Gas mileage calculator: convert fuel gallons used and miles driven into MPG or L/100km instantly for any trip or fill-up. Free, runs locally, no signup.
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Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for the formulas used.
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Gas mileage calculator: convert fuel gallons used and miles driven into MPG or L/100km instantly for any trip or fill-up. Free, runs locally, no signup.
From 25.0 mpg baseline, unit and season errors swing results widely.
Transparent methodology
How this calculator works
Reviewed August 2026 · BoringToolsKit Editorial Team
Formula
MPG = miles ÷ gallons (US). L/100km = liters ÷ km × 100.
BoringToolsKit. “Gas Mileage Calculator.” boringtoolskit.com/gas-mileage-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: 300 miles on 12 gallons
Suppose you reset the trip odometer, fill the tank, drive 300 miles, then refill and the pump shows 12 gallons. Fuel economy = distance / fuel, so 300 / 12 = 25.0 mpg. No other inputs are involved; if the trip meter read 300.0 and the receipt said 11.87 gallons, the same division gives 300 / 11.87 = 25.27, which rounds to 25.3 mpg.
The same tank expressed the European way is 9.4 L/100km. That comes from converting first: 300 miles is 300 x 1.60934 = 482.8 km, and 12 US gallons is 12 x 3.78541 = 45.4 liters. Fuel per 100 km = (45.4 / 482.8) x 100 = 9.4. You can cross-check with the standard shortcut for US mpg: 235.215 / 25.0 = 9.41, which matches. Note the two scales run in opposite directions: cutting L/100km from 9.4 to 8.4 raises mpg from 25.0 to about 28.0, so a one-unit drop is worth roughly 3 mpg at this level of consumption.
Where real-world tank math goes wrong
Pump-to-pump fill levels cause most errors. If you brim the tank one time and stop at the first nozzle click the next, the recorded gallons are off by 1 to 2 gallons, which is 8 to 17% of a 12-gallon fill. On a 300-mile loop that alone swings the result between roughly 23 and 28 mpg. Fill the same way every time, ideally brim-full, to cancel this out.
Unit mixing is the other trap. A UK-spec 30 mpg figure uses the imperial gallon (4.546 L), which is 20% larger than the US gallon (3.785 L), so 30 mpg imperial converts to 36 mpg US. Entering UK numbers as US gallons inflates the answer the same 20%. Fuel type matters too: winter gasoline and E85 both carry less energy per gallon, and EPA testing shows short trips at -7 C can cut economy about 22% versus 25 C. If one tank reads 22 mpg in January and 27 in July, the engine is probably fine.
