How to Calculate Your Carbon Footprint from Your Bills

Turn kWh, therms, gallons, miles and flights into tonnes of CO2e with EPA 2025 and DESNZ 2025 factors, and see which part of a household footprint is biggest.

2026-10-29 · 11 min read

Most carbon footprint quizzes ask a dozen lifestyle questions and return a number you cannot check. You can do better with a year of bills and a few published emission factors. Every kilowatt-hour, therm, gallon and passenger-kilometre has an official conversion to kilograms of carbon dioxide equivalent (CO₂e), and multiplying is all it takes. This guide walks through each part of a household footprint with the factors the US EPA and the UK government published in 2025, shows a worked example, and explains what the number does and does not include. The Carbon Footprint Calculator applies the same factors and shows them on the page.

What CO₂e means

Carbon dioxide is not the only greenhouse gas that burning fuel releases. Methane (CH₄) and nitrous oxide (N₂O) come out in small amounts, and each traps far more heat per kilogram. To add them up, each gas is multiplied by its global warming potential (GWP) over 100 years. The EPA GHG Emission Factors Hub uses the values from the IPCC Fifth Assessment Report: 1 for CO₂, 28 for methane and 265 for nitrous oxide. The total is reported as CO₂e.

For fossil fuels burned at home or in a car, CO₂ dominates: methane and nitrous oxide add well under 1% to the total. They matter more for food, where methane from cattle and nitrous oxide from fertiliser are large parts of the footprint.

Step 1: electricity

Find twelve months of kilowatt-hours on your bills and add them up. Then multiply by an emission rate for the grid that supplies you.

In the US the standard source is EPA's eGRID, the Emissions and Generation Resource Integrated Database. The 2025 Emission Factors Hub uses eGRID2023, which gives a US average of 771.5 lb CO₂, 0.057 lb CH₄ and 0.008 lb N₂O per megawatt-hour. With the GWPs above that is 775.2 lb CO₂e per MWh, or 0.352 kg CO₂e per kWh.

The average hides a wide spread. eGRID divides the country into 26 subregions, and the cleanest emits a sixth as much per kWh as the dirtiest:

In the UK, the DESNZ 2025 conversion factor for grid electricity is 0.177 kg CO₂e per kWh.

If you have rooftop solar or a certified renewable tariff, you can reduce the electricity line by the share it covers. That is the "market-based" method used in corporate reporting; the grid you are connected to still emits at its regional rate, which is the "location-based" view.

Step 2: heating fuels

Gas, oil and propane are burned in your home, so their emissions depend only on how much you use.

A therm is 100,000 Btu, or 0.1 million Btu, which is why the gas factor is about a tenth of the per-million-Btu figure. If your bill shows ccf (hundreds of cubic feet), multiply by about 1.036 to get therms; if it shows cubic metres, use about 1.94 kg CO₂e per m³ (based on EIA's average of 1,036 Btu per cubic foot).

Step 3: driving

For a petrol or diesel car, work out the fuel burned in a year and multiply. EPA's mobile combustion factors are 8.78 kg CO₂ per gallon of gasoline and 10.21 kg per gallon of diesel.

A car driven 11,000 miles at 28 mpg burns 11,000 / 28 = 393 gallons, which is 393 x 8.78 = 3,449 kg, or 3.45 tonnes.

For an electric car, multiply its electricity by your grid factor. At 30 kWh per 100 miles, 11,000 miles uses 3,300 kWh; on the US average grid that is 1.16 tonnes, and on the California grid 0.66 tonnes. The EV vs Gas Cost Calculator compares the two directly, and our EV vs gas guide goes into the costs.

These factors cover only what comes out of the tailpipe (or the power station). Extracting, refining and transporting fuel adds more, as does building the car.

