Fuel Economy Calculator
Find your car’s real fuel economy from a fill-up, or convert MPG (US or UK), L/100 km and km/L both ways.
Fill to the brim, reset the trip meter, drive, then fill to the brim again. The fuel that second fill takes is what you used.
Type in any box and the others change as you type.
Only changes the CO2 estimate.
Your car is the one above. Enter the other car to see the yearly fuel cost of each and how long a pricier, thriftier car takes to pay for itself.
Home charging is usually cheapest.
Add each fill-up: the distance since the fill before, and the fuel put in. Uses the same units as above.
Your average will show here.
How your fuel economy is worked out
Economy = fuel used ÷ distance driven, then turned into each unit.
- 1
Distance driven
trip meter, fill to fill= 500 km
- 2
Fuel to refill
second fill, tank brim full= 35 L
- 3
Fuel ÷ distance
35 L ÷ 500 km× 100= 7.00 L/100 km
- 4
In other units
33.6 MPG (US)40.4 MPG (UK)= 14.3 km/L
Read more: the formula, worked examples, UK vs US MPG, conversion table, CO2 and MPGe
Fuel economy says how far a car goes on its fuel. Most of the world uses litres per 100 km (fuel used for a set distance, so lower is better). India and much of Asia use km per litre, and the USA and UK use miles per gallon (distance on a set amount of fuel, so higher is better). The US and UK gallons are different sizes, so their MPG figures are different too. Every unit is the same measurement turned round or scaled, so one number gives all the others.
The formula
L/100 km = litres put in ÷ km driven × 100
km/L = km driven ÷ litres put in = 100 ÷ L/100 km
MPG (US) = miles ÷ US gallons = 235.21 ÷ L/100 km
MPG (UK) = miles ÷ UK gallons = 282.48 ÷ L/100 km
miles per litre = 62.14 ÷ L/100 km
US gallons per 100 miles = L/100 km ÷ 2.352
cost per km = L/100 km ÷ 100 × price per litre
CO2 per km = L/100 km ÷ 100 × kg CO2 per litre
MPGe = 33.7 ÷ kWh per mile
Worked example
With the starting inputs you drove 500 km and the next fill took 35 litres. That is 35 ÷ 500 × 100 = 7.00 L/100 km, or 500 ÷ 35 = 14.3 km/L. In miles and gallons: 235.21 ÷ 7 = 33.6 MPG (US), 282.48 ÷ 7 = 40.4 MPG (UK), 62.14 ÷ 7 = 8.88 miles per litre, and 7 ÷ 2.352 = 2.98 US gallons per 100 miles.
In Advanced mode, with fuel at $1.00 a litre, the car costs 7 ÷ 100 × 1.00 = $0.07 a km ($0.11 a mile, $7.00 per 100 km). As a petrol car it gives off about 0.07 × 2.31 = 0.162 kg, or 162 g of CO2 per km. (The starting price follows your currency: $1.00, £1.40, €1.75, ₹100, AED 2.70 a litre and so on. Type in what you pay today.)
Comparing two cars
Driving 15,000 km a year, your 7 L/100 km car burns 15,000 × 7 ÷ 100 = 1,050 litres ($1,050 at $1.00 a litre). A car using 5 L/100 km burns 750 litres ($750). The thriftier car saves $300 a year. If it costs $2,000 more, it pays for itself in 2,000 ÷ 300 = 6.7 years; over 5 years it saves $1,500, so you would still be $500 behind.
Electric cars and MPGe
The US EPA counts 33.7 kWh of electricity as the same energy as one US gallon of petrol. An electric car using 17 kWh/100 km uses 17 × 1.609 = 27.36 kWh per 100 miles, so it gets 33.7 ÷ 0.2736 = 123 MPGe. On the same scale a litre of petrol holds 33.7 ÷ 3.785 = 8.90 kWh, so the electric car is like a petrol car using 17 ÷ 8.90 = 1.91 L/100 km, and your 7 L/100 km car uses the energy of 62.3 kWh per 100 km. At $0.17 a kWh, the electric car costs 17 ÷ 100 × 0.17 = $0.029 a km (charging losses not included).
