Wondering how a carbon footprint calculation actually works? You multiply activity data, such as fuel burned or electricity bought, by an emission factor, then convert each gas into carbon dioxide equivalent and add it all up.
This guide is part of the wider topic of what is sustainability, and it focuses on the maths behind measuring emissions.
| Item | Value |
|---|---|
| Key steps in a GHG inventory process | 4 steps |
| Global warming potential of carbon dioxide | 1 (the baseline) |
| Global warming potential of methane | 25 (CO₂ = 1) |
| Global warming potential of nitrous oxide | 298 (CO₂ = 1) |
| Global warming potential of sulfur hexafluoride | 22,800 (CO₂ = 1) |
The step count comes from EPA’s archived guidance, and the warming values from the WBDG greenhouse gas accounting page. Figures can change, so check the source before you rely on them.
The basic maths behind a carbon footprint calculation
Every footprint starts with activity data: how much fuel was burned, how much electricity was bought, how far a vehicle travelled. Each activity is multiplied by an emission factor to give emissions. The WBDG page writes this as Activity x Emission Factor = Emissions, measured in tons of greenhouse gas (WBDG).
An emission factor is the quantity of a greenhouse gas released to the atmosphere for a specific activity, according to EPA’s archived inventory guidance. Two of its examples are the carbon dioxide released per unit of natural gas burned in a boiler and the carbon dioxide released per megawatt-hour of electricity generated.
EPA also publishes an Emission Factors Hub, described in the same archived page as a regularly updated, consolidated set of default factors for organizational reporting. Using a published set like this keeps your numbers traceable.
Which emissions count: scopes 1, 2 and 3
Before you calculate anything, you sort emissions into scopes. This is the standard grouping used in greenhouse gas accounting, and it decides what goes into your total (WBDG).
- Scope 1 covers direct emissions from sources an organization owns or controls, such as fuel combustion in boilers, furnaces and vehicles.
- Scope 2 covers indirect emissions linked to purchased electricity, steam, heat or cooling.
- Scope 3 covers everything else in the value chain, from assets the organization does not own or control but indirectly affects.
WBDG lists business travel, including ground transportation and air travel, among its scope 3 examples. EPA notes that scope 3 emissions, also called value chain emissions, often represent the majority of an organization’s total emissions, so leaving them out can hide most of the picture (EPA archive).
Step by step: how an inventory is built
A greenhouse gas inventory is a list of emission sources and the emissions quantified using standardized methods. EPA’s archived page, which reflects the site as it existed on January 19, 2025, describes a process of four key steps.
- Review accounting standards and methods, determine organizational and operational boundaries, and choose a base year.
- Collect data and quantify emissions.
- Develop an inventory management plan to formalize data collection procedures.
- Set an emission reduction target, then track and report progress.
The boundary decision comes first. EPA says setting organizational boundaries is necessary before identifying any emission sources or collecting data (EPA archive).
The data step is where the calculation happens. The same source breaks it into tasks:
- Compile and review facility data, such as electricity and natural gas use.
- Estimate missing data to fill gaps.
- Choose emission factors.
- Calculate emissions.
Smaller organizations have a shortcut. EPA’s Simplified GHG Emissions Calculator is designed to help small businesses and low emitters estimate and inventory their annual emissions.
Why gases are converted to carbon dioxide equivalent
Burning fuel releases more than one gas, and the gases do not warm the atmosphere equally. To add them together, each is multiplied by its global warming potential (GWP) to give carbon dioxide equivalent (CO₂e). WBDG writes this as Emissions x Global Warming Potential = Total Emissions in tons of CO₂e (WBDG).
The same page lists several common values:
- Carbon dioxide: 1
- Methane (CH₄): 25, linked to coal mining, fuel combustion, landfills and wastewater treatment
- Nitrous oxide (N₂O): 298, linked to fuel combustion and fertilizers
- Hydrofluorocarbons (HFCs): 12-14,800, found in refrigerants and fire suppressants
- Sulfur hexafluoride (SF₆): 22,800, used in electrical equipment
Those values come with a caveat. WBDG says each protocol will have recommended GWP factors, so the same inventory can be reported differently depending on the protocol used. Always note which set was applied.
A worked example for a small workshop
Picture a small workshop that wants a footprint. It has a gas boiler, a delivery van, a refrigeration unit and a utility bill, and staff sometimes fly to meetings.
- Set the boundary: decide which sites and vehicles belong in the inventory.
- Sort by scope: the boiler and van are scope 1, the purchased electricity is scope 2, and staff flights are scope 3.
- Gather activity data: cubic metres of gas, litres of fuel, kilowatt-hours of power, and distances flown.
- Apply emission factors: for electricity, WBDG points to the U.S. EPA eGRID database for factors that convert purchases into emissions.
- Convert and sum: multiply each gas by its GWP, then add everything into one CO₂e total.
Refrigerant leaks matter here too. WBDG classes fugitive emissions from refrigerants as scope 1, and the HFC range above shows why even small leaks can carry weight.
How to read a published footprint critically
A single headline figure tells you little without its method. Knowing the steps above makes it easier to spot vague claims, and the guide on what is greenwashing covers the wider pattern.
Ask these questions of any number you see:
- Which boundary was used, and which entities were included?
- Were scope 3 emissions counted, or only scopes 1 and 2?
- Which base year and emission factors were applied, and where did they come from?
- Did a third party verify the result? EPA lists third-party verification as something to consider at the start (EPA archive).
A footprint measures emissions, not how materials are kept in use. That separate question is explored in the guide to the circular economy.
Frequently asked questions
What does CO₂e mean?
Show answer
It stands for carbon dioxide equivalent, a single unit that lets different greenhouse gases be added together. Each gas is multiplied by its global warming potential first (WBDG).
Where do emission factors come from?
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They can be calculated from direct measurement, the measured carbon or energy content of a fuel, or the chemical formula. WBDG also points to EPA’s Emission Factors Hub for an easy-to-use set of defaults (WBDG).
Is the GHG Protocol the only way to calculate a footprint?
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No. WBDG notes that a variety of accounting protocols are available, and names the GHG Protocol Corporate Standard from the World Resources Institute and the World Business Council for Sustainable Development as one of them. As of EPA’s archived page dated January 19, 2025, EPA described that standard as the global standard for calculating corporate emissions (EPA archive).
Do I have to include scope 3?
Show answer
EPA defines scope 3 as all sources outside scopes 1 and 2 and says it often makes up the majority of an organization’s total emissions. Leaving it out can therefore hide most of a footprint, so state clearly if you do.
