Fuel Economy Calculator

What the Fuel Economy Calculator Does

This Fuel Economy Calculator turns two numbers you already have, the distance you drove and the amount of fuel you used, into clear efficiency figures. It reports your result in two common formats: kilometers per liter (km/L), where higher is better, and liters per 100 kilometers (L/100km), where lower is better.

It is useful for drivers tracking real-world consumption between fill-ups, fleet operators comparing vehicles, people deciding whether a car matches its advertised rating, and anyone budgeting for a trip. You do not need an on-board computer; a trip odometer and a full tank are enough.

How It Works: The km/L and L/100km Formulas

The calculator uses two simple, exact formulas. Both rely on the same inputs, so they always agree with each other.

Kilometers per liter measures how far you travel on each liter of fuel. Liters per 100 km measures how much fuel you burn over a fixed distance, which is the standard reporting unit in most of Europe.

• km/L = distance (km) / fuel used (liters)
• L/100km = (fuel used (liters) / distance (km)) x 100

Worked Example With Real Numbers

Suppose you filled the tank, reset your trip meter, drove 480 km, and then refilled the tank with exactly 36 liters. Plug those numbers in:

km/L = 480 / 36 = 13.33 km/L. L/100km = (36 / 480) x 100 = 7.5 L/100km.

So this car returns about 13.3 km per liter, or 7.5 liters for every 100 km. As a quick sanity check, the two figures are reciprocals scaled by 100: 100 / 13.33 = 7.5, and 100 / 7.5 = 13.33.

Common Mistakes to Avoid

Most bad readings come from input errors, not the math. Watch for these:

• Mixing units: do not enter miles with liters. Convert to a consistent system first (1 mile = 1.609 km, 1 US gallon = 3.785 L, 1 UK gallon = 4.546 L).
• Using a partial fill: for an accurate tank-to-tank result, fill completely at both the start and the end so the fuel figure matches the distance.
• Measuring over too short a distance: a single short trip exaggerates the effect of cold starts. Use at least one full tank for a representative number.
• Confusing the two metrics: a higher km/L is good, but a higher L/100km is bad. They move in opposite directions.

Factors That Affect Your Result

Real-world economy varies from one tank to the next, even in the same car. The biggest influences are driving style and conditions rather than the vehicle alone.

Hard acceleration and high motorway speeds raise consumption, while steady, anticipatory driving lowers it. Short trips, cold weather, heavy loads, roof racks, low tire pressure, and frequent air-conditioning use all push L/100km up. City traffic with constant stop-and-go is typically less efficient than open highway cruising.

Tips for Accurate Tracking

To get trustworthy figures, measure consistently. Always fill to the same point (the first auto-stop of the pump works well), reset the trip meter at each fill, and record the liters added.

Calculating economy for several consecutive tanks and averaging the results smooths out one-off effects like a single long highway trip or a cold week. Over time this gives you a reliable baseline you can use to spot problems, compare seasons, or check claimed manufacturer figures.