A fill-up log
One fill-up can mislead: the pump may click off early, or one week may be all town driving. Log several and add them all up. With fills of 500 km / 35 L, 480 km / 33 L and 520 km / 38 L, the average is (35 + 33 + 38) ÷ (500 + 480 + 520) × 100 = 106 ÷ 1,500 × 100 = 7.07 L/100 km. This is fairer than averaging the three separate figures, because longer stretches count for more.
UK vs US MPG
A UK (imperial) gallon is 4.546 litres and a US gallon is 3.785 litres. Because the UK gallon is 20% bigger, UK MPG is always 1.2 times US MPG for the same car: 30 MPG (US) is 36.0 MPG (UK). Always check which gallon a figure uses before comparing cars.
Conversion table
| L/100 km | km/L | MPG (US) | MPG (UK) |
|---|---|---|---|
| 3 | 33.3 | 78.4 | 94.2 |
| 4 | 25.0 | 58.8 | 70.6 |
| 5 | 20.0 | 47.0 | 56.5 |
| 6 | 16.7 | 39.2 | 47.1 |
| 7 | 14.3 | 33.6 | 40.4 |
| 8 | 12.5 | 29.4 | 35.3 |
| 10 | 10.0 | 23.5 | 28.2 |
| 12 | 8.3 | 19.6 | 23.5 |
| 15 | 6.7 | 15.7 | 18.8 |
CO2 figures
Burning a litre of fuel gives off roughly 2.31 kg of CO2 for petrol, 2.68 kg for diesel and 1.51 kg for LPG. These are typical rounded tailpipe figures, not the official factor for any one country: the US EPA uses about 8,887 g per US gallon of petrol (about 2.35 kg a litre), and pump fuel that contains some biofuel comes out a little lower.
Tips
- Your real economy is often worse than the official figure, especially in town or at motorway speeds. A fill-up log gives your true number.
- Fill to the same point each time (the first click of the pump) so each fill is measured the same way.
- To turn your economy into the cost of a journey, use the fuel cost calculator. For charging an electric car at home, see the electricity cost calculator.
Unit conversions are exact (1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L). Starting prices are examples, not live prices. CO2 figures are estimates. Checked 10 October 2026.
Common questions
How do I convert MPG to L/100 km?
Divide 235.21 by MPG (US), or 282.48 by MPG (UK), to get litres per 100 km. So 30 MPG (US) is 235.21 ÷ 30 = 7.84 L/100 km. For km per litre, divide 100 by the L/100 km figure. Remember that a lower L/100 km is better, while a higher MPG or km/L is better.
How do I work out my car’s mileage in km per litre?
Fill the tank to the brim and reset the trip meter. Drive as normal, then fill to the brim again and note how much fuel went in. Divide the kilometres by the litres. For example, 500 km on 35 litres is 500 ÷ 35 = 14.3 km/L, which is the same as 7.00 L/100 km or 33.6 MPG (US).
Why is UK MPG higher than US MPG for the same car?
A UK (imperial) gallon is 4.546 litres but a US gallon is only 3.785 litres, so a UK gallon is about 20% bigger. The car goes further on a bigger gallon, so its UK MPG is always 1.2 times its US MPG. For example, 30 MPG (US) is 36.0 MPG (UK), and both are 7.84 L/100 km.
What is MPGe for electric cars?
MPGe means miles per gallon of gasoline equivalent. The US EPA counts 33.7 kWh of electricity as one US gallon of petrol. An electric car using 17 kWh per 100 km uses 27.4 kWh per 100 miles, so it gets 33.7 ÷ 27.4 × 100 = about 123 MPGe.
Is a more fuel-efficient car worth the extra price?
Compare the yearly fuel cost of both cars. Driving 15,000 km a year at $1.00 a litre, a car using 7 L/100 km costs $1,050 in fuel and one using 5 L/100 km costs $750, so the thriftier car saves $300 a year. If it costs $2,000 more, it takes 2,000 ÷ 300 = 6.7 years to pay back.
How much CO2 does my car give off per km?
Multiply the litres per km by the CO2 from one litre. Burning a litre of petrol gives off about 2.31 kg of CO2 and diesel about 2.68 kg. A petrol car using 7 L/100 km gives off about 0.07 × 2.31 = 0.162 kg, or 162 g of CO2 per km (260 g per mile). These are typical figures, not official ones.
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