Fuel Economy Unit Converter

How to use this calculator

Four inputs. The last one is optional but useful.

Fuel economy value is the number you’re starting with. Type it in. Works with decimals.

From unit is the unit your number is in. The dropdown has 4 options: MPG (US), MPG (UK), km/L, and L/100km. Pick the one that matches your source data.

To unit is your target unit. Same 4 options. Select whichever format you need.

Fuel price (per litre/gallon) is optional. Enter the current fuel price in your local currency per litre or per gallon, matching the unit system you’re working in. The calculator uses this to output a fuel cost per 100 km estimate. Leave it at the default or adjust it to your actual pump price.

Click Calculate and the blue results panel shows 5 numbers at once: the converted value in your target unit, plus all 4 formats side by side (MPG US, MPG UK, km/L, L/100km), and the fuel cost per 100 km based on your price input.

That “all formats at once” display is the most useful part. Instead of converting separately for each format, one calculation gives you everything.

What problem this calculator solves

Car specs cross borders constantly. A Japanese import lists fuel economy in km/L. A US car review quotes MPG (US). A European spec sheet uses L/100km. A British government document quotes MPG (UK). None of these convert to each other with a simple multiplication because they use different gallon sizes and different mathematical relationships.

The US gallon and the UK (imperial) gallon are not the same. 1 US gallon = 3.785 litres. 1 UK gallon = 4.546 litres. So 30 MPG in the US and 30 MPG in the UK are not the same fuel economy, even though they look identical written down.

And L/100km is inversely related to all the other units. You can’t go from 30 MPG to L/100km by multiplying or dividing by a fixed number without also inverting the fraction. This is where mental math fails and where people get it wrong most often.

The concept explained simply

Think of fuel economy as asking: “How far does this car travel for a given amount of fuel?” or “How much fuel does this car burn for a given distance?”

MPG (any variant) and km/L answer the first question. L/100km answers the second. They’re measuring the same relationship from opposite directions, which is why one goes up as the other goes down.

A car that gets 5 L/100km is more efficient than one that gets 8 L/100km. A car that gets 40 MPG is more efficient than one that gets 25 MPG. These two facts feel obvious in their own units. Comparing them across unit systems is where everyone gets confused.

L/100km tells you the cost of travelling a fixed distance. MPG tells you the range you get from a fixed amount of fuel. Same efficiency, opposite question, opposite direction on the scale.

The formulas explained

The conversions aren’t multiplicative because of the inversion in L/100km. Here’s exactly what the calculator is doing:

Starting point: convert everything through litres per 100 km as the common denominator.

The constants 235.214 and 282.481 come from the respective gallon sizes multiplied by 100:

• US gallon = 3.78541 L, so 100 km ÷ miles per litre factor = 235.214
• UK gallon = 4.54609 L, same logic = 282.481

Working backwards to get MPG from L/100km:

The fuel cost output uses:

MPG (US) to all other units: full conversion table

The most commonly needed table. US EPA ratings converted to every other format.

MPG (US)	MPG (UK)	km/L	L/100km	Cost/100km at $1.50/L	Cost/100km at $2.00/L
10	12.01	4.25	23.52	$35.28	$47.04
15	18.01	6.38	15.68	$23.52	$31.36
20	24.02	8.50	11.76	$17.64	$23.52
25	30.02	10.63	9.41	$14.12	$18.82
30	36.03	12.75	7.84	$11.76	$15.68
35	42.03	14.88	6.72	$10.08	$13.44
40	48.04	17.01	5.88	$8.82	$11.76
45	54.04	19.13	5.23	$7.85	$10.46
50	60.05	21.26	4.70	$7.05	$9.40
60	72.06	25.51	3.92	$5.88	$7.84
70	84.07	29.76	3.36	$5.04	$6.72
80	96.08	34.02	2.94	$4.41	$5.88

L/100km to MPG: the European-to-US table

European spec sheets use L/100km. US buyers frequently need to translate this when evaluating imported vehicles or EU market reviews.

L/100km	MPG (US)	MPG (UK)	km/L	Cost/100km at $1.50/L	Cost/100km at $1.80/L
3.0	78.40	94.16	33.33	$4.50	$5.40
4.0	58.81	70.62	25.00	$6.00	$7.20
5.0	47.04	56.50	20.00	$7.50	$9.00
6.0	39.20	47.08	16.67	$9.00	$10.80
7.0	33.60	40.35	14.29	$10.50	$12.60
7.84	30.00	36.03	12.76	$11.76	$14.11
8.0	29.40	35.31	12.50	$12.00	$14.40
9.0	26.13	31.39	11.11	$13.50	$16.20
10.0	23.52	28.25	10.00	$15.00	$18.00
12.0	19.60	23.54	8.33	$18.00	$21.60
15.0	15.68	18.83	6.67	$22.50	$27.00
20.0	11.76	14.12	5.00	$30.00	$36.00

km/L to other units: the Japan/Asia table

Japan, South Korea, India, and much of Asia rate fuel economy in km/L. If you’re buying a Japanese domestic market vehicle or reading an Asian car review, this is the table you need.

km/L	MPG (US)	MPG (UK)	L/100km
5.0	11.76	14.12	20.00
8.0	18.82	22.60	12.50
10.0	23.52	28.25	10.00
12.0	28.22	33.89	8.33
12.8	30.10	36.14	7.81
15.0	35.28	42.36	6.67
18.0	42.33	50.84	5.56
20.0	47.04	56.48	5.00
25.0	58.81	70.62	4.00
30.0	70.57	84.73	3.33
35.0	82.33	98.84	2.86
40.0	94.09	112.95	2.50

Real fuel economy figures for common vehicles

These are approximate real-world figures (not manufacturer claims) to give you a calibration point when reading converted numbers.

