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Formula
CO2 = (Distance ÷ Fuel economy) × Emission factor
- Distance
- miles (or km) driven on the trip
- Fuel economy
- the car's miles per gallon; L/100 km and km/L are converted to gallons used
- Emission factor
- 8,887 g CO2 per gallon of gasoline or 10,180 g per gallon of diesel (EPA)
What the car carbon emissions calculator measures
Every gallon of fuel your car burns turns into a predictable amount of carbon dioxide. The car carbon emissions calculator uses that fact to estimate the CO2 coming out of your tailpipe for one drive, or for a whole year of driving, from three things you already know: how far you go, how many miles per gallon the car gets and which fuel it uses.
Results come in pounds or kilograms, with metric tons for yearly totals, grams per mile or per kilometer, the fuel burned, and a comparison with the number of young city trees it would take a decade to absorb the same CO2.
How to use the calculator
- Enter the distance for one trip in miles or kilometers. For a commute, use the round-trip distance.
- Optionally enter trips per year. About 250 suits a five-day commute. You can also enter your annual odometer mileage as one trip and leave this blank.
- Enter the fuel economy as mpg, L/100 km or km/L. Your own measured average is more realistic than the window sticker.
- Pick gasoline or diesel and the unit you want the CO2 shown in.
With trips filled in, the headline figure is your yearly CO2; otherwise it’s the CO2 for a single trip. The working underneath shows the gallons and the multiplication.
How to calculate car CO2 emissions by hand
There are two steps: find the fuel, then multiply by the emission factor.
Gallons = Miles ÷ mpg, then CO2 (g) = Gallons × 8,887 for gasoline or × 10,180 for diesel.
Take a 180-mile drive in a car that gets 24 mpg:
- Fuel: 180 ÷ 24 = 7.5 gallons
- CO2: 7.5 × 8,887 = 66,653 g, or about 66.7 kg
- In pounds: 66.7 × 2.2046 = about 147 lb
Metric shortcut
A liter of gasoline produces about 2,348 g of CO2 and a liter of diesel about 2,689 g (the EPA gallon figures divided by 3.785). A 400 km drive at 6.5 L/100 km uses 26 liters, so 26 × 2.348 = about 61 kg of CO2.
A year of driving
For an annual footprint, run your yearly miles through the same formula. Driving 12,000 miles at 30 mpg burns 400 gallons, which is 400 × 8.887 = 3,555 kg, or about 3.55 metric tons (7,837 lb). Using the EPA’s tree factor, that’s the CO2 taken up by roughly 59 urban seedlings over 10 years.
Car CO2 per mile by fuel economy
Grams per mile, which the car carbon emissions calculator shows under every result, is the easiest way to compare cars, because it doesn’t depend on how far you drive. The table uses the EPA gasoline factor; yearly figures assume 10,000 miles.
| Fuel economy | g CO2 per mile | lb CO2 per mile | lb per 10,000 mi | Metric tons per 10,000 mi |
|---|---|---|---|---|
| 15 mpg | 592 | 1.31 | 13,062 | 5.92 |
| 20 mpg | 444 | 0.98 | 9,796 | 4.44 |
| 25 mpg | 355 | 0.78 | 7,837 | 3.55 |
| 30 mpg | 296 | 0.65 | 6,531 | 2.96 |
| 35 mpg | 254 | 0.56 | 5,598 | 2.54 |
| 40 mpg | 222 | 0.49 | 4,898 | 2.22 |
| 50 mpg | 178 | 0.39 | 3,918 | 1.78 |
For diesel, the same mpg gives about 15% more CO2: 509 g per mile at 20 mpg and 339 g at 30 mpg.
Notice the shape of the curve. Moving from 15 to 20 mpg cuts nearly 150 g per mile, while going from 40 to 50 mpg saves only 44 g. Replacing the thirstiest vehicle in a household usually does more than upgrading an already efficient one.
