Fuel Consumption Calculator

How it works

Fuel consumption is a ratio between the fuel used and the distance driven. The same measurement can be written three ways, and the calculator shows all of them:

L/100 km = liters / km × 100
km per liter = km / liters
mpg (US) = miles / US gallons

You enter the distance and the fuel in either metric (kilometers and liters) or imperial units (miles and US gallons). Everything is converted once, to kilometers and liters, using the exact definitions of 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L, and the three figures are then calculated from the same two numbers, so they always agree with each other.

The scales run in opposite directions. L/100 km measures how much fuel it takes to cover a fixed distance, so a smaller number is better. Miles per gallon and km per liter measure how far a fixed amount of fuel goes, so a bigger number is better. To switch between L/100 km and mpg, divide 235.215 by the number you have.

Measuring it with the full-tank method

The calculator needs two honest numbers: the distance and the fuel actually burned. The full-tank method gets them:

1. Fill the tank at a pump until the nozzle clicks off the first time. Do not top it up.
2. Reset the trip meter, or write down the odometer reading.
3. Drive as you normally would, until the tank is at least half empty. A full tank or more is better.
4. Fill up again at the same pump if you can, to the first click, with the car parked level.
5. Note the liters the pump added, and the distance on the trip meter.

The liters added in step 5 replace exactly what the engine burned since step 1. The fuel you put in at step 1 does not count: it only sets the starting level. Enter the distance and those liters into the calculator.

To average several fill-ups, add the liters and add the kilometers, then divide once. For 520 km on 36.4 L, 480 km on 34.8 L and 500 km on 35.8 L the individual figures are 7.00, 7.25 and 7.16 L/100 km. The correct combined figure is 107.0 L over 1,500 km, 107.0 / 1,500 × 100 = 7.13 L/100 km. Averaging the three figures gives 7.14, which is slightly off because it treats a short stretch and a long one as equal.

Why a single tank is noisy

The pump's cutoff depends on how the fuel foams and how the nozzle sits, so two fill-ups of "full" can differ by a fraction of a liter, in either direction. That error is fixed in size, so it matters more on a short distance. If the fill-ups differ by half a liter, that is 0.5 / 500 × 100 = 0.10 L/100 km over 500 km, but 0.5 / 150 × 100 = 0.33 L/100 km over 150 km. The longer you measure, or the more fill-ups you combine, the smaller that error is compared with the total.

Converting between units

With the exact constants (1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L):

mpg (US)  = 235.215 / (L/100 km)       L/100 km = 235.215 / mpg (US)
mpg (UK)  = 282.481 / (L/100 km)       L/100 km = 282.481 / mpg (UK)
km per L  = 100 / (L/100 km)           L/100 km = 100 / (km per L)
mpg (UK)  = mpg (US) × 1.20095         mpg (US) = mpg (UK) × 0.832674
mpg (US)  = km per L × 2.35215         km per L = mpg (US) × 0.425144

A worked example for each:

• 7.0 L/100 km is 235.215 / 7 = 33.6 mpg (US), or 282.481 / 7 = 40.4 mpg (UK), or 100 / 7 = 14.3 km per liter.
• 40 mpg (UK) is 282.481 / 40 = 7.06 L/100 km, or 40 × 0.832674 = 33.3 mpg (US).
• 12.5 km per liter is 100 / 12.5 = 8.0 L/100 km, or 12.5 × 2.35215 = 29.4 mpg (US).
• 30 mpg (US) is 235.215 / 30 = 7.84 L/100 km, or 30 × 0.425144 = 12.75 km per liter.

US gallon or imperial gallon

A US gallon is 3.785 liters and an imperial (UK) gallon is 4.546 liters, about 20 percent bigger. A UK car quoted at 40 mpg is therefore a 33 mpg car in US terms, and the two numbers describe the same fuel use. This calculator uses the US gallon. If you counted fuel in UK gallons, convert it first: 8 UK gallons is 8 × 1.20095 = 9.61 US gallons, and 300 miles on them is 300 / 9.61 = 31.2 mpg (US), which is 37.5 mpg in UK terms (300 / 8) and 7.53 L/100 km.

Examples

Metric. You drove 500 km and filled 35 L. The consumption is 35 / 500 × 100 = 7.0 L/100 km, which is 14.3 km per liter, or 33.6 mpg.

Imperial. You drove 300 miles and used 10 US gallons. That is 30 mpg, or 7.84 L/100 km, or 12.75 km per liter.

Comparing cars. 6 L/100 km is 39.2 mpg, 8 L/100 km is 29.4 mpg and 10 L/100 km is 23.5 mpg. The same two-liter gap is worth far more mpg at the efficient end than at the thirsty end, which is why mpg can make small differences between efficient cars look bigger than they are.

Things to watch for

• Fill up the same way each time. The figure is only as good as the two fill-ups it comes from. Stop at the first click, keep the car level, and use the same pump if you can. A partly filled tank makes the result wrong by a lot.
• Use a full tank or more. One fill-up can be off by a liter or more, which on a 200 km trip is half a liter per 100 km. Measure over a few hundred kilometers.
• Know which gallon you mean. US and UK gallons differ by 20 percent. If a UK source quotes 40 mpg, that is about 33 mpg in US gallons. The calculator uses US gallons.
• The trip computer is an estimate. It is usually a little optimistic. Checking it against a full-tank calculation is worthwhile once.
• Many things move the number. High speed, short cold trips, air conditioning, a roof box, extra load and low tire pressure all raise consumption. A single figure describes one kind of driving, not the car.
• Do not mix the fill-up and the odometer. If you forgot to reset the trip meter, subtract the first odometer reading from the second. Using the total odometer reading gives nonsense.
• Everything runs in your browser. The numbers you enter are not sent to a server.

To turn consumption into money, use the fuel cost calculator, and for other conversions the unit converter.