How Emissions Are Calculated: Data, Emission Factors, and Basic Methodology

The Basic Formula

In its simplest form: activity data × emission factor = emissions. Activity data describes the scale of a process: litres of fuel, kilowatt-hours of electricity, tonne-kilometres of freight, kilograms of material, or the quantity of refrigerant used. The factor represents the average amount of a gas associated with one unit of that activity.

The formula is simple, but an error in boundaries, units, or factor selection can change the result by several times.

Step 1 — Define the Purpose

Methodology depends on the question. A company may prepare an annual corporate inventory, compare two transport options, assess a product, meet a regulatory requirement, or estimate the effect of a project. The purpose determines the required accuracy: a preliminary screening may use average factors, while a public claim or mandatory disclosure requires better data, a validated method, and often independent assurance.

Step 2 — Set Boundaries

For a company, the first decision is which legal entities, facilities, and operations are included. Scope 1, Scope 2, and relevant Scope 3 categories are then defined. The selected approach must be applied consistently across years, or the trend cannot be compared fairly. Boundaries should be broad enough to prevent major emissions from being shifted outside the assessment.

Step 3 — Collect Activity Data

For fuels: litres, cubic metres, kilograms, or energy content. For electricity: meter readings or bills in kWh. Transport: actual fuel, kilometres, passenger-kilometres, or tonne-kilometres. Materials: mass, type, recycled content, and country of production.

Data should be checked for completeness, duplication, and unit consistency. Missing data should be estimated using a documented rule rather than hidden.

Step 4 — Select Emission Factors

Factors may come from national inventories, intergovernmental guidance, official government databases, electricity-grid operators, life-cycle databases, or supplier-specific information. They should match the fuel, technology, country, year, and unit of measurement.

An electricity factor should not be transferred from one country to another without review. An aviation factor depends on class, distance, and methodology. A material factor depends on life-cycle boundaries and production technology.

Step 5 — Convert Gases into CO₂e

If a source produces CO₂, CH₄, and N₂O, the mass of each gas is first calculated. Methane, nitrous oxide, and other gases are then multiplied by the appropriate global warming potentials (GWPs). The converted results are added to obtain total CO₂e. Reports should state the GWP version and time horizon used.

Example 1 — Electricity

An office consumes 10,000 kWh and the selected factor is 0.40 kg CO₂e/kWh. Result: 10,000 × 0.40 = 4,000 kg CO₂e (4 tonnes CO₂e). This is illustrative only — the real factor must match the country, year, and chosen Scope 2 method.

Example 2 — Fuel and Transport

A vehicle consumes 1,000 litres of fuel. Multiply by the factor for that specific fuel. The factor may cover only direct combustion or a broader life cycle. When actual fuel is unknown, distance can be multiplied by a vehicle, passenger-km, or tonne-km factor. Load data matters: the same truck has a different result per tonne depending on capacity utilisation and empty running.

Example 3 — Materials and Purchasing

For a material, mass × factor (e.g. kg CO₂e per kg of steel). Check whether the factor covers only production up to the factory gate or also transport, use, and end of life. If only expenditure is known, a spend-based method can be used for initial Scope 3 screening — but price changes can affect the result even when physical quantities do not change.

Avoiding Double Counting

Double counting can occur within one inventory if the same purchase or journey enters two categories. Across a value chain, the same physical emission may legitimately be Scope 1 for one company and Scope 3 for another — this is not an error because the inventories have different organisational boundaries.

Uncertainty and Documentation

An emissions estimate is almost never perfectly exact. Uncertainty should not be hidden: sources should be explained and, where possible, a range estimated. A good inventory can be reproduced — retain primary documents, units, periods, formulas, factor sources, assumptions, and responsible persons. Methodological changes between years should be explained, and the base year recalculated where appropriate.