How to calculate emissions from activity data: factors, units and GWPs, worked through

A greenhouse gas inventory is mostly multiplication. What makes it trustworthy is everything around the multiplication: the right activity data, in the right unit, matched with a factor that fits it, with a record of where each number came from. This guide works through the formula with real factors.

What is the basic formula?

Emissions = activity data × emission factor

• Activity data is how much of something happened: litres of diesel burned, kilowatt-hours of electricity used, tonnes of waste sent to landfill, kilometres flown.
• An emission factor is the mass of greenhouse gas released per unit of that activity: kilograms of CO₂ per litre, tonnes of CO₂ per megawatt-hour.

For fuels, the IPCC's Tier 1 method works through energy: fuel consumed in mass or volume is first converted to its energy content, then multiplied by a factor per unit of energy for each gas¹. Emission factors you find already expressed "per litre" have simply done that conversion for you — with assumptions you should know.

Worked example 1: diesel in a generator, from litres to tCO₂e

A plant's DG set burned 2,200 litres of diesel in a month. Using IPCC 2006 default values for gas/diesel oil² and its factors for stationary combustion in manufacturing¹:

So the month's diesel is 5.91 tCO₂e — about 2.685 kg CO₂e per litre. The AR5 line uses methane's general value of 28; its fossil value, 30, gives 5,908.1 kg. With AR6 values (fossil CH₄ 29.8, N₂O 273)³ the total is 5,908.4 kg: for combustion, the choice of GWP set barely moves the result, because almost all of it is CO₂.

Worked example 2: grid electricity

The same plant used 120,000 kWh from the grid in FY 2025-26. The Central Electricity Authority's CO2 Baseline Database version 22.0 gives a weighted average emission factor of 0.675 tCO₂/MWh for FY 2025-26, including renewables and cross-border transfers⁵:

120,000 kWh is 120 MWh, and 120 MWh × 0.675 tCO₂/MWh = 81.0 tCO₂

Three things to notice. The factor is CO₂ only. It is published per financial year, months after the year ends — version 22.0, with FY 2025-26, appeared in August–September 2026 — and it changes: 0.727 for FY 2023-24, 0.710 for FY 2024-25 and 0.675 for FY 2025-26. Use the activity year's factor if it is out before you report; otherwise use the latest year available, say which, and decide in your methodology whether to recalculate when the new one appears. And the database describes itself as for general information, planning and policy analysis⁵; using it for a corporate inventory is a methodological choice your report should state. This is the location-based Scope 2 figure; Scope 1, 2 and 3 explained covers the market-based one.

Which emission factor should you use?

Use the most specific factor you can support with evidence, roughly in this order:

1. Measured or supplier-specific: a lab's calorific value for your coal, your supplier's verified emission factor for a material, an electricity supplier's certified figure.
2. Country- or region-specific: India's CEA factors for grid electricity; Indian factors for road transport where they exist.
3. International defaults: IPCC defaults for fuels and industrial processes; recognised databases for spend-based Scope 3.

Whatever you choose, record for each factor:

• Publisher, publication and version (for example, "CEA CO2 Baseline Database v22.0");
• The unit and the gases it covers (CO₂ only, or CO₂e — and with which GWPs);
• The period it applies to, and the period of the activity;
• Why it applies — the fuel grade, region or technology it represents.

An assessor or assurer will ask for exactly this, and a factor that cannot be traced to its source is a finding.

Which global warming potentials should you use?

Global warming potentials (GWPs) convert each gas into CO₂ equivalent over 100 years. The GHG Protocol's rules, from its 2013 amendment, are that companies shall use 100-year GWPs from the IPCC, should use the most recent Assessment Report, should use one set consistently across the inventory and the base year, and shall report which they used⁴. Its 2024 table recommends the latest values, from AR6³:

The ISSB's IFRS S2 requires the latest IPCC values available at the reporting date; an amendment effective from 1 January 2027 allows other values where a regulator or exchange requires them⁶. GWP choice matters most where methane, nitrous oxide or fluorinated gases are a large share — landfill and waste water, agriculture, refrigerant leaks, switchgear.

What mistakes distort inventories?

• kWh and MWh. A factor in tCO₂/MWh applied to kWh overstates by a thousand times; the opposite understates.
• Gross and net calorific value. IPCC factors are on a net calorific value basis¹. Indian fuel data — especially for coal — is often stated as gross calorific value; convert before applying an IPCC factor.
• tCO₂ and tCO₂e. Adding a CO₂-only electricity figure to CO₂e fuel figures is normal, but say so; mixing GWP sets within one inventory is not.
• An out-of-date or undisclosed factor. A grid factor older than the latest one available when you report, or no record of which year's factor was used.
• Stock instead of consumption. Diesel purchased is not diesel burned: reconcile purchases with opening and closing stock.
• Double counting. The same fuel recorded by the stores and by the site, or electricity counted at both a subsidiary and its parent.

How ESGTrack helps

ESGTrack's factor library holds more than ten thousand emission factors, each with its publisher, edition, unit, gases and GWPs, and chooses the factor in force for each month by date — your own site and company factors first. Every result keeps the factor and the calculation beside it, so it can be traced line by line. See carbon accounting in ESGTrack.