Gas Mileage and Fuel Economy, Explained
How to calculate gas mileage with the full-tank method, convert MPG to L/100 km and km/L, US vs UK gallons, and work out the fuel cost per mile or km.
By Udit Agarwal · Updated October 17, 2026
Gas mileage is the distance a vehicle travels on a given amount of fuel: divide the miles driven by the gallons used, so 300 miles on 10 gallons is 30 miles per gallon (MPG). Most of the world states fuel economy the other way round, as liters used per 100 kilometers (L/100 km), and 30 US MPG is 7.84 L/100 km. To convert, divide 235.21 by the MPG figure. Dividing 235.21 by an L/100 km figure converts it back.
There are three common measures. Two are distance per unit of fuel, where a higher number is better, and one is fuel per distance, where a lower number is better.
| Measure | Formula | Better when | Common in |
|---|---|---|---|
| MPG | miles ÷ gallons | Higher | US (US gallons), UK (imperial gallons) |
| km/L | kilometers ÷ liters | Higher | India, Japan |
| L/100 km | liters ÷ kilometers × 100 | Lower | Europe, Canada, Australia |
A trip of 300 miles that uses 10 US gallons is 300 ÷ 10 = 30 MPG. In metric units the same trip is 482.8 km on 37.85 liters, which works out to 12.75 km/L or 7.84 L/100 km. All three numbers describe the same car on the same trip.
A US gallon is 231 cubic inches, exactly 3.785411784 liters. An imperial gallon, the one used for UK fuel economy figures, is exactly 4.54609 liters, about 20% larger. A car that covers 30 miles on a US gallon covers 36.03 miles on an imperial gallon, so the same car has a higher MPG figure in the UK than in the US.
UK fuel is sold by the liter even though economy is quoted in imperial MPG, so a UK driver measuring their own mileage usually has liters from the receipt and miles from the odometer. 400 miles on 45 liters is 45 ÷ 4.54609 = 9.90 imperial gallons, or 40.41 UK MPG, which is 33.65 US MPG and 6.99 L/100 km. Always check which gallon a figure uses before comparing two cars.
Because L/100 km is fuel per distance and MPG is distance per fuel, one is the reciprocal of the other. There is no single factor to multiply by: doubling the MPG halves the L/100 km.
The constant 235.215 is 100 × 3.785411784 ÷ 1.609344, built from the exact sizes of the US gallon and the international mile (1.609344 km). With the imperial gallon it becomes 282.481.
| US MPG | L/100 km | km/L | UK MPG |
|---|---|---|---|
| 15 | 15.68 | 6.38 | 18.01 |
| 20 | 11.76 | 8.50 | 24.02 |
| 25 | 9.41 | 10.63 | 30.02 |
| 30 | 7.84 | 12.75 | 36.03 |
| 35 | 6.72 | 14.88 | 42.03 |
| 40 | 5.88 | 17.01 | 48.04 |
| 50 | 4.70 | 21.26 | 60.05 |
The same formulas work in reverse. 6 L/100 km is 235.215 ÷ 6 = 39.20 US MPG, and 15 km/L is 100 ÷ 15 = 6.67 L/100 km.
With L/100 km, the fuel for any trip is the figure times the distance divided by 100, a straight line. With MPG, the fuel is the distance divided by the MPG, a curve that flattens as MPG rises. That makes MPG differences hard to judge. Over 10,000 miles:
| Upgrade | Gallons per 10,000 miles | Fuel saved | L/100 km |
|---|---|---|---|
| 15 → 20 MPG | 666.7 → 500.0 | 166.7 gallons | 15.68 → 11.76 |
| 30 → 50 MPG | 333.3 → 200.0 | 133.3 gallons | 7.84 → 4.70 |
Replacing a 15 MPG vehicle with a 20 MPG one saves more fuel than replacing a 30 MPG car with a 50 MPG one. The L/100 km column shows it directly, with a drop of 3.92 against 3.14. Richard Larrick and Jack Soll called this misreading the MPG illusion in a 2008 paper in Science. US fuel economy labels also show gallons per 100 miles, a fuel-per-distance figure that behaves like L/100 km.
