MPGtoLiters

MPG Calculator

Work out the miles per gallon your car is really getting from two numbers: the distance driven since the last fill and the fuel you have just put in. Enter them in miles or kilometres and in gallons or litres; the result comes out in mpg (US), mpg (UK), L/100km and km/L at the same time.

Miles per US gallon

30.89

mpg (US)

Miles per imperial gallon

37.10

mpg (UK)

Litres per 100 km

7.61

L/100km

Kilometres per litre

13.13

km/L

Add a fuel price to see the cost per mile.

For a reliable figure, fill to the pump's first click both times and use at least half a tank between fills.

How the calculator works

The distance is converted to kilometres and the fuel to litres, using the exact definitions of the mile (1.609344 km), the US gallon (3.785411784 litres) and the imperial gallon (4.54609 litres). Dividing one by the other gives kilometres per litre, and every other unit is derived from that through the same module the converters use. Nothing is rounded until it is displayed.

The price box is optional. If you enter what you paid per gallon or per litre, the calculator multiplies it by the fuel added to give the cost of the fill, and divides by the distance to give the cost per mile or per kilometre, which is the number that matters when comparing two cars you might buy.

Getting a figure you can trust

The method is the fill-to-fill one: fill until the pump clicks off, note the odometer or reset the trip meter, drive at least half a tank, fill again the same way, and enter the distance and the fuel from the second receipt. Stopping at the first click each time matters more than anything else, because topping up past it on one fill and not the other can move a single-tank result by 5%.

One tank is a data point. Traffic, weather and a single long motorway run all shift it by 10% or more. Three or four tanks averaged over similar driving describe the car. The manual calculation guide walks through a full example and lists the mistakes that make a result meaningless.

Reading the four results

The two mpg figures differ by 20.095% because the gallons do; neither is more correct, and you should compare each with figures from its own country. L/100km runs the other way, with lower meaning better, and it is the honest unit for comparing two cars because fuel per distance is what you pay for. km/L is the unit used in Japan, India and Brazil and behaves like mpg. The MPG or L/100km guide explains why the choice of unit changes how a comparison looks.

A result well below the car's official figure is normal. The real-world MPG guide explains why, and the improving fuel economy guide puts numbers on what closes the gap.

Frequently asked questions

How do I calculate miles per gallon?
Divide the miles driven since your last fill by the gallons you have just added. 380 miles on 12.3 US gallons is 30.9 mpg. If the pump gave litres, the calculator converts them for you; by hand, divide litres by 3.785411784 for US gallons or 4.54609 for imperial gallons first.
How do I work out miles to the gallon from litres?
Convert the litres to the right gallon, then divide miles by gallons. 400 miles on 40 litres is 400 ÷ 8.80 = 45.5 mpg on imperial gallons, or 400 ÷ 10.57 = 37.9 mpg on US gallons. Choose 'litres' as the fuel unit above and both come out at once.
Which gallon should I use?
The one your country's figures use. American mpg figures use the US gallon; British ones use the imperial gallon. The calculator shows both so you can compare with either.
Why does my result differ from the trip computer?
Trip computers estimate fuel from injector timing and are usually a few percent optimistic. The pump is a calibrated meter. If the two differ by more than about 5% over several tanks, trust the pump.
How accurate is one tank?
Not very. Pump shut-off varies by up to half a litre, and weather and traffic move a single tank by 10% or more. Average three or four fills for a figure that describes the car rather than the week.

Converters