MPG to L/100km Conversion: Exact Formula & Table

In the United States, cars are rated in miles per gallon (mpg). In Europe, Canada, Australia, and most of Asia, the same car is rated in liters per 100 kilometers (L/100km). The mpg to L/100km conversion trips up even careful readers — because the two scales run in opposite directions, and you cannot convert with a simple multiplier. Here is the one formula you need, plus how to avoid the US-versus-UK gallon trap. And if you would rather skip the math, our fuel economy converter does the whole thing instantly.

The confusion is understandable. Miles per gallon measures how far you go on a fixed amount of fuel: bigger numbers mean better economy. Liters per 100 kilometers measures how much fuel you burn over a fixed distance: smaller numbers mean better economy. So a car that is "efficient" scores high in the US and low in Europe — and the mpg to L/100km conversion is not intuitive at all. Getting it right matters when comparing cars across borders, reading international reviews, or working out what an imported car will really cost to run.

Why the US and Europe Measure Fuel Differently

The split comes down to history. The United States adopted miles per gallon early in the automobile era, when fuel was cheap and the question buyers asked was simple: "How far can I go on a tank?" The metric world later standardized on liters per 100 kilometers, which answers a different question: "How much fuel does this trip cost me?" Each framing made sense in its own market — but they produce the strange mirror-image effect that trips everyone up.

With mpg, efficiency goes up with the number: 40 mpg beats 25 mpg. With L/100km, efficiency goes down with the number: 5.9 L/100km beats 9.4 L/100km. This inverse relationship is the source of nearly every mistake. People try to convert by multiplying or dividing by a "factor" the way they would with kilometers and miles — but one scale is distance-per-fuel and the other is fuel-per-distance, so the conversion is reciprocal, not proportional. No single multiplier turns mpg into L/100km for every value. There is, however, one exact formula — and it always works.

The Exact MPG to L/100km Conversion Formula

Here is the exact formula for US miles per gallon:

L/100km = 235.214583 ÷ mpg (US)

That constant, 235.214583, is not a magic number. One mile is exactly 1.609344 kilometers, and one US gallon is exactly 3.78541 liters — combining those definitions yields the single constant. Because it comes from exact definitions, the formula is exact, not an approximation.

To go the other direction — from L/100km back to US mpg — just rearrange it:

mpg (US) = 235.214583 ÷ L/100km

Same constant, used the same way, in both directions — a handy way to check your work: convert a figure there and back, and you should land on the number you started with (allowing for rounding). If you convert figures often, bookmark the fuel economy converter and never do the division by hand again.

Why there is no simple "multiply by X" rule

With a normal unit conversion — say kilometers to miles — you multiply by one constant, because the relationship is linear: twice the kilometers is twice the miles. But mpg and L/100km are reciprocals. Doubling the mpg does not double the L/100km figure; it halves it: a car going from 20 to 40 mpg cuts its fuel use per 100 km from 11.76 to 5.88 L/100km. Any "rule of thumb" that ignores this will be wrong by a wide margin, especially at the extremes. Use the formula every time.

US vs UK Gallons: Don't Mix Them Up

This is where the mpg to L/100km conversion genuinely trips people up. There are two different gallons in common use:

US gallon = 3.78541 liters
UK (Imperial) gallon = 4.54609 liters

The Imperial gallon is about 20% larger, so the same car's fuel economy reads roughly 20% higher in UK mpg than in US mpg. A car rated 30 mpg in the US would be rated about 36 mpg in the UK — identical car, identical fuel use, different number on the sticker. Mixing the two up is one of the most common errors in online comparisons, because many British sources simply say "mpg" without specifying the gallon.

If you are converting from UK mpg, use this formula instead:

L/100km = 282.481 ÷ mpg (UK)

The constant 282.481 comes from the same derivation, using the Imperial gallon's 4.54609 liters instead of the US gallon's 3.78541 liters. If you ever convert a UK figure with the US constant, your L/100km result will come out about 20% too low — making the car look far more efficient than it is. When a source just says "mpg," check the country: American sources mean US gallons; British sources mean Imperial gallons.

A Worked Example: MPG to L/100km Conversion Step by Step

A US car review rates a sedan at 30 mpg (US gallons) — here is the full mpg to L/100km conversion, step by step:

Identify the formula. The figure is in US mpg, so: L/100km = 235.214583 ÷ mpg.
Plug in the number. L/100km = 235.214583 ÷ 30.
Divide. 235.214583 ÷ 30 = 7.8404861…
Round sensibly. Fuel economy figures are conventionally given to one or two decimal places, so: 7.84 L/100km.

So 30 mpg (US) = 7.84 L/100km. Now suppose you saw the same car quoted at 36 mpg in a British magazine. Using the UK formula: 282.481 ÷ 36 = 7.8467…, which rounds to 7.85 L/100km — effectively the same car, confirming the two gallons explain the different mpg numbers. Make this cross-check a habit: if both a US and a UK figure for the same model land within a few tenths of a liter, you used the right gallon each time.

Quick Reference Table

For the most common US mpg ratings, here are the exact equivalents in L/100km (235.214583 constant, rounded to two decimals) — handy when scanning American car listings:

US mpg	L/100km	What it feels like
20	11.76	Thirsty SUV or truck
25	9.41	Average older sedan
30	7.84	Decent modern compact
35	6.72	Efficient family car
40	5.88	Hybrid territory
50	4.70	Top-tier hybrid economy

Notice the reciprocal effect: the jump from 20 to 25 mpg saves 2.35 L/100km, but the jump from 40 to 50 mpg — twice the mpg gain — saves only 1.18 L/100km. Turn any consumption figure into real money with our fuel cost calculator — cost per trip or per year.

Frequently Asked Questions

Is a higher mpg always better?

For mpg, yes — higher means more miles per gallon, i.e. better efficiency. But be careful comparing across countries: a higher mpg in UK gallons is not the same achievement as in US gallons, because the Imperial gallon is about 20% larger. Always confirm which gallon a figure uses before comparing, or convert both to L/100km with the correct formula.

Why is L/100km "backwards" from mpg?

They measure opposite things. Miles per gallon is distance divided by fuel (output per input), so bigger is better. Liters per 100 km is fuel divided by distance (input per output), so smaller is better. Because one is the reciprocal framing of the other, the conversion is reciprocal too — that is why the formula divides a constant by the mpg figure instead of multiplying.

Which gallon do UK car specs use?

Officially, the UK has used liters per 100 km on new-car labels since 1992. But British drivers and media still commonly talk about fuel economy in miles per Imperial gallon (4.54609 liters). If a UK source says "mpg" without specifying, assume Imperial gallons — and use the 282.481 constant, not the 235.214583 one.

How do I convert L/100km back to mpg?

Use the same constant in reverse: mpg (US) = 235.214583 ÷ L/100km. For example, a car rated 6.0 L/100km converts to 235.214583 ÷ 6.0 = 39.2 mpg (US). For UK mpg, use 282.481 ÷ L/100km instead. And as always, a fuel economy converter does both directions instantly.

Conclusion

The mpg to L/100km conversion only feels tricky because the two scales run in opposite directions and two different gallons share the same "mpg" label. Remember the essentials: divide 235.214583 by the figure for US gallons, 282.481 for UK gallons, and never shortcut it with a plain multiplier. With the formula, the worked example, and the reference table above, you can convert any figure confidently. For instant, error-free results in both directions, use our fuel economy converter and skip the calculator entirely.