MPG to L/100km Converter

MPG (US) to L/100km — quick reference

The formula, worked line by line

Fuel economy flips between the two systems — distance per fuel against fuel per distance — so the conversion is a reciprocal through a fixed constant rather than a multiplication by a factor. The constant is not arbitrary.

Start from one mile per gallon: that is 1.609344 km per 3.785411784 L, so covering 100 km takes 100 × 3.785411784 ÷ 1.609344 liters, which is 235.2145833… That single number bundles the gallon-to-liter and mile-to-kilometer factors and the scaling to a 100 km basis.

Because the relationship is x times y equals a constant, the two scales are hyperbolas of each other, and that has consequences no linear intuition prepares you for.

The product of the two figures is always the constant: 28 × 8.4 is about 235, and so is 50 × 4.7 and 15 × 15.68. There is exactly one point where the two readings coincide, at the square root of the constant, 15.34 — a car doing 15.34 MPG is also doing 15.34 L/100km.

L/100km = 235.214583 ÷ MPG (US)
MPG (US) × L/100km = 235.214583
235.214583 ÷ 28 = 8.4 L/100km
For imperial MPG, use 282.480936 instead

The reciprocal is also the reason for the so-called MPG illusion, and it is worth doing the arithmetic once. Over 10,000 miles a 15 MPG vehicle uses 666.7 gallons and a 20 MPG vehicle uses 500, so that 5 MPG improvement saves 166.7 gallons.

A 40 MPG car uses 250 gallons and a 50 MPG car uses 200, so a 10 MPG improvement saves only 50. Twice the MPG gain, less than a third of the fuel saved. The reference table makes the same point column by column: one extra MPG is worth 2.14 L/100km at the bottom of the scale and 0.09 at 50 MPG, a factor of more than twenty.

Two limits worth stating plainly. First, MPG is two different units. The imperial gallon is 4.54609 L against the US gallon at 3.785411784, so imperial MPG figures are 20.09 percent higher for the same vehicle and need dividing by 1.20095 before they belong on this page; the equivalent constant for imperial MPG is 282.480936.

Second, none of this addresses whether a rated figure is achievable. Test cycles differ between the United States, the EU and the UK, and a converted number carries whatever the original test measured — the conversion is exact, the underlying rating is a laboratory result, and comparing two cars fairly means comparing figures from the same test regime.

How to use the MPG to L/100km converter

Type a miles-per-gallon figure and the liters-per-100-km equivalent appears as you type, to one decimal place; Swap reverses the direction. Most of the world rates fuel economy as fuel per distance and the United States rates it as distance per fuel, so the two run in opposite directions and a European road test reads backwards to an American eye.

The conversion is a division into a fixed constant rather than a multiplication by a factor, which has a consequence worth absorbing before you use the number: there is no mental shortcut. Halving the MPG doubles the liters per 100 km, it does not shift it by a fixed amount, so the arithmetic that works at 50 MPG is useless at 15. That is the whole case for a calculator here rather than a rule of thumb.

The direction flip is the first thing to internalise. With MPG, bigger is better, because it counts distance covered per unit of fuel. With liters per 100 km, smaller is better, because it counts fuel burned per fixed distance.

The third thing is that MPG is two units, not one. The imperial gallon is 4.54609 liters against the US gallon at exactly 3.785411784, so an imperial MPG figure is 20.09 percent higher than the US MPG figure for exactly the same car: 28 US MPG is 33.6 imperial MPG.

Older British road tests and some Canadian sources quote imperial MPG without saying so, and a figure entered here unadjusted will come out roughly a fifth too optimistic. Divide a UK MPG figure by 1.20095 first, or convert through the imperial constant 282.48 instead. The reference table below carries both MPG columns side by side, along with how much one more MPG is actually worth at each point on the scale.

What is the MPG illusion?

The second thing is the MPG illusion, and it is the reason much of the world abandoned distance-per-fuel in the first place. Because MPG is a reciprocal, equal steps up the MPG scale are not equal steps down in fuel used.

Over 10,000 miles, going from 15 to 20 MPG cuts consumption from 666.7 gallons to 500, a saving of 166.7 gallons. Going from 40 to 50 MPG cuts it from 250 to 200, a saving of just 50 gallons — one third as much, for double the MPG improvement.

In metric terms the same story reads directly off the numbers: 15 to 20 MPG is a drop of 3.92 L/100km, while 40 to 50 MPG is a drop of only 1.18. Replacing the worst vehicle in a household is where the fuel actually goes.

Every MPG (US) value, worked out

71 common MPG (US) figures each get their own page, with the answer at full precision, the arithmetic, what rounding costs, and the nearest real-world reference point on the scale.

Questions people ask