What Does the MPG ↔ L/100km Converter Do?
This converter translates fuel economy between UK imperial MPG and litres per 100 kilometres. The two systems describe the same consumption from opposite directions: MPG counts how far you travel on a gallon, while litres per 100 kilometres counts how much fuel a fixed distance uses. Consequently a higher MPG number and a lower L/100km number both represent better economy. The conversion is arithmetic, not an adjustment for driving conditions.
How to Use This Calculator
- Identify whether the source is imperial MPG or litres per 100 kilometres. The MPG input on this page is UK imperial, so a US publication's MPG must not be entered as though it were the same unit.
- Select the direction of conversion. Enter the numeric value from your source without changing its unit beforehand. The two directions use the same reciprocal relationship.
- Calculate and read the destination unit carefully. A higher MPG figure means less fuel per distance, while a lower litres-per-100-kilometres figure means less fuel per distance.
- Keep enough precision for comparison without implying more precision than the source measurement supports. If feeding the answer into another calculator, use the underlying result rather than repeatedly rounding intermediate conversions.
The Maths & Assumptions Behind the Tool
One imperial gallon is 4.54609 litres and one mile is 1.609344 kilometres. The reciprocal conversion constant is 100 × 4.54609 ÷ 1.609344, approximately 282.481. Therefore litres per 100 km = 282.481 ÷ UK MPG, and UK MPG = 282.481 ÷ litres per 100 km. Inputs must be positive; zero consumption would imply an unlimited MPG value and is not a usable conversion for a fuel-consuming journey.
Worked example
The imperial constant is 100 × 4.54609 ÷ 1.609344 = 282.4809363. Dividing by 45 UK MPG gives 6.28 L/100km. Dividing the same constant by an exact 6.28 L/100km gives 44.98 UK MPG; the small round-trip difference is display rounding.
| UK MPG | L/100km |
|---|---|
| 20 | 14.12 |
| 30 | 9.42 |
| 40 | 7.06 |
| 50 | 5.65 |
| 60 | 4.71 |
| 70 | 4.04 |
Lower L/100km and higher MPG both mean better fuel economy. A US MPG figure needs a different gallon constant.
Why This Matters for UK Drivers
Check the origin of the MPG figure before using it. A UK brochure, an imported vehicle display and an American owner report can use different gallon definitions even when they all say MPG. This page uses imperial gallons. It cannot identify the unit from the number alone. If comparing two reviews, establish the gallon standard first and make sure both describe comparable test or road conditions.
The relationship is inverse rather than a fixed multiplier. Doubling MPG halves litres per 100 kilometres; adding ten MPG does not create the same litres saving at every starting point. Rounding also matters when converting back. Keep the original unrounded number if doing several calculations, and treat small changes beyond the displayed precision cautiously. A correct conversion preserves whatever measurement quality the original figure had; it does not make a laboratory result a forecast of your next journey.
Interpreting the result in practice
The reciprocal form explains why a fixed MPG improvement has different fuel effects at different starting points. Moving from 20 to 30 MPG cuts fuel per mile by one third, while moving from 50 to 60 MPG cuts it by one sixth. Both changes add ten MPG, but they do not save the same litres over a fixed distance. Converting to litres per distance makes that distinction easier to see.
For a fleet or mixed journey, do not convert an unweighted average of individual MPG results and call it overall consumption. Add the distance and fuel first. Total litres divided by total kilometres, multiplied by 100, gives the correct combined litres per 100 kilometres. A brochure figure and a measured result can use the same units while representing different driving conditions; changing units does not remove that difference.
Frequently asked questions
Is this imperial MPG?
Yes. It is designed for UK imperial gallons, not US gallons.
Is lower L/100km better?
Yes. Lower litres per 100 km means less fuel used.
Understanding the two efficiency formats
What the number tells you
UK MPG and L/100km describe the same idea in opposite directions. Higher MPG is better because it means more distance per gallon; lower L/100km is better because it means less fuel used to cover 100 kilometres. That reversal is why comparing the raw numbers directly can be confusing.
Use realistic inputs
The converter uses UK imperial gallons, not US gallons. This matters when reading overseas reviews or owner reports because a US MPG figure is numerically lower for the same fuel consumption. Check the source of any MPG value before comparing it with a UK figure.
What is outside the calculation
Conversion does not make an official test result into a real-world figure. If you are budgeting fuel cost, use the converted value only if the underlying consumption figure represents the driving conditions you expect.