How US gas mileage is calculated
US MPG = miles travelled ÷ US liquid gallons used. With odometer readings, trip distance is the ending reading minus the starting reading. The distance and fuel quantity must both be positive. Use fuel added between comparable full-tank fill-ups so the purchased fuel represents the amount consumed over the recorded distance.
One US liquid gallon equals 231 cubic inches, or 3.785411784 litres. An imperial gallon is 4.54609 litres, so an imperial MPG figure is numerically higher for the same physical journey. The calculator keeps the two conventions explicit and uses 1 mile = 1.609344 km for metric results. A unit conversion does not adjust for driving conditions or how an economy figure was measured.
Worked US fill-up example
Driving 300 miles and using 10 US liquid gallons gives 30.00 US MPG. The same consumption is 36.03 imperial MPG or 7.84 L/100 km after rounding. At a hypothetical gas price of $3.50 per US gallon, the fuel costs $35.00; the displayed cost per mile is $0.12 when rounded to cents. Enter your actual local price rather than treating this example as a market quote.
Comparing trips and entering other units
For an overall figure across multiple tanks, add the miles and gallons across the whole period and divide those totals. Averaging individual MPG numbers directly can misrepresent the combined result. An odometer interval should cover the same fuel purchases included in the fuel total.
The fuel unit button accepts litres or imperial gallons and converts them to US liquid gallons. Distance controls accept kilometres. Distance conversions round to 0.1 mile and fuel conversions retain six decimal places, so preserve your original records for calculations requiring greater precision. US dry gallons are excluded.
Higher MPG and kilometres per litre mean more distance from the fuel used. Lower litres per 100 kilometres mean less fuel for the same distance. Your measured result can change with the route, traffic, vehicle load and driving conditions; this tool does not forecast a future trip. Optional dollar results cover fuel only.
For a headline figure in imperial gallons and a UK price per litre, use the UK MPG calculator.
For IRS mileage amounts rather than fuel economy, use the US mileage calculator.
Inputs for the US MPG calculator
Trip distance or odometer readings
Enter miles driven, or subtract the starting reading from the ending reading. The distance controls can convert kilometre records in place.
Gas used
Enter US liquid gallons added to restore the original tank level. Use the unit converter when your records are in litres or imperial gallons.
Pump price
Optionally enter US dollars per US liquid gallon using the price you actually paid. Zero represents free fuel, and economy can be calculated without a price.
CALCULATOR QUESTIONS
US MPG calculator questions
Which gallon does the US MPG result use?
The primary result uses the US liquid gallon, equal to 3.785411784 litres. The separate imperial result uses the larger UK gallon. Neither result uses a US dry gallon.
Can I enter fuel measured in litres?
Yes. Open the unit button beside Gas used and enter litres. The field converts the amount to US liquid gallons and keeps six decimal places.
Why does L/100 km decrease as MPG increases?
MPG describes distance per fuel volume, while L/100 km describes fuel volume per fixed distance. They are reciprocal ways to describe consumption.
Can this calculate a combined MPG across several tanks?
Yes. Enter total miles and total fuel over the same interval. Combine the underlying quantities rather than averaging the individual MPG figures.
Does this calculate electric-vehicle MPGe?
This page measures distance per liquid-fuel volume. It does not calculate electric-vehicle MPGe or convert electricity use into a gasoline-equivalent rating.