Fuel Consumption Converter
Convert any fuel-economy reading: US mpg, Imperial mpg, litres per 100 km, or kilometres per litre. See all four at once, exact to NIST factors.
30 Miles per US gallon (mpg, US) =
7.84
- Miles per Imperial gallon (mpg, UK)
- 36.03
- Litres per 100 km (L/100km)
- 7.84
- Kilometres per litre (km/L)
- 12.75
Economy units (mpg, km/L) and consumption units (L/100km) are reciprocally related, so doubling fuel economy halves consumption. Conversion uses the exact factors 1 mi = 1.609344 km, 1 US gallon = 3.785411784 L, and 1 Imperial gallon = 4.54609 L (NIST SP 811 / UK Weights and Measures Act 1985), pivoting through km/L.
How to use this calculator
Type the figure you already have, choose what unit it is in ("From"), then pick the unit you want ("To"). The headline shows your chosen target; the breakdown beneath it shows the same value in every other unit, so you only need one trip through the converter to read off mpg-US, mpg-UK, L/100km and km/L together.
How the calculation works
Fuel economy (mpg, km/L — higher is better) and fuel consumption (L/100km — lower is better) are reciprocal sides of the same physical quantity. A straight multiplicative factor table would not work because going from mpg to L/100km inverts the relationship. The converter pivots every value through a single canonical "kilometres per litre" measure, using exact constants: 1 mile = 1.609344 km (1959 IYPA), 1 US gallon = 3.785411784 L (NIST SP 811), 1 Imperial gallon = 4.54609 L (UK Weights and Measures Act 1985). All four output units derive from km/L with no rounding until display.
Worked example
30 mpg (US) — a typical compact car. In km/L: 30 × 1.609344 ÷ 3.785411784 = 12.754 km/L. In L/100km: 100 ÷ 12.754 = 7.84 L/100km. In mpg (Imperial): 12.754 × 4.54609 ÷ 1.609344 = 36.03 mpg. The US EPA "30 mpg = 7.84 L/100km" label on fueleconomy.gov matches to two decimals.
Frequently asked questions
Why is mpg (US) lower than mpg (UK) for the same car?
A US gallon is smaller than an Imperial gallon — 3.785 L versus 4.546 L. So the same car going the same distance on one Imperial gallon would have gone further than on one US gallon, which makes the Imperial mpg figure look bigger. Multiply US mpg by about 1.20095 to get the equivalent Imperial mpg.
Why does the rest of the world use L/100km instead of mpg?
L/100km is a direct measure of how much fuel you actually need — and most regulatory and fleet reporting wants "fuel used" as the headline, not "distance covered". It also scales sensibly: a 5 L/100km car uses half the fuel of a 10 L/100km car over the same journey, whereas the equivalent mpg jump (from 47 to 23.5) is less intuitive at a glance.
What is a good fuel economy figure?
For a modern petrol car: under 7 L/100km (35 mpg-US, 40 mpg-UK) is decent, under 5 L/100km (47 mpg-US, 56 mpg-UK) is good, under 4 L/100km (59 mpg-US, 71 mpg-UK) is hybrid territory. Pure EVs use kWh/100km or miles-per-kWh instead — different physical units, not convertible by these factors.
Why does converting 0 L/100km give an error?
Zero litres per 100 km would mean the car uses no fuel at all — infinite mpg, mathematically undefined. The converter rejects zero or negative inputs because they do not correspond to a real fuel-economy reading.
Are the factors used here exact or approximate?
Exact. 1 mile = 1.609344 km is defined exactly by the 1959 International Yard and Pound Agreement; 1 US gallon = 3.785411784 L is the NIST SP 811 definition (231 cubic inches, with the inch defined as 25.4 mm exactly); 1 Imperial gallon = 4.54609 L is fixed by the UK Weights and Measures Act 1985. The internal pivot through km/L preserves full floating-point precision; rounding happens only at display.
How do I convert miles per kWh (electric cars) to mpg?
You can't convert directly — petrol and electricity have different energy content per unit volume. The closest standardised comparison is "MPGe" (miles-per-gallon equivalent), published by the US EPA, which uses 33.7 kWh = 1 gallon of petrol as the energy-equivalent factor. This converter handles only liquid-fuel economy.