Vehicle type	MPG (US)	MPG (UK)	km/L	L/100km
City bus (diesel)	4-6	5-7	1.7-2.5	40-60
Large pickup truck (V8)	12-16	14-19	5-7	15-20
Mid-size SUV (petrol)	22-27	26-32	9-11	9-11
Family sedan (petrol)	28-35	34-42	12-15	7-9
Compact hatchback (petrol)	32-40	38-48	14-17	6-7.5
Small diesel hatchback	45-55	54-66	19-23	4.3-5.3
Petrol-electric hybrid	45-60	54-72	19-25	4-5.3
Plug-in hybrid (petrol mode)	35-50	42-60	15-21	4.7-6.7
Electric vehicle (MPGe)	80-130	96-156	34-55	1.8-3.0
Sports car (V8, petrol)	15-22	18-26	6-9	11-16
Motorcycle (medium)	45-65	54-78	19-28	3.6-5.3

Common mistakes people make

Treating MPG (US) and MPG (UK) as the same. They’re not. A British car tested under UK standards at 40 MPG is not the same as a US car rated 40 MPG by the EPA. The UK figure is about 20% higher for the same actual fuel consumption because the UK gallon is larger. If a British person tells you their car does “40 miles to the gallon,” and you convert that assuming a US gallon, you’ll get a consumption figure that’s 20% too optimistic.

Forgetting that L/100km inverts when comparing. A car improving from 10 L/100km to 8 L/100km saves more fuel than a car improving from 6 to 4 L/100km, even though both improve by 2 L/100km. That’s because the relationship is non-linear. The calculator’s output across all 4 formats makes this visible: watch how the MPG equivalent changes more dramatically at the low-consumption end.

Using manufacturer claimed figures without adjustment. Official fuel economy ratings from the EPA, NEDC, or WLTP test cycles consistently overstate real-world economy. US EPA figures tend to run 10-20% optimistic. European WLTP figures run 20-35% optimistic. European NEDC figures (pre-2018) were even more inflated. When comparing specs, check which test standard applies.

Entering fuel price in the wrong unit. The fuel price field uses per litre by default. If you’re in the US and thinking in gallons, dividing your price by 3.785 before entering it will give you the correct cost estimate. Or convert to per-litre first: $4.00/gallon ÷ 3.785 = $1.057/L.

Assuming higher MPG is always better value. Fuel economy matters, but fuel price varies by country. A car doing 35 MPG in the US with cheap fuel may cost less to run than a car doing 50 MPG in the UK or EU where fuel is 2-3x more expensive per litre. The fuel cost per 100 km output in the calculator is what actually determines running cost.

The cost estimate output: how to use it

The fuel cost per 100 km figure is the most practically useful output in the calculator. Here’s how to work with it.

Enter your local fuel price per litre. If your pump shows price per gallon, divide by 3.785 (US gallon) or 4.546 (UK gallon) to get per litre.

The output gives cost per 100 km. To get cost for any other distance:

• Cost per 1 km: divide by 100
• Cost per 500 km trip: multiply by 5
• Annual cost at 15,000 km/year: multiply by 150

That’s the calculation this tool sets up. Enter your fuel economy, set the price, multiply the result by your annual km divided by 100. That’s your yearly fuel bill.

Why fuel economy ratings differ by country

The same physical vehicle gets a different fuel economy rating depending on where it’s sold and which test standard applies.

US EPA: uses a two-cycle test (city and highway). The combined figure is a weighted average (55% city, 45% highway). Results tend to run 10-20% below real-world for highway drivers, closer to accurate for urban driving.

European WLTP (since 2018): Worldwide Harmonised Light Vehicle Test Procedure. More realistic than the older NEDC standard it replaced, but still 20-30% optimistic compared to real-world mixed driving.

European NEDC (pre-2018): the old standard that allowed manufacturers to optimise heavily for the test. Notoriously disconnected from real-world results; often 30-40% better than actual use.

Japanese JC08 / WLTC: Japan has progressively moved from the older JC08 test to the WLTC standard (similar to European WLTP). Figures under JC08 were similarly optimistic; WLTC is more realistic.

The practical takeaway: when comparing a US-spec car to a European-spec car, you’re not just dealing with unit differences (MPG vs L/100km) but also test standard differences. A European car claiming 5.0 L/100km under WLTP might actually get 6.5-7.0 L/100km in real mixed driving, which converts to closer to 34-40 MPG (US), not the 47 MPG (US) the raw conversion would suggest.

The unit conversion is exact. The test standard discrepancy is the hidden variable that makes direct comparisons misleading. Always adjust for test standard before comparing real-world economy across regions.

The bottom line

Fuel economy looks like a simple number until you’re comparing cars across borders. Four different units, two different gallon sizes, multiple test standards, and an inversion in the relationship between L/100km and everything else: that’s where quick mental math stops working.

Enter your number, pick your units, and read all 4 formats at once. Add a fuel price and you get the annual cost comparison directly. That’s the calculation most people need, and it’s the one that makes a real difference when choosing between vehicles.