Fuel Economy Converter
Convert between MPG (US and UK), L/100km, km/L, and miles per liter.
Why this isn't a simple multiplication
MPG and km/L measure how far you travel per unit of fuel — a bigger number means better efficiency. L/100km measures the opposite: how much fuel you use per fixed distance — a smaller number means better efficiency. Converting between the two families requires a division (100 ÷ value), not a multiplication, which is why L/100km can't share a simple conversion factor with the others.
Common conversions
- 30 MPG (US) ≈ 7.84 L/100km ≈ 12.75 km/L
- A US gallon (3.785 L) is smaller than a UK/Imperial gallon (4.546 L), so the same car gets a higher MPG(US) number than MPG(UK) for identical real-world fuel use
- 40 MPG (UK) ≈ 7.06 L/100km
- Lower L/100km = more efficient; higher MPG or km/L = more efficient
Why a bigger MPG gain is not always a bigger saving
Moving from 15 to 20 MPG (US) takes consumption from 15.68 to 11.76 L/100km, a saving of about 3.92 liters every 100 km. Moving from 30 to 40 MPG, the same 10 MPG step, goes from 7.84 to 5.88 L/100km and saves only about 1.96 liters. Because fuel used is the reciprocal of MPG, gains at low efficiency are worth far more than the same gain on an efficient car, which is why L/100km makes comparisons easier to judge.
Converting a European figure for a US or UK reader
A car rated at 8 L/100km is 29.40 MPG (US) or 35.31 MPG (UK/Imperial), and 16.67 km/L is the same as 6 L/100km. The UK figure is higher because the Imperial gallon is larger. Whichever pair you choose, enter the number once and read the line below it, which carries the unit name.
How the converter behaves
- Results update as you type or change a unit, and there is no calculate button.
- The value must be above zero. Zero and negative numbers give an error message.
- Results show up to four decimal places in your browser's own number format, so the decimal mark and digit grouping follow your locale.
What it does not do
This is a unit converter, not a fuel-use estimator. Quoted ratings come from different test procedures in different regions, so a figure from one standard is not directly comparable to a real-world result or to a figure from another test. Electric and hybrid ratings such as MPGe or kWh per 100 km are not included.
Frequently asked questions
Why do US and UK gallons give different MPG numbers for the same car?
A US gallon (3.785 liters) is about 17% smaller than a UK/Imperial gallon (4.546 liters). Since MPG is miles per gallon, the same car burning the same amount of fuel per mile will show a higher MPG(US) number than MPG(UK), purely because the US gallon is a smaller unit of volume.
Which unit should I use to compare cars?
L/100km is often considered the most intuitive for comparing absolute fuel costs, since it directly represents fuel used per fixed distance — the difference between a 5 and a 6 L/100km car is a consistent real amount of fuel, whereas the same absolute MPG difference means very different things at low vs. high MPG values.
Why does a higher MPG mean a lower L/100km?
They measure opposite things. MPG counts distance per unit of fuel, so more is better. L/100km counts fuel per fixed distance, so less is better. The conversion between them is 100 divided by the value, in the right units, not a multiplication.
Does this convert electric vehicle ratings like MPGe?
No. It covers US and UK MPG, L/100km, km/L and miles per liter only. Electric ratings use energy units such as kWh, which are outside this tool.
Which units do different countries use?
The US uses MPG (US). The UK still quotes MPG (Imperial) for cars while selling fuel by the liter. Most of continental Europe, Australia and Canada use L/100km, and India and Japan commonly use km/L. Check which gallon a source means before converting.