Why fuel is the whole story at the tailpipe
The emission factor is basic chemistry. Gasoline is mostly carbon and hydrogen. When it burns, nearly all the carbon combines with oxygen from the air, and because each CO2 molecule weighs about 3.7 times as much as its carbon atom, a 6-pound gallon of gas produces about 19.6 pounds of CO2. Diesel holds more carbon per gallon, so it produces about 22.4 pounds.
That means the only ways to lower tailpipe CO2 are to burn less fuel: drive fewer miles, drive a more efficient car, or drive the one you have more efficiently. Fueleconomy.gov notes that aggressive driving can cut highway mileage by 15% to 30%, and that mileage usually drops quickly above 50 mph.
Real-life uses
- Personal carbon footprint: enter your annual mileage to see how much of your footprint comes from driving.
- Comparing two cars: run the same yearly mileage with each car’s mpg before you buy.
- Trip planning: check the CO2 of a road trip, or compare driving with other options. The gas cost calculator prices the same gallons.
- Commuting choices: see what carpooling two days a week would save in a year.
Common mistakes
- Using the sticker rating instead of real mpg. Measure your own with the MPG calculator at your next fill-up.
- Mixing UK and US mpg. A UK gallon is about 20% bigger, so UK mpg figures overstate US efficiency.
- Entering one-way commutes. Double the distance, or the yearly figure comes out at half.
- Treating the result as lifecycle emissions. This is tailpipe CO2 only, not the full footprint of making the car and the fuel.
Glossary
- Emission factor: grams of CO2 released per gallon (or liter) of fuel burned.
- Metric ton (t): 1,000 kg, about 2,205 lb. Carbon footprints are usually given in metric tons.
- Tailpipe emissions: gases leaving the exhaust while driving, as opposed to upstream or lifecycle emissions.
- g/km and g/mile: CO2 per unit of distance; 1 g/mile equals about 0.62 g/km.
Worked examples
A weekend road trip
A 320-mile round trip in a gasoline car averaging 27 mpg.
Inputs: Distance per trip = 320; Distance per trip unit = Miles; Fuel economy = 27; Fuel economy unit = Miles per gallon (mpg); Fuel type = Gasoline (8,887 g CO2/gal, EPA); Show CO2 in = Pounds (lb)
Fuel = 320 mi ÷ 27 mpg = 11.8519 gal CO2 = 11.8519 gal × 8.887 kg/gal = 105.33 kg = 232.21 lb Trees = 0.1053 t ÷ 0.060 t per tree = 1.8
Answer: Driving 320 mi at 27 mpg on gasoline burns 11.85 gallons and emits about 232.2 lb of CO2.
A year of commuting
A 34-mile round-trip commute, 240 working days a year, in a 31 mpg sedan: 21.5 lb a day, about 2.34 metric tons a year.
Inputs: Distance per trip = 34; Distance per trip unit = Miles; Trips per year (optional) = 240; Fuel economy = 31; Fuel economy unit = Miles per gallon (mpg); Fuel type = Gasoline (8,887 g CO2/gal, EPA); Show CO2 in = Pounds (lb)
Fuel = 34 mi ÷ 31 mpg = 1.0968 gal CO2 = 1.0968 gal × 8.887 kg/gal = 9.75 kg = 21.49 lb Per year = 9.75 kg × 240 = 2,339.3 kg Trees = 2.3393 t ÷ 0.060 t per tree = 39
Answer: Driving 34 mi at 31 mpg on gasoline burns 1.1 gallons and emits about 21.5 lb of CO2, or 5,157 lb a year over 240 trips.
A diesel pickup in metric units
500 km in a diesel pickup using 11 L/100 km.
Inputs: Distance per trip = 500; Distance per trip unit = Kilometers; Fuel economy = 11; Fuel economy unit = Liters per 100 km (L/100 km); Fuel type = Diesel (10,180 g CO2/gal, EPA); Show CO2 in = Kilograms (kg)
Fuel for 500 km at 11 L/100 km = 55 L = 14.5295 gal CO2 = 14.5295 gal × 10.18 kg/gal = 147.91 kg Trees = 0.1479 t ÷ 0.060 t per tree = 2.5
Answer: Driving 500 km at 11 L/100 km on diesel burns 14.53 gallons and emits about 147.9 kg of CO2.
