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How do I convert fuel economy between mpg, km/L and L/100km?

Type a value into AnyTool Fuel Economy Converter and pick the units. It instantly converts both ways between miles per US gallon, miles per Imperial (UK) gallon, kilometres per litre, miles per litre, litres per 100 km and US gallons per 100 miles, and shows the value in every unit at once in a copyable table. Economy and consumption units are reciprocal — converting between them inverts the value — and the tool does that inversion exactly. Everything is calculated in your browser, so nothing is uploaded.

  • Live two-way conversion — type in either field and the other updates
  • mpg (US), mpg (UK), km/L, mi/L, L/100km and US gal/100mi
  • Reciprocal-aware: economy (higher = better) vs consumption (lower = better)
  • See the value in every unit at once, each row copyable
  • 100% in your browser — no upload, no signup, works offline

What is

Fuel Economy Conversion

Fuel-economy conversion changes how a vehicle’s efficiency is expressed — for example miles per gallon to litres per 100 km. Economy units (mpg, km/L) measure distance per fixed fuel and consumption units (L/100km) measure fuel per fixed distance, so the two families are reciprocal rather than a single factor: litres per 100 km equal 235.214583 divided by US mpg.

Converters

Related terms

Miles per gallonL/100kmKilometres per litreImperial gallon

Frequently Asked Questions

Divide 235.214583 by the US mpg figure. So 30 mpg (US) is about 7.84 L/100km.

Because miles per gallon and litres per 100 km are reciprocal, you divide rather than multiply: L/100km = 235.214583 ÷ mpg (US). For example 30 mpg (US) ≈ 7.84 L/100km and 40 mpg (US) ≈ 5.88 L/100km. For Imperial (UK) mpg the constant is 282.480936, because the UK gallon (4.546 L) is larger than the US gallon (3.785 L).

The US gallon (3.785 L) is smaller than the Imperial gallon (4.546 L), so the same car reads about 20% higher in US mpg.

A “gallon” is not one fixed size. The US gallon is 3.785411784 litres while the Imperial (UK) gallon is 4.54609 litres. Since UK mpg packs more fuel into each gallon, the same car travels fewer miles per UK gallon, so it reads lower: 1 mpg (US) ≈ 1.201 mpg (UK). AnyTool keeps the two clearly separate.

For mpg and km/L higher is better; for L/100km lower is better — they are reciprocal.

It depends on the unit family. Economy units such as mpg, km/L and mi/L measure distance per fixed fuel, so a higher number means a thriftier car. Consumption units such as L/100km and gallons per 100 miles measure fuel per fixed distance, so a lower number is better. Doubling mpg roughly halves L/100km because the two are reciprocal.

No. Every calculation runs in your browser, so nothing is sent to a server and it works offline.

AnyTool Fuel Economy Converter computes every result locally in JavaScript using exact gallon and mile constants through an L/100km base. No value is uploaded, there is no tracking, and the page keeps working offline after the first load.

Detailed Explanation

Methodology

How the Fuel Economy Converter Works

AnyTool Fuel Economy Converter performs every calculation in the browser. The crucial detail is that fuel economy is not a single linear factor: economy units (distance per fuel) and consumption units (fuel per distance) are reciprocal, so converting between the families inverts the value. Each unit declares an affine round-trip to a shared litres-per-100-km base. Litres per 100 km is the identity base; US mpg uses L/100km = 235.214583 ÷ mpg and Imperial mpg uses 282.480936 ÷ mpg; kilometres per litre uses 100 ÷ value; miles per litre uses 100 ÷ (value × 1.609344); and US gallons per 100 miles scale by 3.785411784 ÷ 1.609344. The two famous constants come straight from the definitions: 235.214583 = 100 × 3.785411784 ÷ 1.609344 and 282.480936 = 100 × 4.54609 ÷ 1.609344.

  • Six units: mpg (US), mpg (UK), km/L, mi/L, L/100km and US gal/100mi
  • L/100km is the shared base; economy units are reciprocals (K ÷ x)
  • Economy is higher-is-better; consumption is lower-is-better
  • Constants 235.214583 (US) and 282.480936 (UK) derive from exact gallon/mile sizes
  • All arithmetic is client-side JavaScript — no server round-trip
How It Works

US Gallons, Imperial Gallons and the Reciprocal Trap

Two pitfalls catch most converters. First, a gallon is not one size: the US gallon is 3.785411784 litres while the Imperial (UK) gallon is 4.54609 litres, so the same car reads about 20% higher in US mpg (1 mpg US ≈ 1.201 mpg UK). Second, you cannot multiply mpg by a constant to get L/100km — they are reciprocal, so you divide a constant by the value. Doubling mpg roughly halves L/100km. AnyTool keeps US and UK mpg distinct and inverts correctly through the L/100km base, so 30 mpg (US) maps to 7.84 L/100km and 5 L/100km maps back to 47.04 mpg (US).

  • US gallon = 3.785411784 L; Imperial gallon = 4.54609 L (both exact)
  • 1 mpg (US) ≈ 1.201 mpg (UK) for the same vehicle
  • 30 mpg (US) ≈ 7.84 L/100km; 5 L/100km ≈ 47.04 mpg (US)
  • Economy ↔ consumption inverts the number, never a straight multiply
  • Editing either field converts both directions live
Privacy & Security

Privacy and Offline Use

Because all conversion runs in the browser, no value is sent to a server, there is no account or tracking, and the page works offline after first load. The converter is built on a shared, unit-tested conversion engine that other AnyTool unit converters reuse, so behaviour is consistent across the site.

Fuel economy conversion: in-browser (AnyTool) vs typical online converters
CapabilityAnyToolTypical online converters
ProcessingRuns in your browserOften server-side
Unitsmpg US, mpg UK, km/L, mi/L, L/100km, gal/100miOften mpg and L/100km only
US vs UK gallonKept distinct, exact constantsFrequently conflated
Reciprocal handlingInverts economy ↔ consumption correctlySometimes mis-multiplied
All units at onceYes, copyable tableOften one target at a time
Works offlineYes (PWA)No
Cost / signupFree, no signupOften ad-heavy or gated

AnyTool inverts economy and consumption correctly through an L/100km base and uploads nothing.