How this calculator works
Miles per gallon is distance divided by fuel volume, with both quantities converted to matching units first. The page uses the exact international mile and the two standard gallon definitions, so US and imperial mpg differ by about 20 percent for the same measurement. Litres per 100 kilometres is the reciprocal measure: fuel per distance rather than distance per fuel. In the fuel and distance modes the same equation is rearranged, so the reported economy remains consistent with the quantity you entered.
Formula
mpg(US) = (km / 1.609344) / (litres / 3.785411784). mpg(imperial) = (km / 1.609344) / (litres / 4.54609). L/100 km = 100 x litres / km. Fuel cost = litres x price per litre. Fuel used = distance / economy, and distance = fuel x economy.
Worked example
A 300 mile trip using 10 US gallons gives 30 mpg US, about 36.03 mpg imperial and about 7.84 L/100 km. At $1.65 per litre the 37.85 litres cost $62.46.
Advertisement
Frequently asked questions
Why are US and imperial mpg different?
The imperial gallon is larger, 4.54609 litres against 3.785411784 litres for the US gallon. The same car covering the same distance on the same fuel therefore shows a higher imperial mpg number.
Which mpg should a Canadian use?
Canada uses litres per 100 km officially. If you compare with US sources, use US mpg; older Canadian material may use the imperial gallon. State which gallon you mean to avoid confusion.
Can I solve for the fuel needed?
Yes. Choose the fuel mode, enter the distance and the target economy, and the page returns the fuel volume and trip cost.
Why is my trip computer different?
A trip computer estimates from injector pulses and may reset between fills. The full-tank method, dividing the fuel you actually bought by the distance you actually drove, is the honest measurement this page is designed for.