What this calculator does
Get a rough idea of how much carbon dioxide (CO2) your household puts out each year from energy and travel. Most of it shows up on a handful of meters and receipts. Those are the electricity bill, the gas or heating-oil bill, the fuel that goes into the car and the flights you take during the year.
Each of those amounts can be turned into CO2 with an emission factor. That's the kilograms of CO2 released per kilowatt-hour, therm, gallon or passenger-mile. This calculator multiplies your yearly use by those factors and adds them up.
The default factors come from the US Environmental Protection Agency's 2025 GHG Emission Factors Hub. (GHG stands for greenhouse gas.) Each one is an input you can change. The electricity factor varies a lot by region. So swapping the national average for your utility's or your grid region's figure gives a better estimate.
Where it's used
Why this math matters outside the classroom:
- Household energy audit. A family multiplies 900 kWh a month by 12 and by 0.35 kg per kWh to find their electricity adds 3,780 kg of CO2 a year.
- Choosing a car. A driver compares 12,000 miles at 25 mpg (480 gallons) with 40 mpg (300 gallons), saving about 1,580 kg of CO2 a year.
- School sustainability projects. Students collect their families' utility bills and compare the yearly CO2 per person across the class.
- Travel decisions. A traveler sees that 2,000 medium-haul flight miles add 258 kg of CO2, about the same as 29 gallons of gasoline.
- Small offices. An office manager estimates the CO2 from the building's gas heating to report it in a simple sustainability summary.
Grade level and standards: Grade 7; Common Core State Standards for Mathematics 7.EE.B.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Enter the number of people in the household, so the result can also be shown per person.
- Enter monthly electricity (kWh) and natural gas (therms) from your bills. Add yearly heating oil or propane in gallons, if you use them.
- Enter the miles your household drives in a year, the average fuel economy and the fuel type. Then enter the miles flown in each distance band.
- Change any emission factor if you have a better one. Then read the yearly total, the per-person figure and the breakdown by source.
What the inputs mean
- Electricity used per month
- Your bill shows the kWh (kilowatt-hours) you used each month. Use a yearly average: add up twelve bills and divide by 12.
- Electricity factor (EPA 2025, US average grid: 0.35)
- The EPA's 2025 hub lists the US average grid at 771.5 lb CO2 per MWh (from eGRID2023). That's 0.35 kg per kWh. Regional grids in the same table range from about a third of that to nearly double it. A renewable-energy plan can be lower still.
- Natural gas used per month
- Gas bills usually show therms. If yours shows hundred cubic feet (ccf), 1 ccf is roughly 1.04 therms. Your bill may state the exact conversion.
- Miles driven per year (all household cars)
- Total miles for all household gasoline or diesel vehicles. Charging an electric car at home is already counted in the electricity figure.
- Short-haul factor (EPA 2025: 0.207)
- Short flights have a higher factor per mile, because take-off and climb use a large share of the fuel.
- Flight miles per year: medium trips (300–2,300 mi)
- Count each passenger's miles: two people flying a 1,000-mile trip there and back is 4,000 passenger-miles. The EPA sorts flights by one-way trip length: under 300 miles, 300 to 2,300, and 2,300 or more.
Formula and method
electricity CO2 = kWh per month × 12 × factor natural gas CO2 = therms per month × 12 × factor heating CO2 = oil gallons × factor + propane gallons × factor driving CO2 = (miles ÷ mpg) × fuel factor flight CO2 = Σ (miles in each band × band factor) total (t) = sum of all sources ÷ 1,000
Each source is how much you used times its emission factor. Monthly bills are multiplied by 12 to make a year. Σ means 'add up', so flight CO2 adds the result for each distance band. The total in kilograms is divided by 1,000 to give tonnes (t).
Driving is turned into gallons of fuel first: miles divided by mpg (miles per gallon). That's because burning a gallon of gasoline always gives the same CO2 (8.78 kg), and so does diesel (10.21 kg). It's fixed by the carbon in the fuel.
The default factors count carbon dioxide only. Methane and nitrous oxide from the same activities add a small extra amount. The extra warming effect of planes at high altitude, beyond their CO2, isn't included.
Worked example
A two-person household uses 900 kWh of electricity and 40 therms of gas a month, drives 12,000 miles a year at 25 mpg on gasoline, and flies 2,000 medium-haul passenger-miles.
- Electricity: 900 × 12 × 0.35 = 3,780 kg.
- Natural gas: 40 × 12 × 5.306 = 2,546.88 kg.
- Driving: 12,000 ÷ 25 = 480 gallons, and 480 × 8.78 = 4,214.4 kg.
- Flights: 2,000 × 0.129 = 258 kg.
- Total: 3,780 + 2,546.88 + 4,214.4 + 258 = 10,799.28 kg, which is 10.80 tonnes, or 5.40 tonnes per person.
Result: The household's listed activities come to about 10.80 tonnes of CO2 a year.
Understanding the result
The breakdown shows where cutting back would matter most. In the example, driving and electricity together make up nearly three-quarters of the total.
This is the direct footprint from energy and travel. Food, goods, services and waste also cause emissions that this calculator doesn't estimate. So a household's full footprint is larger.
Assumptions and limitations
- Default factors are from the US EPA GHG Emission Factors Hub (2025) and count CO2 only.
- Electricity uses one average grid factor for the whole year.
- Fuel economy is an average across all the household's driving.
- Food, shopping, waste and other indirect emissions aren't included.
- The non-CO2 effects of flights and methane leaks from gas systems aren't counted.
- Results depend on the factors chosen. Compare estimates only when they use the same factors.
Frequently asked questions
Where do the default numbers come from?
From the US EPA's GHG Emission Factors Hub, 2025 edition. That covers the US average grid factor (eGRID2023) and the CO2 per therm of natural gas. It also covers the CO2 per gallon of heating oil, propane, gasoline and diesel, and the air-travel factors per passenger-mile. Each label shows the default, so you can see what you change.
How do I find my local electricity factor?
Your utility may publish its emissions per kWh. The EPA's eGRID data also lists factors for each grid region of the US. Enter that figure in place of the national average.
Why is CO2 measured in tonnes?
Household totals run to thousands of kilograms, so metric tonnes (1,000 kg) are easier to read. The total in kilograms is also listed.
Does an electric car count?
Yes, through electricity. If you charge at home, the charging is part of your kWh. So leave its miles out of the driving section, or you'd count it twice.
Sources
- US EPA: GHG Emission Factors Hub (2025) (checked 2026-10-19)
How this calculator is checked
Method: Activity × emission factor, summed. Default CO2 factors from the US EPA GHG Emission Factors Hub (January 2025): Table 1 (natural gas 53.06 kg/mmBtu, distillate No. 2 10.21 kg/gal, propane 5.72 kg/gal), Table 2 (gasoline 8.78, diesel 10.21 kg/gal), Table 6 (US average 771.5 lb CO2/MWh, eGRID2023), Table 10 (air travel 0.207 / 0.129 / 0.163 kg per passenger-mile). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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