Fuel Economy Converter

MPG, L/100km and km/L

Fuel economy is measured as either distance per fuel (MPG, km/L) or fuel per distance (L/100km). These have an inverse relationship, so you cannot convert by simple multiplication. This tool handles the reciprocal math for you.

It supports US MPG, UK (imperial) MPG, liters per 100 km, kilometers per liter and miles per liter — covering car specs from any country.

Common fuel economy conversions

• US MPG to L/100km and back
• US MPG to UK MPG (they differ by gallon size)
• km/L to L/100km for metric specs

Key features at a glance

• Converts US MPG, UK (imperial) MPG, L/100km, km/L, and miles per liter
• Applies correct reciprocal math for inverse units
• Handles the US versus UK gallon difference
• Live conversion table updates as you type
• Covers car specs from any country
• Runs entirely in your browser, even offline

Why fuel economy units are reciprocal

Fuel economy is expressed in two opposite ways, and mixing them up gives nonsensical results. Distance-per-fuel units like miles per gallon and kilometers per liter get bigger as a car becomes more efficient. Fuel-per-distance units like liters per 100 kilometers get smaller as efficiency improves. The two are reciprocals of each other, so you cannot convert between them by simple multiplication — you have to invert.

This is exactly the kind of math that is easy to get wrong by hand. Converting 40 US MPG to liters per 100 km involves dividing a constant by the MPG figure, not scaling it. The converter applies the correct reciprocal formulas automatically, so an efficiency quoted as 'higher is better' translates correctly into one quoted as 'lower is better', and vice versa, without you having to remember which way the relationship runs.

US MPG versus UK MPG

A second trap lurks within miles per gallon itself: the US and UK gallons are different sizes. The imperial gallon is about 20% larger than the US gallon, so the same car achieves a higher MPG figure when measured in UK gallons than in US gallons. A car rated at 30 US MPG is about 36 UK MPG, even though nothing about the vehicle has changed.

This means comparing a US car's economy with a UK car's headline figure is misleading unless you account for the gallon difference. It is a common error in reviews and forums that quote MPG across borders. The converter distinguishes US MPG from UK MPG explicitly, so cross-country comparisons are accurate and you are never fooled into thinking a car is more or less efficient than it really is.

Reading fuel economy and planning costs

Beyond conversion, these figures help you estimate running costs and compare vehicles. Liters per 100 km, the standard in most of the world, maps neatly onto fuel spending: if you know the price per liter and your typical distance, the math is straightforward. MPG requires an extra step but conveys the same information from the distance-per-fuel direction.

Because efficiency gains are non-linear, the reciprocal relationship has a practical lesson: improving a thirsty vehicle from 15 to 20 MPG saves more fuel over a fixed distance than improving an efficient one from 40 to 50 MPG, even though the second looks like a bigger jump. Converting to liters per 100 km makes these real savings clearer, which is one reason the fuel-per-distance form is widely preferred.

Tips for converting fuel economy

• Distance-per-fuel (MPG, km/L): higher is better; fuel-per-distance (L/100km): lower is better
• UK MPG figures are about 20% higher than US MPG for the same car
• Convert to L/100km to estimate fuel costs easily
• You cannot convert these units by simple multiplication — they invert
• Check whether a spec uses US or imperial gallons before comparing

Why choose our fuel economy converter?

Our converter is completely free with no signup, no limits, and no subscription. It runs entirely in your browser, so conversions are instant, it works offline, and the values you enter never leave your device.

It is one of more than 200 free tools on ToopTools, and you can pin it to your My Workspace, a personalized workspace where you keep the tools you use most in a single dashboard. Beside the other unit converters, it handles the tricky reciprocal math of fuel economy in one place.

Common use cases

Car buyers use the fuel economy converter to compare vehicles whose specs are quoted in different units and countries, translating MPG into liters per 100 km and reconciling US with UK figures. It makes cross-border comparisons fair and accurate.

Drivers use it to estimate fuel costs, travelers convert rental-car economy to familiar units, and anyone reading international car reviews uses it to interpret the numbers. Because it applies the correct reciprocal formulas and distinguishes gallon sizes, it avoids the two classic fuel-economy conversion mistakes.

Is the fuel economy converter free?

Yes. It is completely free with no account, no sign-up, and no usage limits. Convert as many figures as you like at no cost.

How do I convert MPG to L/100km?

Because the units are reciprocal, you divide a constant by the MPG value rather than multiplying. The converter does this for you; for US MPG, divide 235.215 by the MPG to get liters per 100 km.

Why is UK MPG higher than US MPG?

Because the imperial gallon is about 20% larger than the US gallon. The same car travels further on a UK gallon, so its MPG figure is higher even though its actual efficiency is identical.

Which is better, a high or low number?

It depends on the unit. For MPG and km/L, higher means more efficient. For liters per 100 km, lower means more efficient, since it measures fuel used over a fixed distance.

Why can't I just multiply to convert fuel economy?

Because distance-per-fuel and fuel-per-distance units are inverses of each other. Converting requires dividing into a constant, not scaling, which is why doing it by hand so often goes wrong.

Are my conversions kept private?

Yes. Everything is computed in your browser and nothing is uploaded, so the values you enter stay private and the tool works without an internet connection.

How do I convert L/100km to MPG?

Enter the figure in liters per 100 km and choose US or UK MPG as the target, and the converter shows the result instantly. Because the two measures are inversely related, it does the reciprocal maths for you, so you get an accurate comparison every time.

Does the fuel economy converter work offline?

Yes. The conversion runs entirely in your browser, so once the page has loaded the tool works without an internet connection, and nothing you enter is uploaded — it stays private on your own device.