How this calculator works
This converter switches between the four fuel economy units used around the world: miles per US gallon, miles per Imperial (UK) gallon, kilometers per liter, and liters per 100 kilometers. Pick a from-unit and a to-unit and type a value to see the converted figure.
Three of these units — mpg (US), mpg (Imperial) and km/L — all measure distance per unit of fuel, so bigger numbers mean better economy and they scale together in the ordinary way. L/100km measures the opposite thing, fuel per fixed distance, so bigger numbers there mean worse economy and the relationship to the others is reciprocal, not linear. This calculator always converts through a common km/L value and inverts explicitly for L/100km, so the direction is never mixed up.
How it converts
Step 1 — convert the input to km/L: km/L = mpg × 1.609344 ÷ (gallon size in liters)Step 2 — convert km/L to the target unit; for L/100km specifically: L/100km = 100 ÷ km/LExact anchors: 1 US gallon = 3.785411784 L, 1 Imperial gallon = 4.54609 L, 1 mile = 1.609344 km. Because L/100km is a reciprocal, doubling your mpg does not halve your L/100km by the same additive amount — always convert through km/L rather than scaling L/100km directly.
Worked example: 23.5 mpg (US) to L/100km
- 23.5 mpg (US) × 1.609344 km/mi = 37.819584 km per US gallon.
- 37.819584 km ÷ 3.785411784 L per gallon = 9.990877 km/L.
- 100 ÷ 9.990877 km/L = 10.009 L/100km.
- So a car rated at 23.5 mpg (US) uses just over 10 liters to travel 100 km.
Frequently asked questions
Why is L/100km 'backwards' compared to mpg?
mpg and km/L measure how far you get per unit of fuel, so a higher number is better. L/100km measures how much fuel you burn over a fixed distance, so a lower number is better. The two are reciprocals of each other, not the same scale flipped.
Is a US gallon the same as a UK (Imperial) gallon?
No. A US gallon is 3.785411784 liters; an Imperial gallon is 4.54609 liters, about 20% larger. The same car reports a higher mpg figure in Imperial gallons than in US gallons even though nothing about the car changed — always check which gallon a published mpg figure uses.
Can I just multiply mpg by a constant to get L/100km?
Not with a single constant across all values, because the relationship is reciprocal. There is a common rule of thumb — 235.21 ÷ mpg (US) ≈ L/100km — which works because 235.21 is 100 × 3.785411784 ÷ 1.609344, but this calculator computes the exact figure directly rather than relying on the rounded constant.
Does a higher km/L always mean a lower L/100km?
Yes, the relationship is monotonic: any increase in km/L (or mpg) always produces a decrease in L/100km, and vice versa. Only the exact rate of change differs from a simple linear scale.
How do I convert L/100km back to mpg?
First convert to km/L by dividing 100 by the L/100km figure, then convert that km/L value to mpg using the same gallon and mile constants this page uses in the other direction. For example, a car using 10 L/100km travels 100 ÷ 10 = 10 km per liter, which converts to roughly 23.5 mpg (US) — reversing this page's own worked example, which converts 23.5 mpg (US) to just over 10 L/100km.