An emission factor is a number that converts an activity into greenhouse gas emissions, for example kilograms of CO₂e per kWh of electricity or per litre of diesel. Multiply your activity data by the right factor and you have your emissions. Every carbon footprint, from a one-person business to a country, is built this way.
Choosing the factor is where most footprint errors happen. This guide explains where the numbers come from, why they differ, and how to pick the right one for each line of your footprint.
The basic formula
Activity data × emission factor = emissions.
If you burn 300 litres of diesel and the factor is 2.68 kg CO₂e per litre, the emissions are 300 × 2.68 = 804 kg CO₂e. The activity data comes from your bills. The factor comes from a published dataset.
What is inside an emission factor
A factor expressed in CO₂e already combines several gases. Burning fuel releases mostly carbon dioxide, plus small amounts of methane and nitrous oxide. The published figure converts those to a single number using their global warming potentials, so you do not have to. Our explainer on CO₂e covers that conversion.
Where emission factors come from
Governments and agencies publish factor sets, usually once a year. These are the ones most companies use:
| Source | Covers | Updated |
|---|---|---|
| UK Government conversion factors (DESNZ) | UK fuels, electricity, travel, freight, waste, water | Yearly, each June |
| US EPA GHG Emission Factors Hub | US fuels, vehicles, business travel | Periodically |
| US EPA eGRID | US electricity by grid region | Yearly |
| National energy or environment agencies | Country-specific electricity and fuels | Varies |
These datasets are free. Use the one for the country where the activity happened. If your country does not publish a full set, use its official electricity factor and take fuel factors from one of the sets above, since those barely change between countries.
Why fuel factors are stable
Fuel factors come from chemistry. A litre of diesel weighs about 0.84 kg and is roughly 86% carbon. When it burns, each kilogram of carbon combines with oxygen to form about 3.67 kg of carbon dioxide. That works out at around 2.65 kg of carbon dioxide per litre, whatever country you are in. Petrol is lighter and gives about 2.31 kg per litre.
Published factors for pump fuel are slightly lower in places where biofuel is blended in, because the biogenic part is counted separately.
Why electricity factors differ so much
A kWh of electricity has no fixed footprint. It depends on how the power was generated. A grid running mostly on coal has a high factor. A grid with a lot of wind, solar, hydro or nuclear has a low one. Between countries the difference can be more than tenfold.
The factor also changes over time. In the UK Government’s 2026 set, grid electricity is about 0.131 kg CO₂e per kWh, roughly 26% lower than the year before, as more renewable power came onto the grid.
Here is what that means in practice. A business using 10,000 kWh a year would report about 1,770 kg CO₂e with the earlier factor and about 1,310 kg with the 2026 one. Its electricity use did not change. Its reported Scope 2 emissions fell by 460 kg.
Want the number without the spreadsheet? Enter four figures from your bills and get your monthly CO₂e split by scope. Open the free Business Carbon Calculator
Location-based and market-based electricity factors
Electricity can be reported with two kinds of factor:
- Location-based: the average for the grid you are connected to. This is the default.
- Market-based: the factor for the specific electricity you bought. A renewable tariff backed by certificates can have a factor of zero. If you have no such contract, the correct market-based factor is the residual mix, which is the grid average after renewable certificates sold to others have been removed.
The two methods are explained further in Scope 1, 2 and 3 emissions explained.
Typical values
These rounded figures give a sense of scale. Check a current official source before you report.
| Activity | Approximate factor | Scope |
|---|---|---|
| Grid electricity (varies by country) | 0.1 to 0.7 kg CO₂e per kWh | 2 |
| Natural gas | 0.18 kg CO₂e per kWh | 1 |
| Diesel | 2.68 kg CO₂e per litre | 1 |
| Petrol | 2.31 kg CO₂e per litre | 1 |
What a factor includes and leaves out
Two factors for the same fuel can differ because they draw the line in different places.
- Combustion only. The standard Scope 1 factor covers the gases released when the fuel burns.
- Well-to-tank. Extracting, refining and delivering the fuel also causes emissions. These are published as separate factors and belong in Scope 3.
- Transmission and distribution losses. Some electricity is lost between the power station and your meter. The emissions from those losses are also a separate Scope 3 factor.
Flights have their own choices. Factors are published by distance band and cabin class, because a business class seat takes up more of the aircraft than an economy seat. Some sets also offer a version that includes the extra warming caused by emissions at altitude. Pick one approach and use it every year.
How to choose the right factor
- Match the country. This matters most for electricity.
- Match the year. Use the factor set for the year your data covers.
- Match the unit. Gas factors exist per kWh and per cubic metre, and per kWh on a gross or net energy basis. UK gas bills use the gross basis. Vehicle factors exist per litre and per kilometre.
- Match the scope. Use combustion factors for Scope 1 and keep well-to-tank factors for Scope 3.
- Write down the source. Anyone reading your report should be able to find the same number.
Activity-based, spend-based and supplier-specific factors
The factors above are activity-based: they use physical quantities such as litres and kWh. They are the most reliable for energy and travel.
When you have no physical data, spend-based factors estimate emissions from money spent, for example kg CO₂e per euro of office supplies. They are built from economic statistics for whole industries, so they are rough. A cheap, high-carbon product and an expensive, low-carbon one get the wrong way round. Use them for a first screen of purchased goods in Scope 3.
Supplier-specific factors are the best option where they exist. If a supplier gives you the measured footprint of the product you buy, use it in place of an industry average.
When factors change
Use each year’s factors for that year’s data. Do not go back and recalculate earlier years with newer factors, because the old ones were correct for their period. The exception is when you discover an error or change your method. In that case, restate the earlier years the same way so the trend stays comparable.
Frequently asked questions
Are emission factors the same every year?
No. Electricity factors change yearly. Fuel factors change little.
What factors does the Ecoki calculator use?
Rounded averages, listed under the calculator on the home page. They give an estimate, not a figure for regulatory reporting. For a full footprint, follow our step-by-step guide with your country’s official factors.
What is the difference between an emission factor and GWP?
GWP converts one gas into CO₂e. An emission factor converts an activity into CO₂e and already has GWP built in.
Can I use UK factors if my business is in another country?
For fuels, the difference is small. For electricity, no. Use the grid factor for your own country.
Where do I find my country’s electricity factor?
Start with your national energy regulator or environment agency. Many publish an annual figure for grid electricity.
Sources
- UK Government conversion factors for company reporting
- US EPA GHG Emission Factors Hub
- GHG Protocol Scope 2 Guidance
About the author
Georgi Todorov is an eco activist and entrepreneur. He runs Ecoki, where he builds free tools and guides that help small businesses measure and cut their carbon footprint.