A short errand in a hybrid (edge case)
A 3-mile errand in a 50 mpg hybrid burns 0.06 gallons; small results are shown to two decimals.
Inputs: Distance per trip = 3; Distance per trip unit = Miles; Fuel economy = 50; Fuel economy unit = Miles per gallon (mpg); Fuel type = Gasoline (8,887 g CO2/gal, EPA); Show CO2 in = Pounds (lb)
Fuel = 3 mi ÷ 50 mpg = 0.06 gal CO2 = 0.06 gal × 8.887 kg/gal = 0.53 kg = 1.18 lb Trees = 0.0005 t ÷ 0.060 t per tree = 0
Answer: Driving 3 mi at 50 mpg on gasoline burns 0.06 gallons and emits about 1.18 lb of CO2.
Assumptions and limitations
- Emission factors are the EPA's 8,887 grams of CO2 per gallon of gasoline and 10,180 grams per gallon of diesel. They assume all the carbon in the fuel is burned to CO2.
- Only tailpipe CO2 is counted. Methane and nitrous oxide from the exhaust, refrigerant leaks, and the emissions from drilling, refining and shipping the fuel or building the car are not included.
- Gasoline means ordinary pump gas. Blends with ethanol (E10 is typical in the US) are treated the same as the EPA factor.
- The fuel economy you enter is treated as constant for the whole distance. Units are converted exactly (1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, 1 lb = 0.45359237 kg).
- When you enter trips per year, the yearly total becomes the main result and is the trip figure multiplied by the number of trips.
- The tree comparison uses the EPA equivalency of 0.060 metric ton of CO2 absorbed by one urban tree seedling grown for 10 years.
Frequently asked questions
How much CO2 does my car produce?
Divide the miles you drive by your mpg to get gallons, then multiply by 8,887 grams for gasoline. A car averaging 25 mpg emits about 355 grams of CO2 per mile, so 12,000 miles a year comes to roughly 4.3 metric tons.
How do you calculate car CO2 emissions per km?
Multiply your L/100 km figure by 23.48 to get grams per km for gasoline, or by 26.89 for diesel. Those numbers are the EPA per-gallon factors converted to grams per liter and divided by 100. A gasoline car using 7 L/100 km emits about 164 g/km.
How much carbon does a typical car emit in a year?
The EPA puts a typical passenger vehicle at about 4.6 metric tons of CO2 a year. That assumes 22.2 mpg and around 11,500 miles, which works out to about 400 grams per mile.
Why does a gallon of gas make about 20 pounds of CO2 when it weighs only about 6 pounds?
Most of the weight comes from the air. Each carbon atom in the fuel joins two oxygen atoms when it burns, and the oxygen makes CO2 about 3.7 times heavier than the carbon alone.
Is diesel worse for CO2 than gasoline?
Per gallon, yes. Diesel releases 10,180 g against 8,887 g for gasoline. Diesel engines usually get more miles per gallon, though, so per mile the gap is smaller and sometimes reversed. Compare cars on grams per mile, not per gallon.
Does this include electric cars?
No. An electric car has no tailpipe, so its CO2 depends on how your grid generates power. Estimate its electricity use and cost with the electricity cost calculator, then apply your utility's emission rate if you know it.
Sources
- Greenhouse Gas Emissions from a Typical Passenger Vehicle — U.S. Environmental Protection Agency. Checked October 10, 2026.
- Greenhouse Gas Equivalencies Calculator - Calculations and References — U.S. Environmental Protection Agency. Checked October 10, 2026.
- Gas Mileage Tips - Driving More Efficiently — U.S. Department of Energy and EPA (fueleconomy.gov). Checked October 10, 2026.