Carbon Accounting Methodology Selector

Choosing the right carbon accounting methodology is harder than most guidance admits. Most of it stops at “estimate your emissions” — as if every category could be calculated the same way. It can’t. The GHG Protocol defines specific calculation methods for each of its 15 Scope 3 categories, each with a different accuracy ceiling depending on the data you actually have. Getting this choice wrong doesn’t just cost you accuracy — under the GHG Protocol’s proposed 2026 revisions, it costs you a visibly weak data-quality disclosure.

This free tool builds you a working carbon accounting methodology in minutes, not weeks. Answer a handful of questions about what you’re calculating and what data you can support, and it tells you exactly which method fits — for Scope 1, Scope 2, all 15 Scope 3 categories, and PCAF financed emissions. No generic four-method wrapper: Category 11 (use of sold products) gets its own use-phase logic, Category 15 (investments) gets PCAF-style attribution, Category 6 (business travel) gets distance-based methods — because that’s what the categories actually require.

Why “just estimate it” stops working in 2026

On 31 March 2026, the GHG Protocol published its Scope 3 Phase 1 Progress Update — the first substantial overhaul of Scope 3 guidance since 2011. Among the proposed revisions: disclosing what proportion of your data sits in each quality tier, and whether it’s been verified. A spend-based estimate isn’t wrong to use — it’s often the only realistic starting point — but under these revisions it has to be labelled as the lowest tier, visibly, rather than blended anonymously into one total. Knowing which tier you’re actually working at, category by category, is no longer optional homework. It’s the disclosure itself.

How the tool decides

Every GHG Protocol Scope 3 category supports a small set of calculation methods, ordered from least to most data-intensive. The tool shows you the methods that actually apply to the category you picked — not a generic list — and asks which one you can support with data you have today, not data you wish you had. Based on your answer, it shows the recommended method, the specific inputs it needs, real factor-database names to use (DEFRA, EPA, ecoinvent, ADEME, GLEC, EPA WARM, IPCC, and the PCAF Standard where relevant), and exactly what upgrading to the next tier would require.

Grounded in the actual standards, not a summary of them

The category-specific logic in this tool is not an AiGreenTools interpretation of carbon accounting methodology — it follows the source documents directly. Scope 3 method options come from the GHG Protocol’s Technical Guidance for Calculating Scope 3 Emissions, and the financed-emissions attribution formulas come from the Partnership for Carbon Accounting Financials (PCAF) Global GHG Accounting and Reporting Standard. When the two disagree with a generic four-method summary you may have seen elsewhere, the standards win — that’s the whole point of building a category-specific tool rather than a one-size-fits-all calculator.

All 15 Scope 3 categories, covered properly

• 1. Purchased goods & services
• 2. Capital goods
• 3. Fuel- & energy-related activities
• 4. Upstream transportation & distribution
• 5. Waste generated in operations
• 6. Business travel
• 7. Employee commuting
• 8. Upstream leased assets
• 9. Downstream transportation & distribution
• 10. Processing of sold products
• 11. Use of sold products
• 12. End-of-life treatment of sold products
• 13. Downstream leased assets
• 14. Franchises
• 15. Investments

Build a plan, not just a one-off answer

Most companies need to assess several categories, not one. As you work through each one, the tool adds it to a running methodology plan — category, recommended method, and data-quality tier — so by the end of a session you have a documented approach across your material categories, not a single throwaway result. Copy the plan as text or print it to PDF before you close the tab; nothing is saved automatically, and nothing you enter is transmitted or stored anywhere.

Who this is for

Sustainability and ESG managers building a Scope 3 inventory, carbon accounting leads preparing a CSRD or SBTi submission, procurement teams asked to support an emissions calculation, and financial institutions working through PCAF asset-class attribution. If you’ve ever been told to “just estimate it” without being told which estimate actually fits your category, this tool answers that question directly. It is equally useful as a training aid for anyone new to the role: working through even a few categories is a fast, concrete way to learn how carbon accounting methodology actually varies from one part of a value chain to another.

Where to go next

Once you know which method fits, the harder work is collecting the data to support it — see our guide on how AI changes Scope 3 supplier data collection. For financed emissions specifically, Persefoni builds PCAF attribution and data-quality scoring into its core platform. For turning a finished inventory into a costed reduction pathway, use the free Net Zero Roadmap Toolkit. For the regulatory context behind the primary-versus-secondary data question, see our CSRD guide. Every tool we review is scored using the AiGreenTools Evaluation Framework™.