What is an emission factor, and how do you choose the right one?

Nobody measures the carbon dioxide coming out of an office boiler. Instead, companies measure an activity they already track, such as kWh of gas or litres of diesel, and multiply it by an emission factor: the average emissions per unit of that activity.

Emissions = activity data × emission factor

For example, 1,000 litres of diesel × 2.58354 kg CO2e per litre (DESNZ 2026) = 2,583.5 kg CO2e.

Where official factors come from

Each source publishes a file; LCALens imports that file, records its address and checksum, and keeps every factor pending until a person has checked it against the source.

Six checks before you use a factor

1. Geography. Grid electricity varies widely by country, so use the factor for the country where the energy was consumed. Fuel factors vary less, but national values exist and are preferable when available.
2. Year. Grid factors change every year. Italy's consumption factor fell from 234.7 g CO2/kWh in 2023 to 192.6 g in 2024 (ISPRA), an 18% fall in one year. Use the factor for your reporting year.
3. Unit. The factor's unit must match your data exactly. Natural gas in DESNZ 2026 is 0.18231 kg CO2e per kWh on a gross calorific value basis and 0.20199 on a net basis; mixing them up is an 11% error.
4. Boundary. Is the factor direct combustion only, or does it include fuel production upstream (a "life-cycle" or "well-to-wheel" factor)? Direct factors belong in Scope 1 and 2; upstream emissions go to Scope 3 category 3. See category 3.
5. Gases. CO2e factors include methane and nitrous oxide; some sources, such as ISPRA's electricity table, publish CO2 only. Report which one you used.
6. GWP basis. CO2e depends on the global warming potentials used. DESNZ 2026 uses AR5 values (AR6 where no AR5 value exists); the GHG Protocol's GWP table states that "the use of the latest values (AR6) is recommended". Keep the basis consistent across years.

Spend-based factors

When you only know what you spent, a spend-based factor gives emissions per unit of currency. The EPA factors used in LCALens are in kg CO2e per 2022 US dollar at purchaser prices, so spend in euros or pounds has to be converted to US dollars and adjusted to 2022 prices first. They are US economy averages and only an approximation for purchases elsewhere, which is why they are best used for screening and replaced with supplier data for the largest suppliers.

Your own factors

Suppliers can give you better data than any average: an energy supplier's fuel mix disclosure, a product's environmental product declaration, or a freight carrier's emission intensity. These are "supplier-specific" factors. Record where each came from and keep the evidence, because an auditor or customer will ask. In LCALens they are stored as private factors, visible only to your organisation and labelled as user-provided.

Converting between units

For gas-to-CO2e conversions and energy and mass unit conversions, see the converters. Every converter shows the value it uses and its source.

Related

• Scope 1, 2 and 3 emissions explained
• Italian electricity emission factors (ISPRA)