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

Convert fuel economy between L/100km, MPG (US), MPG (UK/imperial) and km/L — all four at once, from a single value.

Sources: NIST — Guide to the SI, Appendix B.8 (conversion factors: mile, gallon)

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Fuel Economy Calculator: convert MPG, L/100km and km/L

This fuel economy calculator converts between the four units used around the world to describe how much fuel a car burns: miles per US gallon, miles per UK (imperial) gallon, litres per 100 km, and kilometres per litre. Enter a value in whichever unit you already have — a window sticker, a spec sheet, a forum post, a rental car’s dashboard — and get the other three instantly.

It’s built for the moment you’re comparing two numbers that don’t line up: a US car review quoting 32 mpg next to a European spec sheet quoting 7.3 L/100km, or a UK number that looks suspiciously better than the US figure for what’s supposedly the same car. That gap is usually not an error — it’s explained below.

One thing to keep straight before you start: the four units don’t all point the same way. A lower L/100km means better economy (less fuel per distance). A higher MPG or km/L means better economy (more distance per fuel). Mixing those two directions up is the single most common mistake when comparing figures across regions.

How to use the fuel economy calculator

  1. Pick the unit your number is already in. Use the selector to choose L/100km, MPG (US), MPG (UK/imperial), or km/L.
  2. Enter the value. Type in the number exactly as printed — no conversion needed on your end.
  3. Read off the other three. All four units update instantly, so you can quote whichever one your audience uses.

There’s no separate “convert” button and nothing to configure beyond the unit and the number — this is a straight, instant conversion in every direction, with no advanced settings needed.

How the conversion works: formula and example

Internally, every value is first converted to L/100km, because it’s the only one of the four units that scales linearly with fuel actually burned. From there, the other three are derived:

L/100km = 235.21 ÷ MPG (US) L/100km = 282.48 ÷ MPG (UK) km/L = 100 ÷ L/100km

Those constants aren’t arbitrary: 235.21 comes from converting a US gallon (3.785 litres) over a mile (1.609 km), and 282.48 comes from doing the same with a UK imperial gallon (4.546 litres) — both figures are legally defined units, not estimates.

Step-by-step example: take a car rated at 30 MPG (US).

  1. Convert to L/100km: 235.21 ÷ 30 = 7.84 L/100km.
  2. Convert to km/L: 100 ÷ 7.84 = 12.75 km/L.
  3. Convert to MPG (UK): 282.48 ÷ 7.84 = 36.03 MPG (UK).

So a car rated 30 MPG (US) burns 7.84 L/100km, covers 12.75 km per litre, and would be advertised as roughly 36 MPG if the same figure were quoted in the UK. All three describe exactly the same fuel use — just on different scales.

US MPG vs UK MPG: why the same car gets two different numbers

A UK (imperial) gallon is about 20% bigger than a US gallon, so for the same car, the UK MPG figure is always about 20% higher than the US one. A US gallon is legally 3.785 litres; a UK gallon is 4.546 litres. Since MPG counts how far a car travels per gallon, and the UK gallon holds more fuel, the same real-world fuel consumption produces a bigger MPG number when measured in UK gallons.

This is not a rounding difference and it doesn’t mean the UK car is more efficient — it’s purely a unit conversion. In the example above, 30 MPG (US) converts to 36.03 MPG (UK): 36.03 ÷ 30 = 1.201, a 20.1% jump for identical fuel use. The same gap holds at any level — 40 MPG (US) converts to 48.04 MPG (UK), still a 20% difference. If you see “MPG” quoted without saying US or UK, treat it as ambiguous and check the source before comparing it to another figure.

What counts as good fuel economy

Fuel economy varies enormously by vehicle type, and there’s no single “good” number that applies to everything — but a few reference points help place a figure on the scale. A small hybrid rated at 60 MPG (US) converts to 3.92 L/100km, 72.06 MPG (UK), and 25.51 km/L — near the efficient end of what’s on the road today. At the other end, a large SUV or work truck posting 35 L/100km converts to just 6.72 MPG (US), 8.07 MPG (UK), and 2.86 km/L — several times thirstier for the same distance.

Between those extremes, most everyday petrol and diesel cars sit somewhere around 5–8 L/100km (roughly 30–47 MPG US). Whatever units a listing uses, the same rule always applies: for L/100km, smaller is better; for MPG and km/L, bigger is better.

Frequently asked questions

How do I convert MPG to L/100km?

Divide 235.21 by the MPG (US) figure, or 282.48 if it’s MPG (UK). For a car rated 30 MPG (US): 235.21 ÷ 30 = 7.84 L/100km. The constant is different for US and UK because a US gallon (3.785 L) and a UK gallon (4.546 L) aren’t the same size — always check which one you’re converting before doing the math.

How do I convert L/100km to MPG?

Divide 235.21 by the L/100km figure to get MPG (US), or divide 282.48 by it to get MPG (UK). For a car rated 6.5 L/100km: 235.21 ÷ 6.5 = 36.19 MPG (US), and 282.48 ÷ 6.5 = 43.46 MPG (UK). Both numbers describe the same car — they just use gallons of different sizes.

What’s the difference between US MPG and UK (Imperial) MPG?

A UK gallon (4.546 litres) is about 20% larger than a US gallon (3.785 litres), so the same car’s MPG figure is about 20% higher when measured in UK gallons. A car rated 40 MPG (US) is 48.04 MPG (UK) — same fuel consumption, different-sized unit. Neither number is wrong; they just aren’t interchangeable without converting.

Is a lower or higher number better — MPG, L/100km or km/L?

For L/100km, lower is better, because it measures fuel used per fixed distance. For MPG and km/L, higher is better, because they measure distance covered per fixed amount of fuel. The two scales run in opposite directions, which is why a car that “goes up” in MPG always “goes down” in L/100km — both changes describe the same improvement.

How do I calculate km/L?

Divide 100 by the L/100km figure. For a car rated 6.67 L/100km: 100 ÷ 6.67 = 14.99, rounding to 15 km/L. Going the other way, a car rated 15 km/L converts to 100 ÷ 15 = 6.67 L/100km — the formula works identically in both directions.

Why is my real-world fuel consumption higher than the manufacturer’s rating?

Official ratings come from standardized lab tests run at controlled speeds and temperatures, which real driving rarely matches. Cold starts, short trips, city traffic, air conditioning, cargo weight, and an unusually accelerative driving style all push actual consumption above the rated figure — none of these are measured in the standardized test cycle. A 10–20% gap between rated and real-world figures is common and doesn’t mean the rating was wrong, just that it was measured under different conditions than your daily driving.

What is a good L/100km rating for a car?

There’s no single official cutoff, but as a rough guide, small efficient cars and hybrids often land under 6 L/100km, mid-size petrol and diesel cars typically run 6–8 L/100km, and larger SUVs or trucks commonly exceed 10 L/100km. A hybrid rated 3.92 L/100km sits well below the efficient end of that range; a vehicle at 35 L/100km sits far above it. Compare figures using the same test standard where possible, since lab methods differ between regions.

How do I measure my car’s actual fuel consumption?

The most common method is the “fill-to-fill” approach: fill the tank completely, reset the trip odometer, drive normally until the tank is empty or you’re ready to refill, then fill up again and note exactly how many litres or gallons it took. Divide that amount by the distance driven (and multiply by 100 for L/100km, or divide distance by fuel for km/L or MPG) to get your real-world figure, which you can then enter into this calculator to convert into any other unit.

Sources