What is an emission factor?

Quick Answer: An emission factor is a coefficient that converts a unit of activity (such as kilometres driven, kilowatt-hours of electricity consumed, or pounds spent with a supplier) into a quantity of greenhouse gas emissions, expressed in kilograms or tonnes of CO2 equivalent (CO2e). Emission factors are the building blocks of any carbon footprint calculation, translating real-world activity data into a standardised measure of climate impact. They are published by bodies such as the UK Government (DESNZ/DBET), the US EPA, and the IPCC, and are updated periodically as energy grids and production processes change.

What is an emission factor?

An emission factor is a numerical value that represents the average greenhouse gas emissions associated with a specific activity or process. Multiply your activity data by the relevant emission factor and you get your emissions figure in CO2e.

The basic formula looks like this:

Activity data × Emission factor = GHG emissions (CO2e)

For example, if a company drives 10,000 kilometres in a petrol car, and the emission factor for an average petrol car is 0.17092 kg CO2e per kilometre (UK Government, 2024), the resulting emissions figure is approximately 1,709 kg CO2e, or 1.71 tonnes CO2e.

Emission factors account for all relevant greenhouse gases (carbon dioxide, methane, nitrous oxide and others), each weighted by its global warming potential (GWP) relative to CO2. This weighting is what allows different gases to be expressed as a single CO2e figure.

Where do emission factors come from?

Emission factors are derived from scientific measurement and modelling, then compiled and published by government bodies and research organisations. The most commonly used sources include:

• UK Government (DESNZ/DBET): Annual conversion factors for UK-based carbon reporting, covering electricity, fuels, transport, waste, water, and business travel. Updated each year to reflect changes in the UK grid mix.
• US EPA: Emission factors used widely in North American reporting, published through the Emissions & Generation Resource Integrated Database (eGRID) and related resources.
• IPCC: Provides default emission factors used in national greenhouse gas inventories, particularly relevant for fuel combustion.
• Ecoinvent and similar databases: Life cycle assessment (LCA) databases that provide detailed, process-level emission factors, often used for Scope 3 supply chain calculations.

The source you use matters. UK companies reporting under frameworks such as SECR or PPN 006 are expected to use the UK Government's published conversion factors where available. Using a different source without justification can undermine the credibility of a reported footprint.

What types of emission factors are there?

Not all emission factors are the same. The type you use depends on the data you have available and the level of accuracy you are trying to achieve.

Spend-based factors convert financial spend (for example, £1,000 spent with a haulage supplier) into an estimated emissions figure. These are the least precise option and are typically used as a starting point for Scope 3 categories where more specific data is unavailable.

Average-data factors are the most commonly used type. They represent the average emissions intensity of a product, material, or activity, such as the average emissions per tonne of steel produced or per litre of diesel consumed. These sit in the middle ground: more accurate than spend-based, but less precise than supplier-specific data.

Supplier-specific factors (sometimes called product carbon footprints or PCFs) reflect the actual emissions intensity of a specific supplier's product or process. These require the supplier to have measured and disclosed their own emissions data, which most have not yet done. Where available, they produce the most accurate Scope 3 figures.

Grid emission factors are a specific type used for electricity consumption. They reflect the carbon intensity of the electricity grid in a given country or region, and change over time as the proportion of renewables in the grid mix increases. The UK grid factor has fallen significantly over the past decade as coal generation has been phased out.

Why does the choice of emission factor matter?

Using an outdated or mismatched emission factor introduces error into a carbon footprint at the point of calculation. Two companies with identical energy consumption can report different emissions figures simply by using different factors, or by using a factor from a different year.

This is why methodology transparency matters. A credible carbon footprint should document which emission factors were used, which version or year they are drawn from, and what assumptions were made where a precise factor was unavailable. Without that documentation, a reported figure is difficult to verify, reproduce, or compare year-on-year.

Accuracy also has practical consequences for decarbonisation. If a company uses a spend-based factor to estimate its freight emissions and the figure is significantly understated, it may deprioritise logistics as a reduction target when it should be a primary focus. Better data leads to better decisions.

Seedling applies up-to-date, published emission factors aligned with the GHG Protocol and documents the source of every factor used, so companies can show stakeholders exactly how each figure was calculated.

How do emission factors connect to Scope 1, 2, and 3 reporting?

Emission factors are used across all three scopes of a GHG inventory, but the type and source of factor typically varies by scope.

Scope 1 (direct emissions from owned or controlled sources) uses combustion factors for fuels such as natural gas, diesel, and petrol. These are well-established and relatively stable.

Scope 2 (indirect emissions from purchased electricity, heat, or steam) uses grid emission factors. Companies can report Scope 2 using either a location-based method (the average grid factor for their region) or a market-based method (which accounts for renewable energy certificates and supplier-specific tariffs).

Scope 3 (all other indirect emissions across the value chain) is where emission factor selection becomes most complex. With 15 categories covering everything from purchased goods to employee commuting to end-of-life treatment of sold products, Scope 3 requires a wide range of factors, many of which rely on industry averages rather than precise measurements. The quality of Scope 3 data is directly tied to the quality of the factors applied.

As supplier-level emissions data becomes more widely available, and as databases like Ecoinvent and industry-specific PCF initiatives mature, the accuracy of Scope 3 calculations will improve. For now, being transparent about which factors were used, and flagging where spend-based proxies were applied, is the standard expected by most reporting frameworks.