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MPG to L/100 km: exact conversion

By RoadCost Editorial5 min readUpdated

MPG and L/100 km describe the same idea in opposite directions. MPG tells you how far a car goes on a gallon. L/100 km tells you how much fuel it uses to go 100 kilometres. The conversion is simple, but the UK and US versions use different constants because the gallons are different.

Quick answer

For UK MPG, use L/100 km = 282.480936 ÷ MPG UK. For US MPG, use L/100 km = 235.214583 ÷ MPG US. The UK constant is larger because an imperial gallon is larger than a US gallon.

In this guide

  1. 1. The formulas
  2. 2. Worked examples
  3. 3. How to avoid the classic mistake
  4. Questions

Key takeaways

  • MPG UK and MPG US need different conversion constants.
  • Lower L/100 km means lower fuel consumption.
  • The same MPG number means different consumption in the UK and the US.

The formulas

The constants combine the volume of a gallon with the number of kilometres in a mile. They are fixed, so you can use them without remembering the full derivation.

If you are converting a UK figure, use 282.480936. If the figure came from a US window sticker, use 235.214583. Mixing the two constants is the most common conversion error.

L/100 km = 282.480936 ÷ MPG (UK)
L/100 km = 235.214583 ÷ MPG (US)

Worked examples

The table shows the same MPG number interpreted under each gallon definition. Notice that 40 MPG UK is about 7.06 L/100 km, while 40 MPG US is about 5.88 L/100 km.

MPG to L/100 km under UK and US gallon definitions
MPGL/100 km as MPG UKL/100 km as MPG US
309.427.84
407.065.88
505.654.70
604.713.92

How to avoid the classic mistake

Start by asking where the MPG number came from. A UK brochure, a US window sticker and a rental-car display can all use different conventions. If the source is unclear, convert both candidates to L/100 km and see which result makes sense for the vehicle.

Do not use the UK constant for a US MPG figure. It will make the car look about 20% more efficient than it really is in the US system. That error then flows into fuel-cost estimates.

  • Convert the unit before comparing two cars.
  • Use L/100 km when you want a linear measure of fuel use.
  • Use the same gallon definition throughout a calculation.

Questions

Is 40 MPG good?
It depends on the gallon and the vehicle. In the UK, 40 MPG is about 7.06 L/100 km. In the US, 40 MPG is about 5.88 L/100 km. Vehicle size, fuel type and driving conditions matter too.
How do I convert UK MPG to US MPG?
Multiply UK MPG by 0.8327 to get the approximate US MPG figure. For example, 50 MPG UK is about 41.6 MPG US.
Why not just use an online converter?
You can, but you still need to know which gallon the source used. A calculator with a clear label is safer than a converter that silently assumes one system.

Sources

Try the calculator

Fuel efficiency converter

Convert MPG UK, MPG US, L/100 km and km/L in one place.

Related guides

L/100 km to MPG: exact conversion

Turn litres per 100 kilometres into UK or US MPG with the exact formulas, examples and a simple way to remember the ratio.

US vs imperial gallon: MPG difference

The US and imperial gallons are different volumes. Learn the exact difference and how it changes MPG, fuel prices and cost comparisons.

Fuel economy vs consumption

MPG and km/L measure distance per fuel. L/100 km measures fuel per distance. Here is why the distinction matters for comparisons and costs.

A note on prices and rules

This guide explains arithmetic and units. It does not publish live fuel prices, electricity tariffs or changing tax rules. Use your own price and check official sources for anything that changes.

Page last reviewed: . Formulas are covered by automated tests. Prices are inputs, not live data.