Percentages add to the confusion. Going from 30 to 50 MPG is a 67% rise in MPG but a 40% cut in fuel, and the percentage guide explains why those two numbers differ.
The dashboard figure is the car's own estimate. To measure the real number, use the full-tank method:
With 342 miles on the trip meter and 11.4 gallons to refill, the car managed 342 ÷ 11.4 = 30.0 MPG, or 7.84 L/100 km. In metric, 650 km on 42.25 liters is 42.25 ÷ 650 × 100 = 6.50 L/100 km, or 15.38 km/L.
One tank can mislead, because pumps do not all stop at exactly the same level, so average several fill-ups. Expect the result to differ from the official rating, which comes from a standard laboratory test, and to change with traffic, speed, weather and load. A change of tire size also changes what the odometer reads, and the Tire Size Calculator estimates that error.
Once you know the economy, the fuel cost of each mile or kilometer follows:
With illustrative prices:
| Price | Economy | Fuel cost | Per 1,000 |
|---|---|---|---|
| $3.50 a gallon | 30 MPG | 11.7 cents a mile | $116.67 per 1,000 miles |
| €1.80 a liter | 6.5 L/100 km | €0.117 a km | €117 per 1,000 km |
| ₹100 a liter | 15 km/L | ₹6.67 a km | ₹6,667 per 1,000 km |
A 40-mile daily round trip at 11.7 cents a mile costs $4.67 a day in fuel, which the salary guide can help you set against a day's pay. The Fuel Cost Calculator turns distance, economy and price into a trip cost.
Per-mile reimbursement rates, such as the IRS standard mileage rate in the US, are set to cover the whole cost of running a car, including depreciation, insurance and maintenance, so they are well above the fuel cost alone. The Mileage Reimbursement Calculator multiplies distance by whatever rate applies. For an electric car, the EV Charging Cost Calculator works out the cost of a charge instead.
The Gas Mileage Calculator takes a distance in miles or kilometers and the fuel used in gallons or liters, in any combination, and shows MPG, L/100 km and km/L to two decimal places. It converts with the exact factors, 1 mile = 1.609344 km and 1 US gallon = 3.785411784 liters, and a copy button puts the inputs and all three results into plain text. Commas in the numbers are ignored, and a distance or fuel amount of zero or less shows an error instead of a result.
Its gallon is the US gallon, so its MPG is US MPG. For imperial figures, enter the fuel in liters, or use the Fuel Economy Converter, which converts between MPG (US), MPG (UK), L/100 km and km/L. The calculator handles one trip or tank at a time: it does not keep a log, average several fill-ups or take a fuel price. It runs in your browser and nothing you enter is uploaded. The results are estimates, not financial advice.
How do I calculate gas mileage? Fill the tank, drive, fill it again, and divide the miles driven by the gallons needed to refill. 342 miles on 11.4 gallons is 30 MPG. For L/100 km, divide the liters by the kilometers and multiply by 100.
How do I convert MPG to L/100 km? Divide 235.215 by the US MPG figure, or 282.481 by a UK MPG figure. 30 US MPG is 7.84 L/100 km. The same division converts L/100 km back to MPG.
Why is UK MPG higher than US MPG for the same car? An imperial gallon holds 4.54609 liters and a US gallon 3.785411784 liters, so the UK gallon is about 20% larger and the car goes about 20% further on it. 30 US MPG is 36.03 UK MPG.
How do I convert km/L to L/100 km? Divide 100 by the km/L figure. 15 km/L is 6.67 L/100 km, and 20 km/L is exactly 5 L/100 km. The same division works in the other direction.
Is a higher or lower L/100 km better? Lower. L/100 km measures fuel used over a fixed distance, so a smaller number means less fuel. MPG and km/L are the other way round: higher is better.
Why doesn't my result match the dashboard or the official rating? The dashboard shows the car's own estimate, and the official rating comes from a standard laboratory test, while your result reflects your own driving, roads and weather. Averaging several full-tank measurements gives the most reliable figure.
How do I work out fuel cost per mile? Divide the price per gallon by the MPG. At $3.50 a gallon and 30 MPG, each mile costs about 11.7 cents. In metric, multiply the price per liter by the L/100 km figure and divide by 100.
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