L/100km to MPG Converter

L/100km to MPG (US) — quick reference

The formula, worked line by line

The two systems measure inverse quantities — fuel per distance against distance per fuel — so they are joined by a constant rather than by a factor, and MPG comes from dividing that constant by the liters-per-100-km figure. The constant is derived, not chosen.

One mile per gallon means covering 1.609344 km on 3.785411784 liters, so 100 km costs 100 × 3.785411784 ÷ 1.609344 liters, which is 235.2145833… Every digit of it comes from exact definitions: the international mile, the international inch, and the US gallon at 231 cubic inches.

Because the product of the two readings is always that constant, the conversion is symmetrical — the same number divides in both directions, which is why the Swap button needs no second formula. It also means the curve is a hyperbola, so the spacing of one scale is not the spacing of the other. There is a single point where the two coincide, at the square root of the constant: 15.34 L/100km is 15.34 MPG.

MPG (US) = 235.214583 ÷ L/100km
L/100km × MPG (US) = 235.214583
235.214583 ÷ 8 = 29.4 MPG
imperial MPG = 282.480936 ÷ L/100km

The stretching effect is worth quantifying, because it explains a lot of marketing. A one-liter-per-100-km improvement is the same fuel saving wherever it happens: 10 liters over a 1,000 km drive, every time. But in MPG it looks wildly different.

From 3 to 2 L/100km, MPG rises from 78.4 to 117.6, a jump of 39.2. From 16 to 15, MPG rises from 14.7 to 15.7, a jump of 1.0. The identical physical saving is worth thirty-nine times more MPG at the efficient end, which is why very frugal cars advertise headline MPG numbers that look transformative next to a modest metric gap.

Two limits worth stating plainly. First, decide which gallon before you compare anything. Imperial MPG is 20.09 percent higher than US MPG for the same vehicle because the imperial gallon is 4.54609 L against 3.785411784, so convert with 282.480936 if you want the British figure and 235.214583 if you want the American one; mixing them silently is the classic error in cross-market car comparisons.

Second, the conversion says nothing about whether the rating is realistic. US, EU and UK test cycles differ, so a converted figure inherits whatever the original test measured — compare cars rated under the same regime, and treat any single number as a laboratory result rather than a promise about your own driving.

How to use the L/100km to MPG converter

Type a liters-per-100-km figure and the US MPG equivalent appears as you type, to one decimal place; Swap reverses the direction. This is the inbound conversion: the number came from a rental agreement, a European or Asian brochure, a foreign road test or a manufacturer specification, and you want it in the units you reason in.

The arithmetic is one division into a fixed constant, 235.214583, and there is no mental shortcut available because the relationship is a reciprocal rather than a scaling. Going from 5 to 10 L/100km does not subtract a fixed number of MPG, it roughly halves the MPG. That is worth knowing before you try to eyeball a brochure figure, because the intuition that works at one end of the scale actively misleads at the other.

Start with the direction flip. Liters per 100 km improves as it falls, MPG improves as it rises. A frugal 5 L/100km rental is a strong 47.0 MPG; a thirsty 12 L/100km vehicle is only 19.6 MPG. When the rental desk says about six liters per hundred, that is 39.2 MPG — considerably better than it sounds to an American ear, because the small number is doing the bragging.

Two practical cautions. First, MPG is two units. The imperial gallon is 4.54609 liters against the US gallon at exactly 3.785411784, so imperial MPG figures run 20.09 percent higher than US ones for the identical car: 8 L/100km is 29.4 US MPG but 35.3 imperial MPG. British motoring press figures are usually imperial, so a UK review and a US review of the same car will disagree by a fifth for no mechanical reason.

Second, translate the figure into fuel before you judge it. Multiply liters per 100 km by 10 for a 1,000 km drive: an 8 L/100km car takes 80 liters and a 12 L/100km car takes 120, and since fuel abroad is priced per liter that 40-liter difference is the number that hits your card, not the MPG spread.

Why does MPG stretch at the efficient end?

The subtler effect is that MPG stretches the efficient end of the scale out of proportion. Because the conversion is a division, small absolute improvements in liters per 100 km produce enormous MPG jumps once the figure is already low.

Cutting 5 L/100km to 4 lifts MPG from 47.0 to 58.8, a gain of 11.8 MPG for one liter. Cutting 12 to 11 lifts MPG from 19.6 to 21.4, a gain of just 1.8 for the same one-liter improvement — even though the second change saves exactly the same fuel per 100 km as the first.

This is the same reciprocal distortion as the MPG illusion, seen from the other side, and it is why headline MPG figures for hybrids and small diesels look so dramatic against a modest metric difference.

Every L/100km value, worked out

55 common L/100km 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