Fuel Economy Converter
Convert fuel economy between miles per gallon (US/Imperial), litres per 100km, and kilometres per litre.
How to use this tool
- Enter fuel economy and from unit in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your l/100km and the full breakdown beneath it.
Formula
All values are first converted to US miles per gallon (mpg_us) as an intermediate, then converted to each output:
From L/100km: mpg_us = 235.2145833 ÷ L/100km
From km/L: mpg_us = km/L ÷ 0.42514371
From MPG (Imperial): mpg_us = mpg_imp ÷ 1.20095042
Outputs: L/100km = 235.2145833 ÷ mpg_us | km/L = mpg_us × 0.42514371 | mpg_imp = mpg_us × 1.20095042
How it works
Fuel economy is an inversely proportional quantity when comparing L/100km with distance-per-volume units (mpg, km/L): better efficiency means a higher mpg or km/L value but a lower L/100km value. The converter handles this by pivoting through US mpg, using the constant 235.2145833 (derived from the number of US fluid-ounce millilitres in 100 km per gallon) to perform the reciprocal transformation for L/100km.
Common mistake: Treating US gallons and Imperial gallons as identical. An Imperial gallon is 1.20095 times larger than a US gallon, so 30 mpg (US) equals approximately 36 mpg (Imperial) — a difference that matters when comparing vehicle specs across regions.
Worked example
Convert 30 MPG (US) to all fuel economy units
- Input: 30 mpg_us. This is already the pivot unit, so mpg_us = 30.
- L/100km = 235.2145833 / 30 ≈ 7.84.
- km/L = 30 × 0.42514371 ≈ 12.754.
- MPG (Imperial) = 30 × 1.20095042 ≈ 36.03.
30 MPG (US) = 7.84 L/100km = 12.754 km/L = 36.03 MPG (Imperial).
Common mistakes to avoid
- Treating US MPG and UK MPG as equivalent -- a UK Imperial gallon is 4.546 L versus 3.785 L for a US gallon, so a car rated 40 UK MPG is only about 33 US MPG.
- Assuming a lower L/100km number is always worse -- L/100km is an efficiency measure where smaller is better (less fuel per distance), the opposite of MPG where bigger is better.
- Dividing by zero when entering 0 L/100km -- the MPG conversion formula uses division by L/100km, so a zero entry is mathematically undefined.
Key terms
- MPG (US)
- Miles per US gallon. The standard fuel economy rating used in the United States. One US gallon = 3.785 litres.
- L/100km
- Litres of fuel consumed per 100 kilometres driven. Used in most of Europe, Australia, and Canada. Unlike mpg, a lower number means better efficiency.
- km/L (kilometres per litre)
- Distance travelled on one litre of fuel. Used in parts of Asia and Latin America. Directly proportional to efficiency — a higher number is better.
- Imperial gallon
- The British gallon, equal to approximately 4.546 litres — about 20% larger than the US gallon (3.785 L). MPG figures based on Imperial gallons will always be higher than US MPG for the same vehicle.
Frequently asked questions
- Why does L/100km decrease as efficiency improves, while MPG increases?
- L/100km measures fuel consumed per unit of distance -- less is better. MPG measures distance achieved per unit of fuel -- more is better. They are reciprocal concepts, so a very efficient car might be 4.5 L/100km and 52 US MPG at the same time.
- How do I convert 8 L/100km to MPG?
- Use the formula: US MPG = 235.2145833 / L/100km. So 235.2145833 / 8 = approximately 29.4 US MPG.
- What is km/L and where is it used?
- km/L is the distance travelled per litre of fuel, conceptually similar to MPG. It is commonly used in Japan, India, and some other Asian countries. A car achieving 15 km/L gets about 35 US MPG or 6.67 L/100km.