MPG to Liters per 100 km

Published July 19, 2026By Samson PG

Quick answer

L/100km = 235.215 ÷ mpg (US). Lower L/100km means better efficiency — the inverse of how MPG reads.

MPG (miles per gallon) rises when a car is more efficient. L/100km (liters per 100 kilometers) falls when efficiency improves. Converting between them is a fixed reciprocal relationship once you know which gallon you mean.

US MPG ↔ L/100km

Using the common constant for US liquid gallons:

L/100km = 235.215 / mpg
mpg     = 235.215 / (L/100km)

Example: 30 mpg (US) → 235.215 ÷ 30 ≈ **7.84 L/100km**.

Imperial (UK) gallon note

UK gallons are larger. The constant is different (commonly cited near 282.481 for imperial mpg ↔ L/100km). Mixing US and UK mpg without labeling causes large errors.

Quick reference (US mpg)

MPG (US)	≈ L/100km
20	11.8
25	9.4
30	7.8
35	6.7
40	5.9

Which number to use

Region habit	Prefer
U.S. stickers / many U.S. drivers	MPG
Europe / much of the world	L/100km
Comparing two cars	Convert both to one unit first

Why the two scales run in opposite directions

This is the part that trips people up: higher mpg is better, lower L/100km is better. They are reciprocals, not a linear rescaling, so the same “improvement” is worth wildly different amounts depending on where you start.

From	To	mpg gained	Fuel actually saved per 100 km
15 → 20 mpg	15.7 → 11.8 L/100km	+5	3.9 L
30 → 35 mpg	7.8 → 6.7 L/100km	+5	1.1 L
40 → 45 mpg	5.9 → 5.2 L/100km	+5	0.7 L

Every row gains the same 5 mpg, yet the first saves more than five times the fuel of the last. Upgrading a thirsty vehicle slightly beats making an efficient one slightly better — which is invisible if you only look at mpg.

This is sometimes called the “MPG illusion”, and it is why L/100km is the more honest unit for budgeting: it scales directly with what you actually spend.

Doing it in your head

The exact factor for US mpg is 235.215, but 235 is close enough for any practical purpose:

L/100km = 235 ÷ mpg
mpg     = 235 ÷ L/100km

Because it is the same constant both ways, one number covers both directions. A handy anchor: 235 ÷ 30 ≈ 7.8, so 30 mpg is a bit under 8 L/100km. For the UK gallon, swap 235 for 282.

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