MPG Calculator & Fuel Economy Converter
Convert instantly between UK MPG, US MPG, L/100km, km/L and miles per litre, or work out your car’s real-world MPG from a fill-up.
MPG Calculator & Fuel Economy Converter (UK MPG, US MPG, L/100km, km/L)
Fuel economy is measured differently around the world, which makes comparing figures between UK, US and European sources confusing. Our MPG Calculator converts instantly between UK MPG, US MPG, litres per 100km (L/100km), kilometres per litre (km/L) and miles per litre — or works out your car’s real-world MPG from a simple fill-up. This is a unit conversion tool based on fixed physical constants, not a running-cost estimate; for fuel cost comparisons, see our Fuel Cost Calculator.
Why Is UK MPG Different From US MPG?
“MPG” always means miles per gallon, but the UK and US use different gallons. The UK (imperial) gallon is 4.54609 litres, while the US gallon is 3.78541 litres — about 17% smaller. Because a UK gallon holds more fuel, the same car will always show a higher UK MPG figure than a US MPG figure for identical real-world efficiency.
| Unit | Definition | Higher Number Means |
|---|---|---|
| UK MPG | Miles per imperial gallon (4.54609 L) | More efficient |
| US MPG | Miles per US gallon (3.78541 L) | More efficient |
| L/100km | Litres consumed per 100 kilometres | Less efficient |
| km/L | Kilometres travelled per litre | More efficient |
| Miles/L | Miles travelled per litre | More efficient |
How to Use This Calculator
- To convert a known figure: use the “Convert Units” tab, enter your value, select which unit it’s currently in, and click Convert to see it expressed in all five units.
- To calculate your own MPG from a fill-up: use the “Calculate From Fill-Up” tab, enter the distance you travelled and how much fuel you used to cover it, then click Calculate.
Common MPG Conversion Reference
| UK MPG | US MPG | L/100km | km/L |
|---|---|---|---|
| 30 | 25.0 | 9.42 | 10.62 |
| 40 | 33.3 | 7.06 | 14.16 |
| 50 | 41.6 | 5.65 | 17.70 |
| 60 | 50.0 | 4.71 | 21.24 |
| 70 | 58.3 | 4.04 | 24.78 |
Figures rounded to 2 decimal places using fixed conversion factors: 1 UK gallon = 4.54609 litres, 1 US gallon = 3.78541 litres, 1 mile = 1.609344 km.
How to Calculate Your Own Car’s MPG From a Fill-Up
- Fill your tank completely and reset your trip counter (or note the odometer reading).
- Drive normally until your next fill-up.
- Fill the tank completely again and note exactly how much fuel went in.
- Enter the distance travelled since the last fill-up and the fuel used into the calculator above.
This “brim-to-brim” method gives a more accurate figure than a car’s onboard trip computer, which typically estimates rather than measures fuel used.
Why Manufacturer MPG Figures Differ From Real-World MPG
- Test conditions: official WLTP figures are measured under controlled laboratory conditions that don’t reflect real traffic, weather or terrain.
- Driving style: harder acceleration and higher speeds significantly reduce real-world MPG compared to laboratory testing.
- Load and aerodynamics: roof boxes, heavy loads and open windows all increase fuel consumption.
- Short journeys: engines are less efficient before they reach normal operating temperature, so short trips typically show worse MPG.
- Tyre pressure: under-inflated tyres increase rolling resistance and reduce fuel economy.
Frequently Asked Questions
Why is UK MPG higher than US MPG for the same car?
The UK (imperial) gallon is 4.54609 litres, while the US gallon is only 3.78541 litres — about 17% smaller. Since MPG means miles travelled per gallon, and a UK gallon contains more fuel, the same car covers more miles per UK gallon than per US gallon, even though its real-world efficiency hasn’t changed.
How do I convert MPG to L/100km?
Divide 282.481 by the UK MPG figure to get an approximate L/100km value (this factor combines the imperial gallon size with the miles-to-kilometres conversion). Our calculator does this conversion instantly and also shows US MPG, km/L and miles per litre from the same figure.
Is a lower or higher L/100km better?
Lower is better for L/100km, which is the opposite of MPG. L/100km measures how much fuel is used to cover a fixed distance, so a smaller number means the car uses less fuel over that distance. MPG measures distance covered per unit of fuel, so a bigger number means better efficiency.
How accurate is calculating MPG from a single fill-up?
A single “brim-to-brim” fill-up calculation is generally more accurate than a car’s onboard trip computer, but it can still be affected by how full the pump considers “full” and rounding on the fuel pump display. For a more reliable average, repeat the calculation over several fill-ups and take the average.
What is a good MPG for a car in the UK?
This varies hugely by vehicle type. Small petrol hatchbacks often achieve 45-55 UK MPG, mid-size diesels commonly achieve 50-65 UK MPG, while larger SUVs and performance cars may only achieve 25-35 UK MPG. Hybrid vehicles can exceed 60-70 UK MPG in mixed driving.
Does this calculator work for diesel as well as petrol?
Yes. MPG, L/100km, km/L and miles per litre are unit conversions based on distance and fuel volume, and apply equally to petrol and diesel vehicles. The conversion factors themselves don’t depend on fuel type.