Calculating CO2 emissions for a vehicle comes down to one core relationship: how much fuel the vehicle burns and how much carbon that fuel contains. For gasoline cars, every gallon burned produces 8.87 kilograms of CO2. For diesel, it’s 10.19 kilograms per gallon. Multiply the fuel you use by the right factor, and you have your emissions.
The Basic Formula for Gasoline and Diesel
The simplest calculation requires only two numbers: your fuel consumption and the carbon content of that fuel. The U.S. Energy Information Administration publishes standard emission factors for each fuel type. For finished motor gasoline, the factor is 8.87 kg of CO2 per gallon. For diesel, it’s 10.19 kg per gallon.
The math looks like this:
CO2 emissions = Gallons of fuel used × Emission factor per gallon
If you drove 12,000 miles last year and your car averages 30 miles per gallon, you used 400 gallons of gasoline. Multiply 400 by 8.87, and your vehicle produced roughly 3,548 kg (about 3.55 metric tons) of CO2.
You can also work backwards from fuel economy to get a per-mile figure. Divide the emission factor by your miles per gallon. A 25 MPG gasoline car emits about 355 grams of CO2 per mile (8,870 grams ÷ 25). A 35 MPG car drops to about 253 grams per mile. For context, the average new vehicle sold in the U.S. in model year 2023 produced 319 grams per mile, with preliminary 2024 data showing that dropping to 305 grams per mile.
Converting Between Units
Emissions are reported in different units depending on the context. Individual trips or per-mile comparisons typically use grams of CO2. Annual totals for personal vehicles are usually expressed in metric tons (1 metric ton = 1,000 kilograms = 1,000,000 grams). Corporate and fleet reporting almost always uses metric tons.
If you’re working in liters rather than gallons, divide the per-gallon factor by 3.785 (the number of liters in a gallon). That gives you roughly 2.34 kg of CO2 per liter of gasoline and 2.69 kg per liter of diesel.
Calculating Emissions for Electric Vehicles
Electric vehicles have no tailpipe emissions, but the electricity they consume is generated somewhere, and that generation often involves burning fossil fuels. The CO2 from an EV depends on two things: how efficiently the car uses electricity (measured in kilowatt-hours per mile) and how carbon-intensive your local electrical grid is.
EV CO2 emissions = kWh consumed per mile × Grid emission factor (grams CO2 per kWh)
A typical EV uses about 0.25 to 0.35 kWh per mile. The grid emission factor varies enormously by region. In areas powered mostly by renewables or nuclear, it can be under 100 grams of CO2 per kWh. In coal-heavy regions, it can exceed 700 grams per kWh. The EPA’s eGRID database provides emission factors by zip code, so you can look up the specific number for where you charge your car.
For plug-in hybrids, the calculation splits into two parts: electric miles and gasoline miles. The proportion depends on the vehicle’s all-electric range. A plug-in hybrid with 50 miles of electric range will run more of its total miles on electricity than one with 20 miles of range. You calculate each portion separately and add them together.
Freight and Heavy-Duty Vehicles
For trucks carrying cargo, emissions are calculated per ton-mile, which accounts for both distance and the weight of freight being moved. The average freight truck in the U.S. emits 161.8 grams of CO2 per ton-mile.
Freight CO2 = Weight of cargo (tons) × Distance (miles) × 161.8 grams per ton-mile
A truck hauling 20 short tons over 1,000 miles produces 20,000 ton-miles. Multiply by 161.8 grams, and you get about 3,236,000 grams, or 3.24 metric tons of CO2 for that single trip. This weight-based approach makes it possible to compare efficiency across different loads and routes. A half-empty truck covering the same distance produces fewer total emissions but far more emissions per ton of cargo delivered.
Tailpipe Versus Full Life Cycle
The formulas above measure what comes out of the tailpipe (or smokestack, for EVs). This is called “tank-to-wheel” accounting. It captures the CO2 released when fuel is burned in the engine but ignores the emissions from extracting, refining, and transporting that fuel to the gas station. That upstream phase is called “well-to-tank.”
For gasoline and diesel vehicles, the upstream portion is relatively small compared to combustion, which is why most standard calculations focus on tailpipe emissions. But if you want a complete picture, especially when comparing gasoline cars to EVs, you need the “well-to-wheel” total. This adds roughly 20 to 25% more emissions for fossil fuels, depending on the source and refining process. For EVs, the well-to-wheel view is essentially the full picture, since all emissions happen at the power plant and in fuel supply chains rather than at the vehicle itself.
Beyond CO2: Calculating CO2 Equivalent
Vehicles also emit small amounts of methane and nitrous oxide, both of which trap more heat per molecule than CO2. To account for these, scientists convert them into “CO2 equivalent” using global warming potential multipliers. The most current values from the IPCC’s Sixth Assessment Report assign methane a factor of 27 (meaning one ton of methane warms the climate as much as 27 tons of CO2 over 100 years) and nitrous oxide a factor of 273.
For most passenger vehicles, these non-CO2 greenhouse gases add only a small percentage on top of the CO2 total. But for fleet reporting or regulatory compliance, you may need to include them. The formula is straightforward: multiply the mass of each gas emitted by its global warming potential, then add everything together to get a single CO2-equivalent number.
Putting It Into Practice
For a quick personal estimate, you only need your annual fuel purchases. Check your receipts, your fuel tracking app, or simply divide your annual mileage by your car’s real-world fuel economy. Multiply gallons by 8.87 for gasoline or 10.19 for diesel, and you have your annual CO2 output in kilograms. Divide by 1,000 to convert to metric tons.
If you want a per-trip calculation, the same logic applies on a smaller scale. A 300-mile road trip in a car that gets 28 MPG uses about 10.7 gallons of gasoline, producing roughly 95 kg of CO2. Splitting that among four passengers brings each person’s share down to about 24 kg, which is useful for comparing driving against flying or taking a train.
For fleet managers and businesses tracking emissions across many vehicles, the fuel-based method remains the most accurate starting point. Track total fuel purchased across all vehicles, apply the emission factors, and you have your fleet’s CO2 footprint without needing to monitor individual trips. If you need per-vehicle or per-route breakdowns, telematics systems that log fuel consumption by vehicle make the calculation automatic.