Carbon accounting: what it is and how to implement it in your company

Carbon accounting is the process of systematically measuring, quantifying and recording an organisation's greenhouse gas (GHG) emissions, expressed in CO2 equivalent (CO2e) and usually compiled in an annual inventory. It is the environmental equivalent of financial accounting: instead of euros, it measures tonnes of emissions.

Its result is the company's carbon footprint, the basis on which targets are set, reduction plans are designed and information is reported to clients, investors and public administrations. Without rigorous accounting, any climate target remains a hard-to-defend statement of intent.

Implementing it means choosing a recognised framework, defining the organisation's boundaries, classifying emissions by scope, applying reliable emission factors to activity data and consolidating everything into a verifiable inventory. Here is the full step-by-step guide.

What carbon accounting is

Accounting for carbon means translating a company's activity (fuel consumption, electricity, purchases, transport, use of its products) into a single, comparable emissions figure. To add up very different gases, each one is converted into CO2 equivalent according to its global warming potential, so that methane or nitrous oxide can be aggregated alongside CO2 in a single unit.

It is not a one-off exercise but a system repeated every year with the same method, which makes it possible to compare years and check whether the company is genuinely reducing. That methodological consistency is precisely what the reference frameworks provide.

Reference frameworks: GHG Protocol and ISO 14064-1

There are two dominant frameworks for corporate carbon accounting. They are broadly compatible and are often used together.

• The GHG Protocol Corporate Standard is the most widespread emissions accounting standard in the world. It organises emissions into three scopes and is the methodological reference for reporting initiatives and climate targets.
• ISO 14064-1:2018 is an international standard designed to be verifiable by third parties. It requires defining both organisational and operational boundaries and documenting a base-year recalculation policy.

The practical difference lies in the organisational boundaries. The GHG Protocol requires you to choose one of three named consolidation approaches (operational control, financial control or equity share) and to restate historical data if you change approaches. ISO 14064-1, by contrast, does not force any of those three options: the organisation defines and documents its own boundary approach, with its justification.

If you are unsure which to choose, we cover it in detail in our comparison GHG Protocol vs ISO 14064-1 and in the guide on what the ISO 14064 standard is and what it is for.

How to implement carbon accounting step by step

1. Define organisational and operational boundaries

The first step is to decide which part of the company is included in the calculation. The organisational boundaries determine which facilities, subsidiaries or holdings are included, according to the chosen consolidation approach (operational control, financial control or equity share). The operational boundaries define which emissions are accounted for within those boundaries, classifying them by scope.

2. Classify emissions by scope

Carbon accounting organises emissions into three scopes. Understanding them well is the key to the whole inventory; we detail them in the article on the difference between scope 1, 2 and 3 emissions.

Scope 3 is usually the largest and the hardest to measure. The GHG Protocol organises it into 15 categories; you can see how they are structured in our guide on the 15 categories of scope 3.

3. Apply emission factors to activity data

Emissions are rarely measured with sensors: they are estimated by multiplying an activity figure (litres of diesel, kWh of electricity, kg of material) by an emission factor that converts it into CO2e. These coefficients come from official or recognised databases, such as MITECO's emission factors, the UK government conversion factors (DEFRA and DESNZ), the IPCC's, the International Energy Agency's, or databases such as Ecoinvent.

In scope 2 it is worth distinguishing two methods: the location-based method, which uses the average factor of the local electricity grid, and the market-based method, which uses the factor of the electricity product actually contracted. Good accounting usually reports both.

4. Set the base year and a recalculation policy

The base year is the reference year against which future emissions are compared and targets are set. For that comparison to remain valid over time, you need a recalculation policy that adjusts the base year when there are structural changes (mergers, acquisitions, divestments) or methodological improvements. Without one, growth from acquiring another company could be mistaken for a real increase in emissions.

5. Build the emissions inventory

The emissions inventory is the final result of the process: gathering activity data by scope and category, applying the factors and consolidating everything into tonnes of CO2e. It is the document that summarises the organisation's footprint and on which the report and the reduction plan are built.

6. Take care of data quality

The reliability of the inventory depends on data quality. It is best to prioritise primary or measured data (actual invoices, meter readings, supplier data) over estimates, document all assumptions, keep every data point traceable and control uncertainty. A well-documented figure is a figure you can defend before an auditor.

7. Verify the inventory

An independent third party can verify the inventory with a limited or reasonable level of assurance, which adds credibility with regulators, investors and clients. The review and verification of the report relies on ISO 14064-3, the part of the standard specific to this purpose.

In Spain it is worth being clear about the legal framework: Royal Decree 214/2025 requires in-scope companies to calculate their scope 1 and scope 2 carbon footprint (scope 3 remains voluntary) and to have a reduction plan with a five-year horizon. Registering that footprint in MITECO's carbon footprint registry, by contrast, remains voluntary for private companies: only state public sector bodies are required to register.

Software tools for carbon accounting

Running carbon accounting on spreadsheets works at first, but it becomes fragile as data sources and scope 3 categories grow. An outdated emission factor or a badly copied formula can distort the whole inventory.

Specialised tools automate data collection (for example, by reading invoices), apply up-to-date emission factors, calculate by scope and generate audit-ready reports, reducing errors compared with spreadsheets. This frees up the team's time for what really matters: analysing the results and reducing emissions.

Frequently asked questions

What is the difference between carbon accounting and carbon footprint?

Carbon accounting is the process (measuring, quantifying and recording emissions with a consistent method); the carbon footprint is the result, the total emissions figure expressed in CO2 equivalent. You do accounting in order to obtain the footprint.

Is carbon accounting mandatory in Spain?

It depends on the company. RD 214/2025 requires in-scope organisations to calculate their scope 1 and scope 2 carbon footprint and to have a reduction plan. Registering that footprint in the MITECO registry is voluntary for private companies, although more and more clients and tenders value it.

Which framework should I choose, GHG Protocol or ISO 14064-1?

They are compatible and often combined. The GHG Protocol is the reference for calculation and international comparability; ISO 14064-1 is designed for accredited verification and regulatory compliance. Many companies calculate under the GHG Protocol and verify under ISO.

Why is scope 3 so important?

Because in most companies it concentrates the majority of emissions, as it covers the entire value chain. It is also the hardest to measure, since it depends on supplier and customer data that the company does not directly control.

Implement your carbon accounting

Defining boundaries, classifying by scope and keeping emission factors up to date is much simpler with the right tool. Manglai's carbon footprint software lets you run carbon accounting by scope, with GHG Protocol and ISO 14064 methodology, up-to-date factors and audit-ready reports, and turn that data into an actionable reduction plan.