Step 4: flights

Flights are the most uncertain part of a footprint and often the largest for frequent flyers. The most widely used per-passenger factors come from the UK Department for Energy Security and Net Zero (DESNZ), which publishes them every year for company reporting. The 2025 factors, per passenger-kilometre of great-circle distance, are:

Three things are worth knowing about these numbers. First, DESNZ already adds 8% to great-circle distances for indirect routes and holding patterns, so you enter the airport-to-airport distance without adding anything. Second, business and first class seats take up more of the cabin, so they are allocated more of the plane's fuel. Third, "RF" stands for radiative forcing: aircraft also warm the climate through contrails and nitrogen oxides at altitude. DESNZ represents this with a 1.7 multiplier on the CO₂ part, while noting that the size of the effect is uncertain. The calculator includes it by default and lets you switch it off.

A return flight between London and New York, about 5,570 km each way, is 11,140 passenger-km. In economy that is 1.30 tonnes with RF or 0.77 tonnes without. In business class it is 3.78 tonnes with RF.

Step 5: food

Food emissions are harder to measure from receipts, so most calculators use averages by diet type. The best-known figures come from Scarborough and colleagues, who analysed the diets of more than 55,000 people in the UK EPIC-Oxford study (Climatic Change, 2014). Standardised to a 2,000 kcal diet, they found:

The difference between a high-meat and a vegan diet is about 1.6 tonnes per person per year, comparable to a long-haul return flight with RF.

A worked example

Here is a two-person US household, which is the calculator's default:

• 900 kWh of electricity a month (10,800 a year) on the US average grid: 10,800 x 0.352 = 3.80 t
• 500 therms of gas: 500 x 5.31 = 2.66 t
• One car, 11,000 miles at 28 mpg: 3.45 t
• Two short-haul economy round trips of 900 miles each way (1,448 km), with RF: 4 x 1,448 x 0.12576 = 0.73 t
• Two people on a medium-meat diet: 2 x 5.63 x 365 = 4.11 t

The total is 14.7 tonnes of CO₂e a year, or 7.4 tonnes per person. Food is the largest single slice, followed closely by electricity and driving.

Move the same household to California, swap the car for an EV using 30 kWh per 100 miles, skip the flights and eat vegetarian, and the total falls to about 8.2 tonnes. The gas heating is now the biggest single item, which is where a heat pump would come in.

What this number leaves out

A bill-based footprint covers the emissions you can measure directly: home energy, personal travel and food. It does not cover:

• Goods and services you buy, such as clothes, electronics, furniture and the emissions embedded in their manufacture and shipping.
• Upstream fuel emissions, from extracting, refining and transporting fuel, and electricity lost in transmission.
• Public services, such as roads, schools, hospitals and the military, which national per-capita figures share out across the population.
• Waste and water.

That is why national per-capita figures, which include everything a country emits divided by its population, are higher than a household energy footprint. Treat the calculator's result as the part of your footprint you control most directly, and track how it changes as you change things.

Where to cut first

The breakdown tells you where the leverage is. In most homes, the biggest options are:

1. Heating: a heat pump replacing oil, propane or resistance heat cuts both cost and emissions; see heat pump vs gas running costs.
2. Driving: an EV on a clean grid, or simply fewer miles.
3. Flights: one fewer long-haul return trip can save more than a year of careful recycling.
4. Electricity: a cleaner tariff or rooftop solar.
5. Diet: shifting from high to low meat saves about 0.9 tonnes per person a year.

Sources

• US EPA, GHG Emission Factors Hub (January 2025): stationary and mobile combustion factors, eGRID2023 subregion output emission rates and AR5 global warming potentials.
• US EPA, Emissions and Generation Resource Integrated Database (eGRID2023).
• UK Department for Energy Security and Net Zero, Greenhouse gas reporting: conversion factors 2025, and the 2025 methodology paper (8% great-circle uplift, 1.7 radiative forcing multiplier).
• US Energy Information Administration, "Units and calculators explained": 1,036 Btu per cubic foot of natural gas.
• Scarborough P. et al. (2014), "Dietary greenhouse gas emissions of meat-eaters, fish-eaters, vegetarians and vegans in the UK", Climatic Change 125, 179-192.

Open the Carbon Footprint Calculator