How to Convert MPG to Litres per 100km: The Formula and the Table

Drivers searching for how to convert mpg to litres per 100km usually want one of two things: a number they can trust for their own car, or a way to compare two options fairly. Both need the same foundation — a clear formula, consistent units, and honest assumptions. This page covers all three before it gets to any example, because a worked example built on muddled units helps nobody.

Read the formula section first if you are doing the sums by hand, skip to the table if you just want to look a figure up, and read the mistakes section if your own calculation keeps coming out different from what the dashboard claims. Read the figures on this page about how to convert mpg to litres per 100km as a worked illustration of the constants, not as a figure for any particular car. The three rarely agree perfectly, and the reasons why are worth knowing.

How the MPG to L/100km conversion works — how to convert mpg to litres per 100km

MPG and litres per 100 km measure the same thing in opposite directions. Read the figures on this page about how to convert mpg to litres per 100km as a worked illustration of the constants, not as a figure for any particular car. MPG says how far you go on a fixed amount of fuel; L/100km says how much fuel you use over a fixed distance. To convert, you need the size of the gallon, and that is where most mistakes happen: a US gallon is 3.785 litres, while a UK imperial gallon is 4.546 litres, so the same car has a higher MPG figure in the UK than in the US. The conversion constants are 235.21 for a US gallon and 282.48 for an imperial gallon, so L/100km = 235.21 ÷ MPG (US) or 282.48 ÷ MPG (UK imperial). Going the other way, MPG (US) = 235.21 ÷ L/100km and MPG (UK) = 282.48 ÷ L/100km. Always label which gallon you mean, because a figure without that label cannot be checked. A car doing 40 MPG in the US does about 5.88 L/100km, while the same 40 MPG measured in imperial gallons is about 7.06 L/100km — the same driving, two different-looking numbers, purely because the gallon changed.

Worked example: converting a real figure — how to convert mpg to litres per 100km

Take a car that does 35 MPG. For how to convert mpg to litres per 100km, the same reasoning applies — use the right gallon constant and the conversion stays consistent. Using a US gallon, its consumption is 235.21 ÷ 35 = 6.72 L/100km. Using a UK imperial gallon, the same 35 MPG converts to 282.48 ÷ 35 = 8.07 L/100km. Both are correct; they describe the same fuel use with different gallons. Now go the other way: a car using 6.5 L/100km does 235.21 ÷ 6.5 = 36.2 MPG in US gallons, or 282.48 ÷ 6.5 = 43.5 MPG in imperial gallons. The common mistake is to convert with the wrong constant and then conclude the car is far more or less efficient than it really is. If you are comparing a UK figure with a US figure, convert both to L/100km first — it is the only unit that means the same thing on both sides of the Atlantic — and only then compare. The table below does that conversion for the most common MPG values, with both gallon types shown so the difference is visible at a glance.

MPG to L/100km conversion table — how to convert mpg to litres per 100km

Use the US column if your gallons are US gallons and the UK column if they are imperial gallons. Read the figures on this page about how to convert mpg to litres per 100km as a worked illustration of the constants, not as a figure for any particular car. The two columns describe the same fuel use; the UK figure is always higher because the imperial gallon is larger.

MPG L/100km (US gallon) L/100km (UK imperial gallon)
20 11.76 14.12
25 9.41 11.30
30 7.84 9.42
35 6.72 8.07
40 5.88 7.06
45 5.23 6.28
50 4.70 5.65

Why the UK figure is always higher — how to convert mpg to litres per 100km

An imperial gallon is bigger than a US gallon — 4.546 litres against 3.785 — so a car travelling the same distance on the same amount of fuel records a higher MPG in the UK than in the US. For how to convert mpg to litres per 100km, the same reasoning applies — use the right gallon constant and the conversion stays consistent. The ratio is fixed: UK MPG is roughly 1.2 times US MPG for the same real economy. That is why American and British figures for the same model never match, and why neither can be called wrong. The practical rule is simple: never compare an MPG figure with another MPG figure until you know both gallons. Convert both to L/100km, compare those, and the gallon problem disappears. The table on this page shows both conversions precisely so you can see the gap instead of guessing at it.

