Conversion Runs locally Ready to use Built-in examples No tracking

Fuel Economy Converter

Convert between L/100km, km/L, mpg (US) and mpg (UK). Note fuel economy is inverse: lower L/100km is better, higher mpg is better.

Fuel economy is quoted as L/100km in much of the world, as km/L in Japan and as mpg in the US and UK — and the direction is inverted: a lower L/100km is better, while a higher km/L or mpg is better. This tool converts between all four; the anchor is 8 L/100km = 12.5 km/L ≈ 29.4 mpg (US) ≈ 35.3 mpg (UK).

Two traps. The direction is one: converting from mpg back to L/100km is a reciprocal relationship rather than a multiplication, so doing it by instinct gets the answer backwards. The gallon is the other: the UK gallon is about 1.2 times the US one, so the same mpg label on a British and an American car differs by 20% in real consumption — which is exactly why UK road tests always look more economical.

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How to use

  1. Enter the consumption figure and choose its unit.
  2. Read the other three units.
  3. Check the direction: lower L/100km is better, higher mpg is better.
  4. Confirm which gallon is in play when reading mpg.

How it works

Basic usage

Enter a value, pick the source and target units, and it converts live; the swap button reverses direction in one click.

L/100km vs. mpg

China and Europe use "liters per 100 km" — lower is better; the US uses mpg — higher is better. The direction is reversed.

Conversion examples

Example: 8 L/100km = 100/8 = 12.5 km/L, about 29.4 mpg (US); US and UK gallons differ, so 1 mpg (UK) is about 1.2 mpg (US).

Code example

JavaScript Reciprocal units, with L/100km as the hub 

const KM_PER_MILE = 1.609344;
const L_PER_GAL_US = 3.785411784;
const L_PER_GAL_UK = 4.54609;

// mpg to L/100km: miles per gallon turned into litres per 100 km
const mpgToL100 = (mpg, gal) => 100 * gal / (mpg * KM_PER_MILE);
const l100ToMpg = (l100, gal) => 100 * gal / (l100 * KM_PER_MILE);

mpgToL100(29.4, L_PER_GAL_US);   // ≈ 8 L/100km
l100ToMpg(8, L_PER_GAL_UK);      // ≈ 35.3 mpg(UK)

Python The same formulas in Python 

KM_MILE = 1.609344
GAL_US = 3.785411784
GAL_UK = 4.54609

mpg2l100 = lambda m, g: 100 * g / (m * KM_MILE)
l100_2mpg = lambda l, g: 100 * g / (l * KM_MILE)

mpg2l100(35.3, GAL_UK)   # ≈ 8.0
l100_2mpg(8, GAL_US)     # ≈ 29.4

FAQ

Why is fuel economy a "reciprocal" unit?

L/100km means "how many liters to drive 100 km"; mpg means "how many miles on one gallon" — fuel is in the numerator for one and the denominator for the other, a reciprocal relationship. You can't multiply by a single factor; you must invert.

How many liters per 100 km is 30 mpg (US)?

30 mpg (US) is about 7.84 L/100km. For a quick estimate, divide 235.2 by the mpg value (US).

How much do mpg US and mpg UK differ?

The imperial gallon (4.546 L) is larger than the US gallon (3.785 L), so at the same number, mpg (UK) is about 1.2 times mpg (US). When reading British car media, don't confuse the two.

How do I convert L/100km to cost per kilometer?

Cost per km = (L/100km / 100) x fuel price. For example 8 L/100km at 7.5 per liter gives 0.08 x 7.5 = 0.6 per km. Highway and city congestion differ markedly — estimate from an average over several refuels.

How do mpg and L/100km convert?

They're reciprocal and opposite: L/100km = 235.215 / mpg (US), mpg (US) = 235.215 / L/100km; for imperial use the factor 282.481. So higher mpg means better economy, while lower L/100km means better economy.

How much higher can real fuel use be than the trip computer?

The trip computer usually reads lower than measured, with 5-15% deviation common; idling, air conditioning, low tire pressure and short trips all raise real consumption. For accuracy, use "distance between two fill-ups / fuel added."

How does EV energy use compare with fuel?

EVs use "energy per 100 km" (kWh/100km), typically 12-20 kWh/100km. At home-charging rates that's about 6-15 per 100 km, clearly below a comparable gasoline car. Use your local electricity price — fast charging and home charging differ a lot.