MPG Calculator

Background.

Fuel economy is one division — distance driven over fuel used — and the arithmetic is the easy part. The trouble is the units. A US gallon is 3.785411784 litres and an imperial gallon, the one used for British MPG figures, is 4.54609 litres; both are exact by definition, and the imperial gallon is about 20% bigger. So a car reviewed at 50 mpg in a British magazine is managing roughly 41.6 mpg by the American measure, with nothing about the car different. More confused comparisons start there than anywhere else.

Most of the world avoids the gallon altogether and quotes consumption instead of economy: litres per 100 kilometres, where lower is better. That choice is not just habit. Fuel used, fuel cost and tailpipe CO2 all rise in straight proportion to L/100 km, while MPG is a reciprocal and bends the comparison. Researchers Richard Larrick and Jack Soll named the effect the MPG illusion in Science in 2008: people tend to treat equal MPG gains as equal savings, so a jump from 15 to 25 mpg and one from 40 to 50 look alike, yet over every 100 miles the first saves about 2.667 gallons and the second only half a gallon.

The US fuel-economy label added gallons per 100 miles alongside MPG from the 2013 model year for exactly that kind of comparison. Here all five common units come out side by side — US MPG, UK MPG, kilometres per litre, litres per 100 km and US gallons per 100 miles — so you can read whichever one answers your question, whether that is a budget, a comparison with a window sticker or a figure a friend quoted from abroad. The conversions rest on three exact constants: the international mile of 1.609344 km, fixed by the 1959 agreement on the yard and pound, and the two gallons. The inputs are the uncertain part, and the steps below show how to measure them from one fill-up to the next.

What is mpg calculator?

Miles per gallon is the ratio of distance driven to fuel consumed, and it is the unit of US fuel-economy labels and of the Corporate Average Fuel Economy (CAFE) programme run by the National Highway Traffic Safety Administration, which sets a sales-weighted fleet average each carmaker's new vehicles must meet; the first passenger-car standard, in 1978, was 18 mpg. A car that covers 300 miles on 12 US gallons does 25 mpg. Litres per 100 km turns the ratio upside down and scales it: a car using 12 litres every 100 km does about 19.6 US mpg, and for any car US MPG multiplied by L/100 km comes to about 235.215. British MPG figures divide by the larger imperial gallon, so they run about 20% higher than US figures for the same driving. None of these numbers is an engine efficiency in the thermodynamic sense; they are distance-per-fuel ratios, useful because distance and fuel are what a driver can actually observe.

How to use this calculator.

1. 01Fill the tank until the pump nozzle clicks off the first time and reset the trip odometer. Don't top off past the click: it adds little, risks spills, and what matters is stopping at the same point at both fill-ups.
2. 02Drive normally for at least one tank, ideally two, so that small differences in where the pump shuts off matter less. A tank of mixed town and highway driving is the fairest comparison with a combined rating.
3. 03Refuel the same way, stopping at the first click. The amount on the pump is the fuel you used; the trip odometer reading is the distance.
4. 04Enter the distance and choose miles or kilometres, then enter the fuel and choose the unit on your receipt: US gallons at a US pump, litres almost everywhere else, UK gallons only if a British figure is what you have.
5. 05Read the five outputs. US MPG compares with an American window sticker; L/100 km or gallons per 100 miles feed a fuel-cost or CO2 estimate; UK MPG should come out about 20.1% above US MPG, a quick check that the units went in correctly.
6. 06To track a car over months, keep a running total of distance and fuel and divide the totals, rather than averaging each tank's MPG — one of the questions below shows why the two differ.

The formula.

The core relationship is US MPG = distance in miles ÷ fuel in US gallons. The calculator converts whatever you enter into kilometres and litres first, then derives every unit from the same ratio using exact constants: 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L (NIST SP 811), and 1 imperial gallon = 4.54609 L (UK Weights and Measures Act 1985).

L/100 km = 235.214583 ÷ US MPG km/L = US MPG × 0.4251437 UK MPG = US MPG × 1.20095 gal/100 mi = 100 ÷ US MPG

The constant 235.214583 is not arbitrary: it is 3.785411784 × 100 ÷ 1.609344, the litres in a US gallon times 100, divided by the kilometres in a mile. The 0.4251437 is 1.609344 ÷ 3.785411784, and 1.20095 is the ratio of the two gallons, 4.54609 ÷ 3.785411784.

Two of the five outputs, L/100 km and gal/100 mi, measure consumption and grow in proportion to the fuel you burn. The other three measure economy and are reciprocals of consumption, which is where intuition goes wrong. Two cars 5 mpg apart at the low end, 15 and 20 mpg, differ by about 1.667 gallons every 100 miles; two cars 5 mpg apart at the top, 40 and 45 mpg, differ by about 0.2778 gallons — one sixth as much. Whenever the question is about money or emissions, compare the consumption figures.

A worked example.

Frequently asked questions.

References& sources.

1. [1]U.S. Environmental Protection Agency. Fuel Economy Guide (model year 2025). The authoritative annual publication of EPA combined, city, and highway MPG ratings for every US-market vehicle, plus the gallons-per-100-miles secondary metric introduced in 2013.
2. [2]U.S. National Highway Traffic Safety Administration. Corporate Average Fuel Economy (CAFE) Standards — official regulatory text and history. The federal programme that sets the sales-weighted MPG floor each automaker's new-vehicle fleet must meet, beginning with the 18-mpg passenger-car standard of 1978.
3. [3]Thompson, A. & Taylor, B. N. (2008). NIST Special Publication 811: Guide for the Use of the International System of Units (SI). U.S. Department of Commerce. The authoritative source for the exact conversion constants used in this calculator: 1 mile = 1.609344 km and 1 US gallon = 3.785411784 L, both fixed by international agreement.
4. [4]International Energy Agency. Global Fuel Economy Initiative — annual report on light-duty vehicle fuel-economy trends across IEA member countries. The authoritative cross-country reference for how the US, EU, Japan, China, and India each express and regulate vehicle fuel economy.
5. [5]Larrick, R. P. & Soll, J. B. (2008). 'The MPG Illusion'. Science, vol. 320, no. 5883, pp. 1593–1594. The peer-reviewed paper that documented systematic consumer misperception of fuel savings from MPG improvements, and which motivated the EPA's 2013 addition of gallons-per-100-miles to the window sticker.
6. [6]United Kingdom. Weights and Measures Act 1985, Schedule 1. The statute that fixes the imperial gallon at 4.54609 litres exactly — the legal definition behind every UK window-sticker MPG figure and the source of the roughly 20% gap between UK and US MPG ratings.