Carbon Accounting
Every emissions figure a company publishes — in an annual report, a CDP response, a net zero target — comes out of the same discipline. It is called carbon accounting, and like financial accounting it has standards, principles, boundaries and auditors.
The core calculation is simple: activity multiplied by an emission factor. Most of the work, and most of the errors, sit in deciding what to count.
Carbon accounting is the measurement and reporting of the greenhouse gases an organisation, product or activity is responsible for, in tonnes of CO₂e. It mostly multiplies activity data by emission factors under the GHG Protocol or ISO 14064-1, split into scopes 1, 2 and 3.
What carbon accounting is
Carbon accounting is the practice of measuring, recording and reporting greenhouse gas emissions. It turns operational data — fuel bought, electricity used, goods purchased, distance travelled — into a figure in tonnes of CO₂-equivalent that can be tracked, compared, verified and reduced.
Despite the name, it covers all seven greenhouse gases in the Kyoto basket, not only carbon dioxide. Each is converted into CO₂ equivalent using its global warming potential, so methane from a gas leak and refrigerant from an air-conditioning unit can be added to the CO₂ from a boiler. “GHG accounting” and “greenhouse gas accounting” mean the same thing.
The output for a company is a GHG inventory: a structured list of its emissions, split into scope 1 (direct), scope 2 (purchased energy) and scope 3 (the rest of its value chain). That inventory feeds disclosure rules, targets, investor questionnaires and the company’s own reduction plans.
Definition at a glance
| What it is | Measuring and reporting greenhouse gas emissions in tonnes of CO₂e |
|---|---|
| Also called | GHG accounting; greenhouse gas accounting; emissions accounting |
| Main standards | GHG Protocol Corporate Standard; ISO 14064-1 (organisations); ISO 14064-2 (projects); ISO 14067 (products) |
| Core formula | Activity data × emission factor = emissions |
| Output | A GHG inventory split into scopes 1, 2 and 3, or a product, project or portfolio footprint |
| Checked by | Third-party verifiers or auditors, at limited or reasonable assurance |
| Required by | Disclosure rules such as CSRD, ISSB-based rules and California’s SB 253 |
The five principles
Relevance — “Ensure the GHG inventory appropriately reflects the GHG emissions of the company and serves the decision-making needs of users”.
Completeness — “Account for and report on all GHG emission sources and activities within the chosen inventory boundary. Disclose and justify any specific exclusions.”
Consistency — “Use consistent methodologies to allow for meaningful comparisons of emissions over time.”
Transparency — “Address all relevant issues in a factual and coherent manner, based on a clear audit trail.”
Accuracy — “Ensure that the quantification of GHG emissions is systematically neither over nor under actual emissions, as far as can be judged, and that uncertainties are reduced as far as practicable.”
These are the same ideas as financial accounting’s, applied to emissions. Transparency is the one that most often decides whether an inventory survives an audit: every number needs a traceable source, which in practice means recording which emission factor was used, from which publication and which year.
Five kinds of carbon accounting
| Kind | Measures | Main standard |
|---|---|---|
| Corporate | A company’s emissions over a year, by scope | GHG Protocol Corporate Standard; ISO 14064-1 |
| Product | The life-cycle emissions of one product, per unit | ISO 14067; GHG Protocol Product Standard — see product carbon footprint |
| Project | The reduction or removal a project achieves against a baseline | ISO 14064-2; carbon credit methodologies |
| Financed | Emissions attributable to a bank’s loans or an investor’s holdings | PCAF |
| National | A country’s emissions, reported to the UN | IPCC 2006 Guidelines |
When people say “carbon accounting” without qualification, they usually mean corporate accounting. The other four use the same arithmetic but different boundaries: a product footprint follows one item through its life, a project compares against a world without the project, and a national inventory counts what is emitted within a country’s borders.
How the numbers are calculated
Almost every line in a carbon account is one multiplication:
Emissions (kg CO₂e) = activity data × emission factor
Activity data is how much of something happened: kWh of electricity, litres of diesel, tonnes of steel bought. The emission factor is the emissions per unit of that activity, taken from a published source such as DEFRA, the EPA or IPCC.
Where activity data is not available, accountants fall back on cruder methods. The spend-based method multiplies money spent by an industry-average factor per dollar or pound; it is fast and complete but imprecise. At the other end, the supplier-specific method uses emissions data from the supplier itself. Most real inventories mix all three, starting spend-based and replacing the largest lines with better data over time. A few large sources, such as power plants, measure emissions directly at the stack instead.
The steps in a corporate inventory
- Set the organisational boundary. Decide which entities are included, by equity share or by control.
- Set the operational boundary. Identify the sources in scopes 1, 2 and 3, and which scope 3 categories are relevant.
