Why are US and UK MPG different?

Because the US gallon and the imperial (UK) gallon are different sizes. 1 US gal = 3.785 L; 1 imp gal = 4.546 L (about 20% larger). A car listed as 30 mpg in the UK gets fewer miles per US gallon (about 25 mpg US).

Why is L/100km the EU's preferred form?

Because for budgeting fuel cost on a known trip distance, the math is trivial: 800 km × 6 L/100km = 48 L of fuel. With MPG you'd need a division and unit shuffle. The EU also requires it on every new car's official sticker.

How accurate are these conversions?

Exact within IEEE 754 double-precision floats. The factors used (1 gallon = 3.785411784 L, 1 mile = 1.609344 km) are themselves exact by definition.

Does 'fuel economy' include hybrids and EVs?

For hybrids and plug-ins quoted in MPGe (miles per gallon equivalent), yes — MPGe is mathematically the same as MPG when both sides describe the energy a gallon contains. For pure electrics use kWh/100km or miles/kWh; we'll ship a dedicated EV-efficiency converter later.

What's WLTP vs EPA vs NEDC?

Three different test cycles for the same car will report meaningfully different numbers. WLTP (EU, since 2017) is the strictest. EPA (US) is the closest to real-world driving. NEDC (EU before 2017) is now considered optimistic by 20–30%. This calculator converts units, not standards.

Why does the calculator reject 0?

Zero MPG would mean 'this car uses infinite fuel for zero distance' — undefined. The converter rejects 0 and negative values so you get a clear empty state rather than a misleading number.

Fuel Economy Converter — MPG, L/100km, km/L

Four units, one number — with the inverse math done right.

Fuel-economy units don’t convert with a simple multiply. MPG (miles per gallon) and km/L are distance-per-fuel — “more is better”. L/100km and gal/100mi flip the relationship — “less is better”. Converting between the two styles requires an inverse, which is where most casual calculators get wrong by 5–20%. The widget below goes through a canonical kilometers-per-liter representation, so a 30 mpg (US) car is also 7.84 L/100km, 36 mpg (UK), and 12.75 km/L — all exact within floating-point precision.

Value

Unit

MPG (US)
30.00miles per US gallon
MPG (UK)
36.0285miles per imperial gallon
km/L
12.7543kilometers per liter
L/100km
7.8405liters per 100 kilometers

How to use

Enter a value
Whatever unit your spec sheet uses. US specs are in MPG, EU specs are usually in L/100km, Japan in km/L.
Read all four units
All other forms recalculate the moment your input parses. Quick presets (30 mpg, 7 L/100km, 50 mpg-UK, 15 km/L) cover the most common comparisons.
Sanity-check the direction
If you halve the input on a 'less is better' unit and the 'more is better' values double, the conversion is working. The inverse relationship is the source of most bugs in third-party calculators.

Quick reference

MPG (US)	L/100km	km/L	MPG (UK)
15	15.68	6.38	18.0
20	11.76	8.50	24.0
25	9.41	10.63	30.0
30	7.84	12.75	36.0
40	5.88	17.00	48.0
50	4.70	21.25	60.0
60	3.92	25.50	72.1

Frequently asked questions

Why are US and UK MPG different?: Because the US gallon and the imperial (UK) gallon are different sizes. 1 US gal = 3.785 L; 1 imp gal = 4.546 L (about 20% larger). A car listed as 30 mpg in the UK gets fewer miles per US gallon (about 25 mpg US).
Why is L/100km the EU's preferred form?: Because for budgeting fuel cost on a known trip distance, the math is trivial: 800 km × 6 L/100km = 48 L of fuel. With MPG you'd need a division and unit shuffle. The EU also requires it on every new car's official sticker.
How accurate are these conversions?: Exact within IEEE 754 double-precision floats. The factors used (1 gallon = 3.785411784 L, 1 mile = 1.609344 km) are themselves exact by definition.
Does 'fuel economy' include hybrids and EVs?: For hybrids and plug-ins quoted in MPGe (miles per gallon equivalent), yes — MPGe is mathematically the same as MPG when both sides describe the energy a gallon contains. For pure electrics use kWh/100km or miles/kWh; we'll ship a dedicated EV-efficiency converter later.
What's WLTP vs EPA vs NEDC?: Three different test cycles for the same car will report meaningfully different numbers. WLTP (EU, since 2017) is the strictest. EPA (US) is the closest to real-world driving. NEDC (EU before 2017) is now considered optimistic by 20–30%. This calculator converts units, not standards.
Why does the calculator reject 0?: Zero MPG would mean 'this car uses infinite fuel for zero distance' — undefined. The converter rejects 0 and negative values so you get a clear empty state rather than a misleading number.

About

Why the math is special

Convertitive's main unit framework treats every quantity as a value × factor. Fuel economy mixes ratio types: MPG and km/L are 'distance per fuel'; L/100km and gal/100mi are 'fuel per distance'. You can't simply multiply between the two — you have to take a reciprocal. This widget runs a canonical km/L representation under the hood, so all four units stay consistent regardless of which one you typed into.

Practical use

Comparing US and EU car specs, budgeting fuel costs in unfamiliar units when traveling, sanity-checking what a manufacturer means when they say '40 mpg' (usually US, sometimes UK).