Emission factors (sometimes called GHG conversion factors) are the multipliers used to convert a unit of physical activity into a quantity of greenhouse gas emissions. They are the second half of the foundational equation in carbon accounting:
Activity Data × Emission Factor = GHG Emissions (tCO₂e)
Where they come from
In the UK, the most widely used source is the annual UK Government GHG Conversion Factors dataset, published by the Department for Energy Security and Net Zero (DEECC, formerly DEFRA / BEIS). It contains thousands of factors covering fuels, electricity, transport, refrigerants, materials, waste disposal routes, water, business travel, and homeworking.
Internationally, factors are also published by the IEA (electricity grids), EPA (US factors), Ecoinvent (life-cycle data), and various national inventories. Sector-specific or supplier-specific factors are used where they are more accurate than national averages.
Why it matters
The choice of emission factor materially changes the final result. Using an outdated grid factor, the wrong mode-of-transport factor, or a generic factor where a specific one exists can over- or under-report emissions by significant margins. Credible reports always state which factor source and version was used (e.g., UK Government GHG Conversion Factors 2024) so calculations are reproducible.
A practical example
Burning 1,000 litres of diesel × the DEECC diesel emission factor (~2.51 kgCO₂e per litre) = 2,510 kgCO₂e, or 2.51 tCO₂e. Change the fuel to petrol (~2.18 kgCO₂e per litre) and the same volume produces 2.18 tCO₂e — same activity data, different factor, different result.
We use the latest DEECC factors as our primary source on every engagement. See where they apply on the methodology page.