- Choose a base year. The year against which change will be measured, recalculated if the company changes shape.
- Collect activity data. Invoices, meter readings, fuel cards, travel bookings, procurement records.
- Apply emission factors. Match each activity to a factor of the right geography, year and unit, and record its source.
- Aggregate and check. Total by scope, test against last year, and document assumptions and exclusions.
- Verify and report. Have the inventory checked to the required assurance level, then disclose it.
What is changing in 2026–27
The two most widely used corporate standards are merging. On 29 July 2026 the GHG Protocol announced that it and ISO “will combine their corporate carbon accounting standards into a single, harmonized global accounting standard”, bringing the GHG Protocol’s scope 1, 2 and 3 standards together with ISO 14064-1. A single public consultation on the combined standard is planned for the second quarter of 2027.
Two pieces are being reworked inside that programme. The scope 2 rules for purchased electricity, including how renewable energy purchases are counted, drew nearly 1,100 consultation responses from 56 countries. And a new standard on “actions and market instruments” proposes that companies report physical emissions, market-based emissions and the impact of their own actions as separate statements. Until a new standard is published, the current GHG Protocol and ISO 14064-1 remain the rules in use.
The arithmetic of carbon accounting is settled. What is still being negotiated is what counts, and how certificates and offsets are shown next to the physical number.
Worked micro-example
- Electricity: 40,000 kWh × 0.13096 kg CO₂e/kWh [GreenCalculus grid.gbr.electricity.location_based · DEFRA 2026 'UK electricity'!E25 · v2026.237] (UK grid, location-based) = 5,238 kg — scope 2
- Gas heating: 60,000 kWh × 0.18231 kg CO₂e/kWh [GreenCalculus fuels.gbr.natural_gas.kwh_gcv · DEFRA 2026 'Fuels'!D42] = 10,939 kg — scope 1
- Company vans: 3,000 litres of diesel × 2.58354 kg CO₂e/litre [GreenCalculus fuels.gbr.diesel_average_biofuel_blend.litre · DEFRA 2026 'Fuels'!D72] = 7,751 kg — scope 1
Total: 23.9 t CO₂e — 18.7 t scope 1 and 5.2 t scope 2, with factors from DEFRA’s 2026 conversion factors. Business travel, purchased goods and waste would add scope 3 on top, usually the largest part for an office-based business.
Common mistakes
- Using a factor from the wrong country or year. A US grid factor applied to UK electricity, or a 2019 factor in a 2025 inventory, can move the result substantially.
- Leaving out scope 3. For most companies it is the largest share of emissions; an inventory with only scopes 1 and 2 can miss most of the footprint.
- Mixing GWP sets. Combining factors built on different IPCC assessment reports in one total without saying so.
- Counting the same emissions twice. For example, including a supplier’s delivery both in purchased goods and in transport.
- Netting off offsets. Carbon credits are reported separately, not subtracted from gross emissions.
- Not recording sources. An inventory without a record of which factor came from where cannot be audited or repeated next year.
Build a scope 1, 2 and 3 inventory with every emission factor sourced, or check how mature your carbon accounting is.
Frequently asked questions
Carbon accounting is the method; a carbon footprint is the result. Accounting is the process of collecting activity data, applying emission factors and following a standard such as the GHG Protocol. The footprint is the total it produces, for a company, a product, an event or a person.
For companies, the GHG Protocol Corporate Standard and its scope 2 and scope 3 standards, and ISO 14064-1. For products, ISO 14067 and the GHG Protocol Product Standard; for projects, ISO 14064-2; for banks and investors, PCAF; for countries, the IPCC Guidelines. The GHG Protocol and ISO announced in July 2026 that they will combine their corporate standards into one.
For large companies in a growing number of places, yes. The EU’s CSRD, ISSB-based rules in countries such as Australia and Japan, and California’s SB 253 all require a greenhouse gas inventory. Smaller companies are often asked for their emissions by large customers who must report scope 3, even where no law applies to them directly.
Mostly by multiplying activity data by an emission factor: for example, 40,000 kWh of UK electricity times 0.13096 kg CO₂e per kWh gives 5,238 kg. Each line is then assigned to scope 1, 2 or 3 and the results are added up. Where activity data is missing, spend-based factors per dollar or pound are used as a fallback.
Carbon accounting is the measurement and reporting of the greenhouse gases an organisation, product or activity is responsible for, expressed in tonnes of CO₂-equivalent. It follows standards such as the GHG Protocol and ISO 14064-1, splits emissions into scopes 1, 2 and 3, and calculates most of them by multiplying activity data by published emission factors.
Need someone who does this? 6 carbon accounting & inventory providers in our directory →