Emission factors explained: where the numbers come from
An emission factor is the bridge between what you used and the emissions it caused. Multiply 1 kWh of electricity by the grid's factor and you get kg CO₂e. Every product footprint is built from dozens of these lines, so the factors you choose decide much of the answer.
6 min read · Updated
What an emission factor is
An emission factor is the greenhouse gas emissions per unit of activity: per kilogram of polyester, per kilowatt-hour of electricity in a given country, per tonne-kilometre of road freight. It includes the whole chain behind that unit, for example the fuel mined and burned to make the electricity, and losses on the grid.
Factors are published by governments, industry associations, researchers and database providers, each with a stated scope, year and geography.
Why two factors for the same material disagree
- Boundary: one stops at the fibre, another includes spinning.
- Geography: cotton grown with rain and with irrigation, or electricity from coal and from hydro, give very different figures.
- Year: grids and processes change; a ten-year-old factor may no longer describe today.
- Allocation: how a shared process is split, such as leather between hide and meat.
- Warming factors: older figures use earlier IPCC values; current practice is AR6.
How to judge a factor
Ask four questions: who published it, what it includes, where and when it applies, and whether you are allowed to use it. Data quality ratings put a number on the first three. Our methodology page lists every factor we use with its source, year, geography and licence.
Where ours come from
We use public databases such as the UK government's conversion factors (DESNZ), Ember's electricity data, CEA India's grid database, US EPA eGRID, AGRIBALYSE and the IPCC's AR6 warming factors, plus published studies and product declarations (EPDs) for specific materials. Each one is credited on the footprint that uses it, and the full list is on our home page.
Supplier-specific factors beat all of these when you have them; see primary vs secondary data.
Questions
Are emission factors free?
Many public ones are, under open licences. Large commercial databases need a licence, and some do not allow their values to be shown in other software.
What is a life-cycle electricity factor?
One that includes the fuel supply chain and grid losses as well as the power station's own emissions. Product footprints need this kind.
Can I use my supplier's factor?
Yes, if it covers the same scope. It usually improves the footprint's data quality.