Gas Mileage Calculator
Calculate your vehicle's actual fuel efficiency from distance driven and fuel used — in MPG, L/100km and km/L.
How to use this calculator
- 1Reset your trip odometer at a fill-up, then enter the distance and fuel added at your next fill-up.
- 2The more fill-ups you average, the more representative the figure becomes of your actual driving.
How the calculation works
MPG = miles ÷ gallons. L/100km = (litres ÷ km) × 100. km/L = km ÷ litres- MPG
- Distance per unit of fuel — higher is better
- L/100km
- Fuel per unit of distance — lower is better
All three figures describe the same underlying efficiency; this shows all three so the result is useful regardless of which one your region or vehicle documentation uses.
Worked example
350 miles on 11.5 gallons
- 1.MPG = 350 ÷ 11.5 = 30.43.
- 2.Converting to metric: about 7.73 L/100km, or 12.94 km/L.
Result: 30.43 MPG
What "fuel efficiency" is really measuring
Fuel efficiency describes how far a vehicle can travel for a given amount of energy put into it. Every figure this calculator produces — MPG, L/100km, km/L — is just the same underlying ratio of distance to fuel, expressed in different directions and different units. None of them is more "correct" than the others; they exist because different countries built their fuel-economy conventions around different everyday units of distance and volume.
The number itself is never fixed for a given car — it shifts with driving style, terrain, temperature, tyre pressure, load and even how full the tank was when the trip started. That is why a single fill-up calculation is a snapshot rather than a definitive rating, and why averaging several fill-ups gives a steadier picture of a vehicle's real-world efficiency.
Three units, two opposite directions
These three figures are mathematically related but do not move in the same direction as efficiency changes, which is the single most common source of confusion when comparing them.
- Miles per gallon (MPG) — distance per unit of fuel — a bigger number means better efficiency. Standard in the United States and a few other countries that still use gallons for fuel.
- Litres per 100 km (L/100km) — fuel per unit of distance — the inverse relationship, so a smaller number means better efficiency. Standard across most of Europe, Canada and Australia.
- Kilometres per litre (km/L) — distance per unit of fuel again, like MPG, just in metric units — a bigger number is better. Common in parts of Asia and Latin America.
Why the US measures fuel economy backwards from most of the world
Most countries express fuel consumption as L/100km — literally "how much fuel does this trip cost" — which lines up naturally with how people budget for fuel. The US instead adopted miles per gallon, a distance-per-fuel figure, which reads intuitively as "bigger is better" but has an odd side effect: equal MPG improvements are not worth equally much fuel saved. Going from 10 to 20 MPG saves far more fuel over a fixed distance than going from 40 to 50 MPG does, even though both are "+10 MPG" — a distortion that a L/100km figure does not have, because it scales linearly with actual fuel used.
This asymmetry is part of why fuel-economy regulations and some manufacturers increasingly quote both figures, or use fuel-consumption metrics internally even in MPG-native markets.
Getting an accurate reading from your own vehicle
A few habits make a fill-up-based measurement much more trustworthy.
- Fill to the same point both times — topping off completely at both the start and end fill-up removes the biggest source of error — a partial fill makes the fuel-used figure meaningless.
- Average several tanks, not one — a single tank is affected by whatever mix of driving happened to occur during it; three or more fill-ups smooth out short-term noise from traffic, weather and route.
- Expect a gap from the sticker rating — official window-sticker or brochure figures come from standardised lab tests and are usually optimistic compared to everyday mixed driving, so a lower real-world number does not necessarily mean anything is wrong with the vehicle.
What this assumes, and where it stops
Assumptions
- The fuel used figure is for exactly the distance entered — typically the fuel needed to refill the tank to full after that distance.
Limitations
- A single fill-up is noisy — pump shut-off point and tank fill technique vary. Averaging several fill-ups gives a more reliable figure.
Common questions
How do I measure this accurately?
Fill the tank completely and reset the trip odometer. Drive normally until your next fill-up, then fill the tank completely again and note how much fuel went in — that amount is what you used for the distance shown on the trip odometer. Repeating this over several fill-ups and averaging gives a more reliable figure than any single tank.
Why does my result differ from the manufacturer's rating?
Official ratings come from standardised lab tests that rarely match real-world conditions — driving style, terrain, weather, tyre pressure and load all affect actual fuel economy, usually bringing real-world figures below the rated one.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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