⛽ Gas Mileage Calculator
Work out your fuel economy in seconds. Enter the distance you drove and the fuel you used to see miles per gallon (MPG) and litres per 100 km (L/100km) side by side.
What is this tool?
Fuel economy describes how far a vehicle travels on a given amount of fuel, or equivalently how much fuel it uses over a given distance. It is one of the most useful numbers for any driver: it affects your running costs, your environmental footprint, and even your choice of car. The two dominant units in the world are miles per gallon (MPG), used mainly in the United States and the United Kingdom, and litres per 100 kilometres (L/100km), used in most of the rest of the world. This calculator takes the distance you drove and the amount of fuel you used, and instantly shows both fuel-economy figures side by side. You can enter the distance in miles or kilometres and the fuel in gallons or litres, and the tool converts everything internally so the two standard outputs are always correct. This makes it easy to compare a car quoted in MPG with one quoted in L/100km, or to track your own fuel economy over time regardless of which unit you are used to. Everything runs locally in your browser, so there is nothing to install and no data leaves your device. The tool validates that both inputs are positive numbers and clearly reports the results, including the conversion in both directions so you can see exactly how the two units relate.How it works
The calculator first normalises the inputs into consistent units. For the MPG figure, the distance is converted to miles and the fuel to gallons, then MPG is calculated as miles divided by gallons. For the L/100km figure, the distance is converted to kilometres and the fuel to litres, then the fuel in litres is divided by the distance in kilometres and multiplied by 100 to give litres per 100 km. The two figures are inversely related: a higher MPG means a more efficient car, while a lower L/100km means a more efficient car. The conversion factor between them is fixed — roughly, 1 MPG (US) equals 235.215 divided by L/100km — so the calculator can move freely between the two systems. Input validation ensures the distance and fuel are both greater than zero; a zero fuel amount would cause a division by zero in the L/100km calculation, and a zero distance would make MPG undefined, so the tool prompts you clearly instead of producing a misleading result.How to use
- Enter the distance you drove, then choose miles or kilometres.
- Enter the amount of fuel you used, then choose gallons or litres.
- Press the Calculate button.
- Read your fuel economy in MPG and in L/100km side by side.
- Change the units or values to recalculate and compare.
Frequently Asked Questions
Frequently Asked Questions
What is the difference between MPG and L/100km?
MPG (miles per gallon) measures distance per unit of fuel — higher is better. L/100km (litres per 100 kilometres) measures fuel per unit of distance — lower is better. They are two ways of expressing the same idea, used in different parts of the world.
Are US and UK gallons the same?
No. A US gallon is about 3.785 litres, while an imperial (UK) gallon is about 4.546 litres. Because the gallon sizes differ, the MPG figure for the same car is about 20 percent higher in UK imperial gallons than in US gallons. This calculator uses US gallons; if you need imperial MPG, convert your fuel to litres and select litres instead.
Which result means better fuel economy?
For MPG, a higher number is better because it means you travel further on each gallon. For L/100km, a lower number is better because it means you use less fuel for the same distance. So a car that does 50 MPG is more efficient than one that does 30 MPG, while a car that does 5 L/100km is more efficient than one that does 8 L/100km.
Why does my L/100km look high?
L/100km is fuel per distance, so it is naturally higher for less efficient vehicles like large SUVs or trucks, and lower for small cars and hybrids. Short trips with lots of stopping and starting also raise L/100km because the engine runs inefficiently. Check that you selected the correct units for your distance and fuel — mixing miles with litres, for example, would give a number that does not match either standard system.
Tips & Advice
For the most accurate measurement, fill your tank completely, reset your trip meter, drive normally until the tank is low, then fill up again and record the exact litres or gallons you added — that second fill is the true fuel used over the recorded distance. Fuel economy varies with driving style: smooth acceleration, sensible speeds and properly inflated tyres can improve MPG by 10 to 20 percent compared with aggressive driving. Cold weather, short trips and heavy loads all reduce fuel economy, so compare figures taken under similar conditions. When comparing two cars, make sure both figures use the same gallon (US or imperial), because a 40 MPG UK figure is not the same as a 40 MPG US figure. Tracking your fuel economy over several fill-ups is more reliable than a single measurement, because one tank can be affected by an unusual trip or a different pump.
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