What is a mileage calculator?
A mileage calculator works out a vehicle's real fuel economy from two numbers you record: the distance driven and the fuel used to cover it. This one takes kilometers and liters and reports the result two ways, liters per 100 km and kilometers per liter, each rounded to two decimal places. Both values describe the same economy.
"Mileage" is everyday shorthand for fuel economy in many countries, especially South Asia, where it's usually quoted in km/L. Most of Europe and Australia quote L/100 km instead. Showing both saves you converting.
The formulas
Consumption (L/100 km) = fuel used (L) ÷ distance (km) × 100
Economy (km/L) = distance (km) ÷ fuel used (L)
The two are reciprocals scaled by 100, so either one gives you the other: 100 ÷ 12.5 km/L = 8 L/100 km, and 100 ÷ 8 L/100 km = 12.5 km/L.
Lower L/100 km means a more efficient car. Higher km/L means the same thing. If you compare cars, stick to one measure so you don't misread which is better.
Measuring it: the full-tank method
Trip computers and brochure figures are estimates. To find what your own car actually does:
- Fill the tank until the pump clicks off, and reset the trip meter to zero.
- Drive normally. A longer stretch, ideally most of a tank, gives a more reliable number than a short run.
- Refill at the same pump if you can, again stopping at the first click.
- Enter the trip meter distance and the liters from the second fill-up.
The second fill-up replaces exactly the fuel the car burned since the first, which is why this method works. Repeating it over a few tanks and averaging the results smooths out the variation in how full the tank gets each time.
Worked examples
| Distance | Fuel used | L/100 km | km/L |
|---|---|---|---|
| 540 km | 38.5 L | 7.13 | 14.03 |
| 400 km | 32 L | 8.00 | 12.50 |
| 643.7 km (400 mi) | 45.4 L (12 US gal) | 7.05 | 14.18 |
For the first row: 38.5 ÷ 540 × 100 = 7.13 L/100 km, and 540 ÷ 38.5 = 14.03 km/L.
Converting to miles per gallon
The calculator doesn't output mpg, but km/L converts with a single multiplier:
| From km/L to | Multiply by | Why |
|---|---|---|
| US mpg | 2.352146 | 3.785411784 L per US gallon ÷ 1.609344 km per mile |
| UK (imperial) mpg | 2.824809 | 4.54609 L per imperial gallon ÷ 1.609344 km per mile |
So the 14.03 km/L in the first example is about 33.0 US mpg or 39.6 UK mpg. The mile is exactly 1.609344 km and the US gallon is exactly 3.785411784 L, both listed in NIST SP 811. The imperial gallon is fixed at 4.54609 L by the UK's Weights and Measures Act 1985. If you're starting from miles and gallons, the length converter and volume converter will get you to kilometers and liters.
Why your mileage changes
Your figure will move from tank to tank, and that's normal. The US Department of Energy's fuel economy tips cover the habits that matter most, such as aggressive acceleration, high motorway speeds and long idling. Other things that raise consumption include short trips on a cold engine, underinflated tyres, roof racks and heavy loads. A sudden drop with no change in how you drive can be a sign the car needs servicing.
Putting the result to use
Your measured L/100 km is the most accurate figure to feed into the fuel cost calculator when budgeting a trip. Pair it with the travel time calculator to plan both the cost and the hours of a long drive.
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