Gas mileage is the distance your vehicle covers per unit of fuel, calculated as miles driven divided by US gallons used (MPG) or, in metric form, as liters used divided by kilometers driven and then multiplied by 100 to give L/100km. For a road trip, the number tells you two practical things at once: how efficient your car is on the route you actually drove, and how much fuel a similar future drive is likely to burn. The formula is short — MPG equals miles divided by gallons, and L/100km equals liters divided by kilometers, then multiplied by 100 — but the way you measure the inputs decides whether the answer is trustworthy. This article walks through the trip calculation in three steps, shows how to gather clean numbers on the road, explains why MPG and L/100km move in opposite directions, and clears up the US-versus-UK gallon trap that catches most cross-border comparisons.
The Gas Mileage Formula in Two Unit Systems
Every fuel-economy number is built from the same two ingredients: how far you drove and how much fuel it took to cover that distance. The unit system you pick only changes how the result is written; the underlying division is identical, and a calculator that handles the conversion saves you from doing it by hand.
In the US system, gas mileage is expressed as MPG, where higher is better, and the formula is simply MPG = miles driven ÷ US gallons used. A driver who covers 300 miles on 10 gallons gets 30 MPG. In the metric system used across most of Europe, Asia, and the Americas outside the United States, fuel economy is usually expressed as L/100km, where lower is better, and the formula is L/100km = (liters used ÷ kilometers driven) × 100. That same 30 MPG driver, if measured in metric, would burn roughly 7.84 liters per 100 km. A third unit, km/L (kilometers per liter, higher is better), is popular in parts of Asia and is simply kilometers driven divided by liters used, with no scaling factor needed.
| Unit | Formula | Better when... | Commonly used in |
|---|---|---|---|
| MPG (US) | miles ÷ US gallons | Higher | United States, sometimes the UK |
| L/100km | (liters ÷ km) × 100 | Lower | Europe, most of the world |
| km/L | km ÷ liters | Higher | Japan, India, parts of Asia |
How to Calculate Your Trip's Gas Mileage
The fastest way to turn a recorded trip into a fuel-economy figure is to feed the two numbers you already have into a dedicated calculator. The Gas Mileage Calculator accepts any pair of distance and fuel values and returns all three readings — MPG, L/100km, and km/L — at the same time, so you do not have to convert by hand or pick the unit you think you need.
- Pick your unit system. Choose imperial (miles and US gallons) if you filled up in the United States or want a US-style MPG figure. Choose metric (kilometers and liters) for L/100km or km/L.
- Enter the distance you drove and the amount of fuel you used to cover it. Both numbers should come from the same trip — the same fill-up, the same odometer window — so the ratio is fair.
- Read the fuel economy results below. MPG, L/100km, and km/L are all shown together, so you can quote the figure your country uses and the one the rest of the world uses side by side.
There is no button to press and nothing is uploaded, so the calculation happens in your browser the moment you type. That makes the tool well-suited to logging a figure at the pump, when you have the odometer and the receipt in front of you.
How to Measure Real Mileage on a Trip
A calculator is only as accurate as the numbers you give it. The cleanest way to measure a trip's fuel economy is the tankful-to-tankful method, which uses the gas station itself as your measuring instrument so you never have to estimate partial fills.
Start by filling the tank completely and either resetting the trip meter or writing down the odometer reading. Drive normally until you need fuel again — no special hypermiling, no detours that are not part of your usual routes. When you next fill up, fill the tank completely a second time; the pump shows exactly how much fuel went in, and that figure is the fuel you burned between the two fills. The distance you covered over the same window, paired with the fuel the pump registered, are the two numbers to enter in the calculator.
A single short trip can be skewed by traffic, weather, or a handful of hard accelerations. Averaging three or four full tanks smooths out that noise and gives a figure that holds up against the EPA rating for your car or the manufacturer's brochure claim. If only one trip is available, log it anyway, but treat the result as a ballpark rather than a permanent rating. The more representative your inputs, the more useful the answer becomes for trip planning.
Why Higher MPG and Lower L/100km Both Mean Better Economy
MPG and L/100km describe the same thing — how far you go on a fixed amount of fuel — but they look at it from opposite sides. MPG is a measure of efficiency, so a bigger number means you went further on each gallon. L/100km is a measure of consumption, so a smaller number means you burned less fuel to cover the same 100 km. That is why 30 MPG feels good while 7.84 L/100km feels good, but 30 L/100km does not — the two figures describe the same efficient trip.
The two are linked by a fixed conversion that holds for any US-gallon MPG figure:
L/100km = 235.214583 ÷ MPG (US)
For example, 300 miles on 10 US gallons is 30 MPG. Substituting 30 into the conversion gives 235.214583 ÷ 30, which is roughly 7.84 L/100km, so the same trip is described as 30 MPG in the United States and 7.84 L/100km in Europe. Knowing the conversion means you can read a foreign review, owner's manual, or government fuel-economy label without being misled by the direction of the number.
US Gallon vs UK Gallon: The 20% Trap
Before quoting an MPG figure from anywhere — a brochure, a review, an older British owner's manual — check which gallon is being used. The calculator described above uses the US gallon, which is 3.785411784 liters and matches the figures quoted on American window stickers and by the US Environmental Protection Agency.
The UK, or Imperial, gallon is larger at 4.54609 liters — about 20% bigger than the US gallon. Because the denominator is bigger, the same car scores a higher MPG number in UK gallons than in US gallons even though nothing about the engine has changed. A car rated 30 US MPG works out to roughly 36 UK MPG. Comparing a US figure against a UK figure directly makes one car look far thirstier than it really is, which is the most common source of cross-border fuel-economy confusion.
| Gallon | Liters | Used by | 30 US MPG equals about |
|---|---|---|---|
| US gallon | 3.785411784 | USA, EPA window stickers | 30 US MPG |
| UK (Imperial) gallon | 4.54609 | UK (older literature) | ~36 UK MPG |
Using Your Trip Mileage to Plan the Next Drive
Once you have a reliable trip mileage figure, you can use it the other way around — to estimate fuel needed for a future drive rather than to measure a past one. The calculation is the same formula rearranged: trip distance divided by mileage in the matching units. If your measured economy is 30 MPG, a 450-mile trip will burn substantially less fuel than the same trip in a 20 MPG car, a meaningful planning difference for road trips, towing, or any drive where refueling stops matter. In metric, with 7.84 L/100km, a 500 km drive burns substantially less than a car scoring 12 L/100km on the same route. The exact totals depend on the trip distance and your measured mileage, which is exactly what the Gas Mileage Calculator is built for. For drivers weighing whether an electric vehicle would handle the same route, an EV charging cost versus gas comparison puts the two fueling methods on the same footing.
Your measured figure will always be more honest than the brochure number, because it reflects your real routes, your real load, your real driving style, and your local fuel grade. Keep logging each tank, average several of them, and the trip mileage stops being a guess and becomes a planning tool you can rely on for the next drive.