Common mistakes — how to convert mpg to litres per 100km

The most common mistake is mixing gallons: US and imperial gallons differ by about 20 percent, so a figure converted with the wrong gallon is wrong by the same margin. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. Next is trusting a single tank — one short fill or one unusual journey can tilt it badly — instead of averaging several. Third is reading the trip meter without resetting it at the fill, which mixes miles from before the fill into the calculation. Fourth is using the tank's capacity instead of the gallons actually pumped; the capacity is not the amount you used. And fifth is comparing a dashboard figure with a brim-to-brim figure as if they were the same measurement: the dashboard estimates from the engine's data, the brim-to-brim method measures what actually went in, and they disagree by a few percent in either direction.

Related calculations — how to convert mpg to litres per 100km

If this page answered your question, these pages take the next step — the same method applied to a closely related question, or the calculator that does the arithmetic for you. For how to convert mpg to litres per 100km, the same reasoning applies — use the right gallon constant and the conversion stays consistent.

Putting your how to convert mpg to litres per 100km figure to work

Putting the figure to work is where the method pays off. Read the figures on this page about how to convert mpg to litres per 100km as a worked illustration of the constants, not as a figure for any particular car. Once you know your own MPG, measured brim-to-brim over several tanks, you can price any journey before you set off: multiply the miles by the fuel price divided by your MPG, and the answer is the fuel cost of that trip at that example price. You can also compare cars honestly, because you are comparing measured figures rather than brochure claims. Keep a simple log — date, miles, gallons, price — and review it monthly. Patterns show up quickly: a winter dip, a motorway improvement, a slow decline that points to tyre pressures or a change in routes. The log turns a vague feeling about fuel use into a number you can act on, and that is the whole point of measuring in the first place.

Using your how to convert mpg to litres per 100km figure day to day

A useful habit is to record the figure alongside the conditions. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. Note the kind of driving each tank covered — mostly town, mostly motorway, or a mix — and the season, because both move the result enough to matter. A tank that looks disappointing after a week of short cold trips is not a problem with the car; it is a problem with the comparison. Compare motorway tanks with motorway tanks and winter with winter, and the averaged figure becomes a reliable baseline. Once you have that baseline, a genuine change stands out: a sudden drop after nothing else changed is worth a look at tyre pressures, a roof box left fitted, or a brake that is dragging. The measurement is simple; the discipline of recording it is what makes it useful.

Frequently asked questions

How do I convert MPG to L/100km?

Divide 235.21 by the MPG for a US gallon, or 282.48 by the MPG for a UK imperial gallon. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. So 40 MPG is 5.88 L/100km (US) or 7.06 L/100km (UK). Label which gallon you used, because the two answers differ.

Why is UK MPG higher than US MPG for the same car?

Because the UK imperial gallon is larger than the US gallon — 4.546 litres against 3.785 litres. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. The same distance on the same fuel gives a UK MPG figure about 20 percent higher. Neither figure is wrong; they use different gallons.

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

Divide the constant by the L/100km figure: MPG (US) = 235.21 ÷ L/100km, and MPG (UK) = 282.48 ÷ L/100km. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. A car using 6.0 L/100km does 39.2 MPG (US) or 47.1 MPG (UK).

Which is better, a higher MPG or a lower L/100km?

Both mean the same thing: higher MPG is better, and lower L/100km is better. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. The two scales run in opposite directions, which is why the conversion is a division rather than a multiplication, and why it is easy to read a converted figure backwards if you are not careful.

For how to convert mpg to litres per 100km, can I compare a UK MPG figure with a US MPG figure directly?

No — not until both are in the same units. That is the point to carry into how to convert mpg to litres per 100km: label the gallon every time, and the two systems stop disagreeing with each other. Convert both to L/100km first, using the right constant for each gallon, and then compare. Comparing the raw MPG figures directly will make the UK car look about 20 percent more efficient than it really is relative to the US car.