Fuel Consumption Calculator

What the fuel consumption calculator does

You filled up, you drove, and now you want to know how thirsty your vehicle actually was. That is what this tool is for. You tell it how far you went and how much fuel that took, and it gives you your real consumption, the amount of fuel your vehicle burns to cover the distance. Add the price you paid and it also tells you what each mile or kilometer is costing you.

This is the looking-back number, worked out from fuel you have already used, so it reflects your real driving rather than a figure off a sticker. You can run it in imperial or metric, whichever you think in.

How to use it

Three boxes, and the last one is optional.

1. Unit System. Pick imperial or metric. The labels next to the boxes switch to miles and gallons, or kilometers and liters, so you always know which units you are entering.
2. Distance Travelled. How far you drove on that fuel.
3. Fuel Spent. How much fuel that distance actually used. This is fuel you have already burned, not the size of your tank.
4. Price per Gallon or Liter. Optional. Add it and you also get your cost per mile or per kilometer. Leave it blank and you just get the consumption figure.

Press Calculate for the result, or Reset to clear the boxes.

How your consumption is worked out

The whole thing rests on one division: fuel used divided by distance covered. How that gets written depends on the unit system you picked.

In metric, the standard way to state consumption is liters per 100 km, so the tool works out how much fuel you would use to drive 100 km at the rate you just drove:

Liters per 100 km = fuel used ÷ (distance ÷ 100)

It also flips that around and shows you km per liter, since plenty of people prefer that. In imperial, it reports miles per gallon instead, which is simply the distance divided by the fuel:

Miles per gallon = distance ÷ fuel used

If you entered a price, the cost per mile is the price of a gallon divided by your miles per gallon, and the cost per kilometer works the same way with liters. Nothing more complicated than that is going on.

An example with real numbers

Say you drove 480 km and that took 36 liters, and fuel was 1.30 per liter.

• Liters per 100 km = 36 ÷ (480 ÷ 100) = 36 ÷ 4.8 = 7.5
• The same thing as km per liter = 480 ÷ 36 = 13.33 km/L
• Cost per km = 1.30 ÷ 13.33 = about 0.10 per km

So that drive ran at 7.5 liters per 100 km, which is 13.33 km on a liter, and it cost you roughly ten cents a kilometer. In imperial the steps are the same, you just divide miles by gallons to land on miles per gallon.

Consumption or economy, and which way the number runs

This is the bit that confuses people, so it is worth thirty seconds. There are two ways to describe the same fact, and they run in opposite directions.

Liters per 100 km is a consumption figure. It counts fuel used over a fixed distance, so a smaller number is better. A car at 6 liters per 100 km sips less than one at 9.

Miles per gallon and km per liter are economy figures. They count distance covered on a fixed amount of fuel, so a bigger number is better. A car doing 40 mpg goes further on a gallon than one doing 30.

Both say the same thing about your vehicle. They just look at it from opposite ends, which is why a low liters-per-100-km and a high miles-per-gallon are both good news.

What a normal figure looks like

It depends heavily on the vehicle, but a rough guide helps you sanity check your result. A modern petrol sedan tends to sit somewhere around 6 to 8 liters per 100 km, which is roughly 30 to 40 mpg. A larger SUV or truck drinks more, often 10 to 14 liters per 100 km. A hybrid can dip well under 5. If your figure is wildly outside the range you expected for your vehicle, it is usually worth a second measurement before you trust it.

What pushes your consumption up

If the number came out higher than the figure on your car's label, that is normal, and a few things explain most of the gap.

• Short, cold trips. An engine uses more fuel before it warms up, so lots of short hops burn more per kilometer than one steady run.
• City traffic. Stopping and starting uses far more fuel than holding a steady speed, so a city tank reads thirstier than a highway one.
• Weight and drag. A loaded boot, a roof box, even a full roof rack you forgot to take off, all add fuel to every kilometer.
• Air conditioning and a heavy right foot. Both quietly raise consumption, especially in town.

Questions people ask

How is this different from a fuel cost calculator?

A fuel cost calculator looks forward. You tell it a distance and a known efficiency and it predicts what a trip will cost. This tool looks back. You tell it fuel you have already used and it works out your real consumption rate from that.

Should I use liters per 100 km or miles per gallon?

Whichever your country and your car use. They measure the same thing in opposite directions. Liters per 100 km is fuel per distance, so lower is better. Miles per gallon is distance per fuel, so higher is better. The tool shows you the figure that matches the unit system you pick.

How do I know exactly how much fuel I used?

The cleanest way is to fill the tank at the start, drive, then fill it again at the end. The amount it takes to top it back up is what that drive used. Pump receipts give you the figure to the hundredth of a liter or gallon.

Why is my consumption worse than the official rating?

Official ratings are measured in a lab under steady conditions. Real driving has cold starts, traffic, weather, and your own habits, all of which use more fuel, so a real figure a bit worse than the rating is exactly what to expect.

References

1. National Institute of Standards and Technology (NIST), Special Publication 811: Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
2. U.S. Department of Energy and U.S. Environmental Protection Agency, FuelEconomy.gov: Gas Mileage Tips. https://www.fueleconomy.gov/feg/drive.shtml