Find the fuel economy your car really gets. Fill the tank, note the miles on your trip meter at the next fill-up, then enter the miles and the gallons it took to refill.
Some fields need a fix, so the result shows your last valid numbers.
- This fill-up cost
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- Liters per 100 km
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- Kilometers per liter
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How the MPG calculator works
Miles per gallon is the distance you drove divided by the fuel you used. The calculator takes the miles since your last fill-up and divides them by the gallons it took to fill the tank again. With a price per gallon, it also shows what that fill-up cost, what each mile cost you and the cost per 100 miles. For anyone who thinks in metric, it converts the result to liters per 100 km and kilometers per liter.
The formula in plain English
- MPG = miles driven ÷ gallons to refill
- Cost per mile = (gallons × price per gallon) ÷ miles driven
- L/100 km = 235.215 ÷ mpg
With the default example, 320 miles on 10.5 gallons works out to about 30.5 mpg. At $3.40 a gallon, the fill-up cost $35.70 and each mile cost about 11 cents. Those defaults are placeholders, not averages. Use the numbers from your own pump receipt.
How to measure mpg accurately
- Fill the tank until the pump clicks off. Do not top off.
- Reset your trip meter, or write down the odometer reading.
- Drive as you normally would, ideally until the tank is at least half empty.
- Fill up again at the same pump if you can, stopping at the first click.
- Enter the miles on the trip meter and the gallons shown on the pump.
One tank gives you a snapshot. Repeat it over three or four fill-ups and average the results for a figure you can trust. A long highway tank and a tank of short city errands can look very different, and the average is closer to what you will pay over a year.
Why your mpg may differ from the sticker
Window sticker figures come from standardized lab tests. Real roads add things those tests leave out: hard acceleration, high highway speeds, air conditioning, cold starts, short trips, roof racks, extra weight and underinflated tires. If your number is a little below the sticker, that is normal. If it drops sharply compared with your own past readings, something may be wrong.
A sudden fall in mpg is worth looking into. Common culprits include low tire pressure, a dragging brake and engine faults such as a misfire. A flashing check engine light is the clearest warning sign; it usually means a misfire that wastes fuel and can overheat the catalytic converter. Stop driving hard and get the code read soon.
How to use the result
Plug your real mpg into the fuel cost calculator to see your yearly gas bill, or into the car ownership cost calculator to see fuel next to depreciation, insurance and maintenance. Tracking mpg over time is also a cheap way to spot problems early.
Shopping for a new car? Compare your measured mpg with the rated figure of the car you are considering, then run both through the fuel cost calculator with the same yearly miles. The difference shows what better fuel economy would really save you each year.
Frequently asked questions
How do I calculate my car’s mpg?
Fill the tank, reset the trip meter, drive, then refill. Divide the miles on the trip meter by the gallons it took to refill. That is your miles per gallon.
Is the dashboard mpg readout accurate?
It is an estimate from the car’s computer and can read a little high or low. The fill-up method uses the actual fuel pumped, so it is the better check.
How do I convert mpg to L/100 km?
Divide 235.215 by the mpg figure. For example, 30 mpg is about 7.8 liters per 100 km. Lower L/100 km means better economy, the opposite of mpg.
Are my numbers saved?
No. The calculation runs in your browser and nothing you enter is stored or sent anywhere.