Zerdly

Fuel Consumption Calculator

Work out your real fuel economy from a trip. Enter the distance you drove and the fuel you used, and get the result in L/100km, kilometres per litre, and imperial miles per gallon.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the distance travelled in kilometres (read it from your trip meter between two fills).
  2. Enter the fuel used in litres — the amount it took to refill the tank at the end of the trip.
  3. Read the fuel economy in L/100km, plus km per litre and imperial mpg for cross-reference.

How it works

Fuel economy (L/100km) = fuel used / distance × 100. Distance per litre = distance / fuel. The imperial miles-per-gallon figure is 282.481 / (L/100km), using the 4.546 L UK gallon. For 500 km on 40 L: 40 / 500 × 100 = 8 L/100km, 500 / 40 = 12.5 km/L, and 282.481 / 8 = 35.31 mpg.

Worked example

Worked example. Drive 500 km and use 40 L: economy = 40 / 500 × 100 = 8.0 L/100km, or 12.5 km/L, or 35.31 imperial mpg.

Common mistakes

  • Confusing imperial and US mpg — this tool reports imperial (UK) mpg, which is about 20% higher than US mpg for the same car.
  • Measuring only a partial fill — for accuracy, brim the tank at both ends of the trip so the fuel figure matches the distance exactly.
  • Judging economy from one short or cold trip — a full tank of mixed driving gives a far more representative figure.

Frequently asked questions

Is a lower or higher number better?

For L/100km, lower is better — it means less fuel per distance. For km/L and mpg it is the opposite: higher is better.

Why does the mpg look high compared with a US figure?

This uses the imperial (UK) gallon of 4.546 litres. The US gallon is only 3.785 litres, so US mpg for the same car is about 20% lower. US mpg would be 235.215 / (L/100km).

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