MPG calculator
Fill up, note the miles on your trip meter at the next fill-up and the gallons it took, and you'll see your car's real fuel economy. Add a few fill-ups for an average, and the price to see your cost per mile.
On the trip meter since the last full tank.
On the pump or the receipt.
For your cost per mile and per 100 miles.
Combined figure from the window sticker or fueleconomy.gov.
In every unit
- Miles per gallon (US)
- 27.4 MPG
- Liters per 100 km
- 8.59 L/100 km
- Kilometers per liter
- 11.64 km/L
- Miles per gallon (UK)
- 32.9 MPG
UK MPG uses the larger imperial gallon (4.546 liters), so it comes out about 20% higher than US MPG.
CO₂ from this fuel
- Total
- 101.3 kg (223 lb)
- Per mile
- 325 g
EPA figures: about 8,887 g of CO₂ per gallon of gasoline burned.
- Nothing is uploaded
- Free, no sign-up
- Works on phone and desktop
How to measure your car's real MPG
- 1
Fill the tank until the pump clicks off, then reset your trip meter.
- 2
Drive as you normally do. At the next fill-up, fill until it clicks off again.
- 3
Enter the miles from the trip meter and the gallons from the pump. Add more fill-ups for a steadier average.
- 4
Add the price per gallon for your cost per mile, and your rated MPG to see how you compare.
Features
Every unit at once
MPG, liters per 100 km, km per liter and UK MPG side by side, in miles and gallons or kilometers and liters.
Average of several fill-ups
Add up to 12 fill-ups. The average is total miles over total gallons, so short tanks don’t skew it.
Cost per mile
Enter what you pay at the pump to see fuel cost per mile, per 100 miles and per 1,000 miles.
CO₂ and the rating
See the CO₂ from the fuel you burned, using EPA figures, and how your mileage compares with the sticker.
The full-tank method, step by step
Your trip computer gives an estimate. To measure what your car really uses, track the fuel you put in:
- Fill up until the pump clicks off. Don't round up or squeeze in more, just stop at the first click.
- Reset the trip meter, or write down the odometer reading.
- Drive normally. The longer you go before the next fill-up, the smaller any error from the pump's click-off becomes, so wait until the tank is at least half empty.
- Fill up again until it clicks off, at the same station and pump if you can. Note the gallons on the pump and the miles on the trip meter.
- Divide the miles by the gallons. That's your MPG for that tank.
A worked example with three fill-ups
Say your log shows 312 miles on 11.4 gallons, then 298 miles on 10.9 gallons, then a short tank of 120 miles on 5.1 gallons in town. The three tanks work out at 27.4, 27.3 and 23.5 MPG.
The right average is total miles over total gallons: 730 ÷ 27.4 = 26.6 MPG. Averaging the three MPG figures would give 26.1 MPG, because it treats the short town tank as if it mattered as much as the long highway ones. The calculator always uses the totals.
MPG in other units
| US MPG | L/100 km | km/L | CO₂ per mile |
|---|---|---|---|
| 15 | 15.7 | 6.4 | 592 g |
| 20 | 11.8 | 8.5 | 444 g |
| 25 | 9.4 | 10.6 | 355 g |
| 30 | 7.8 | 12.8 | 296 g |
| 35 | 6.7 | 14.9 | 254 g |
| 40 | 5.9 | 17 | 222 g |
| 50 | 4.7 | 21.3 | 178 g |
CO₂ uses the EPA's figure for gasoline. UK MPG is based on the imperial gallon, which is about 20% bigger than the US gallon, so 30 US MPG is 36 UK MPG.
What lowers gas mileage
- Speed: fueleconomy.gov notes that gas mileage usually drops quickly above 50 mph.
- Driving style: hard acceleration and braking can cost roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic, according to fueleconomy.gov.
- Short trips and cold starts: an engine uses more fuel until it warms up, so winter and short errands bring the average down.
- Weight and drag: roof boxes, bike racks, towing and a full trunk all cost fuel.
- Tires: underinflated tires add rolling resistance. Check the pressure against the placard in the driver's door jamb, with the tires cold.
From MPG to cost per mile
Divide the price per gallon by your MPG to get the fuel cost of each mile: at $3.25 a gallon and 26.6 MPG that's about 12.2 cents a mile. Plug the same MPG into the gas cost calculator to price a road trip, and check the tire size calculator before you change tire sizes, since a taller or shorter tire throws the odometer off.
Frequently asked questions
How do I calculate my MPG?
Divide the miles you drove by the gallons it took to fill the tank again. 312 miles on 11.4 gallons is 27.4 MPG. For it to be accurate, fill the tank completely at the start and at the end.
Why do I have to fill the tank all the way both times?
Because the second fill-up only measures the fuel you burned if the tank was full when you started and is full again when you stop. If you top up halfway, add those gallons to the next full fill-up and count all the miles since the last full tank.
Why is my MPG lower than the EPA rating?
EPA ratings come from standardized lab tests. Short trips, cold weather, idling, high highway speeds, roof racks, heavy loads and underinflated tires all cost fuel. fueleconomy.gov notes that aggressive driving can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic.
Is my car's trip computer accurate?
It's usually close, but it estimates fuel from the engine's injectors rather than measuring what goes into the tank, and it can read a little high or low. Check it against the full-tank method over a few fill-ups.
How do I convert MPG to L/100 km?
Divide 235.2 by the MPG: 25 MPG is 9.4 L/100 km and 40 MPG is 5.9 L/100 km. The same division works the other way. For km per liter, multiply MPG by 0.425.
How much CO₂ does my car produce?
The EPA puts it at about 8,887 grams of CO₂ per gallon of gasoline and 10,180 grams per gallon of diesel. At 25 MPG that's about 355 grams per mile. The EPA's typical car, at 22.2 MPG over about 11,500 miles, emits about 4.6 metric tons a year.
Can new tires change my MPG reading?
Yes. Your odometer counts wheel turns, so a tire with a different overall diameter makes it read a little high or low, and the MPG with it. The tire size calculator shows how big the difference is.
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