better economy.
How do I average mpg over several fill-ups?
Add up the distance and the fuel for every tank after the first, then divide. Tanks of 320, 335 and 325 miles on 10.5, 11.0 and 10.2 gallons give 980 ÷ 31.7 = 30.9 mpg. Averaging each tank’s mpg separately can give a slightly different answer.
Why is the first fill-up not counted?
With the full-tank method, the fuel bought at each fill-up replaces what was burned since the previous one. The first fill-up only sets the starting point: its fuel was used for driving before the log began, so only its odometer reading matters.
How do I work out fuel cost per mile?
Divide what you paid for fuel by the miles driven. 11.2 gallons at $3.50 is $39.20, and over 350 miles that is 11.2 cents a mile. Over the three-tank log, $107.86 of fuel over 980 miles is about 11 cents a mile.