MPG Calculator

Enter the miles you drove and the gallons you used to find your real miles per gallon, plus L/100km and km/L conversions. This is the true fuel economy of your car and your driving, not a lab estimate.

Miles driven

Gallons used

Your fuel economy

0 MPG

L/100km0

km / L0

Miles per tank-gallon0

Range on 12 gal0 mi

Tools · By Mustafa Bilgic · Last updated 20 June 2026

How to measure your real MPG

Your true fuel economy is simply the miles you covered divided by the gallons you burned. The formula is MPG = miles ÷ gallons. To get an accurate reading, fill the tank all the way, reset your trip odometer (or write down the mileage), drive normally for a full tank, then fill up completely again. The pump tells you exactly how many gallons it took, and your trip meter tells you the miles. Divide one by the other and you have a number that reflects your real roads, your real driving, and your real car.

Worked example: You drive 310 miles and the next fill-up takes 11 gallons. 310 ÷ 11 = 28.2 MPG. In metric that is 235.215 ÷ 28.2 = 8.3 L/100km, or about 12.0 km/L. The calculator above does this instantly and adds your estimated range on a 12-gallon tank.

Why measure it yourself

The MPG figure on a window sticker comes from EPA lab tests under controlled conditions. It is useful for comparing one model to another, but it rarely matches what any individual driver sees. Measuring your own MPG over a few tanks reveals how your habits, your commute, and your maintenance affect the number. A sudden drop in MPG is also one of the earliest warning signs of a problem — a dragging brake, a clogged air filter, low tire pressure, or a failing oxygen sensor — so tracking it is a cheap form of preventive diagnosis.

Watch the trend: A 15–20% MPG drop with no change in driving often points to a mechanical issue. See how to improve fuel economy and when to replace your air filter.

Converting between MPG and metric

If you compare cars rated in metric units, two conversions matter. To get litres per 100 kilometres, divide 235.215 by your US MPG. To get kilometres per litre, multiply US MPG by 0.42514. Note that US gallons and Imperial (UK) gallons differ: one Imperial gallon is about 1.201 US gallons, so a UK MPG figure looks higher for the same real efficiency. This calculator uses US gallons throughout.

More fuel and cost tools

Once you know your real MPG, plug it into our fuel cost calculator to project annual gas spending, the cost per mile calculator to see your true driving cost, or the commute cost calculator to price a daily drive. Thinking of switching power sources? Compare with the gas vs electric cost calculator, and use the total cost of ownership calculator for the full picture.

Frequently asked questions

How do I calculate MPG by hand?

Fill the tank completely and reset your trip odometer or note the mileage. Drive normally until you need fuel again, then fill up completely and note how many gallons it took. Divide the miles you drove by the gallons you added. If you drove 310 miles and added 11 gallons, that is 310 divided by 11, or about 28.2 MPG. Averaging two or three tanks gives a more reliable number than a single fill.

Why is my MPG lower than the window sticker?

EPA window-sticker figures come from standardized lab tests that do not match every driver. Cold weather, short trips before the engine warms up, aggressive acceleration, highway speeds above 65 mph, roof racks, underinflated tires, and a heavy right foot all lower real-world economy. Seeing 10 to 15 percent below the sticker is common and usually nothing is wrong with the car.

How do I convert MPG to L/100km?

Divide 235.215 by your US MPG figure. For example, 30 US MPG is 235.215 divided by 30, which is about 7.84 L/100km. This calculator shows both L/100km and km/L automatically so you can compare against vehicles rated in metric units.