Getting started with Scope 3 emissions: where to begin on a small budget

Someone has asked you to report on Scope 3, the budget is small, and five software demos later the starting point is still not obvious. Start with a spend-based estimate across all fifteen GHG Protocol categories, then improve the categories that matter. Perfect data is not the entry requirement.

What is Scope 3, and which categories does it cover?

Scope 3 is the emissions from a company's value chain rather than from its own operations or its purchased energy. The GHG Protocol Corporate Value Chain (Scope 3) Accounting and Reporting Standard splits Scope 3 into fifteen categories, covering upstream activities such as purchased goods and services and downstream activities such as use of sold products.

Category 1, purchased goods and services, covers the cradle-to-gate emissions of everything an organisation bought in the reporting year. Category 2 covers capital goods. The GHG Protocol asks companies to screen all fifteen categories and report the ones that are relevant to the business, so a first inventory is a screening exercise before it is a measurement exercise.

Where should you start if the budget is tight?

Start with one platform that covers Scopes 1, 2 and 3 together, then narrow. A single place to upload and review data costs less to run than three separate tools, and it gives the team somewhere to show progress while the Scope 3 picture is still coarse.

The question to put to a vendor is whether Scope 3 is additive or overwhelming. A tool that demands supplier-specific data for every line before it will produce a number stalls in week one. A tool that produces a screening number on day one, then lets you replace the weakest parts, keeps moving.

Is spend-based data good enough to start with?

Yes, for a first inventory. The GHG Protocol sets out four calculation methods for Category 1: the supplier-specific method, the hybrid method, the average-data method and the spend-based method. The spend-based method applies emission factors to procurement spend, and the standard treats it as a legitimate method rather than a shortcut.

Product carbon footprints and full life cycle assessments are useful, and neither is required to publish a first number. What matters more is whether the tool accepts hybrid data, meaning spend-based estimates for most lines and actual supplier figures where you already hold them, and whether it can swap one for the other later without a rebuild.

Buyers call the estimated part proxy data. The problem with proxy data is usually provenance rather than accuracy: nobody can see which factor set produced the number. Ask about factor sets by name, because CEDA, Ecoinvent, GaBi, DEFRA and EPA behave differently in a spend-based calculation, and a tool that will not tell you which set produced a figure cannot tell you why the figure moved.

DitchCarbon publishes how its own factors are built and applied in its calculation methodology. The emission factor methodology for spend-based Scope 3 categories 1 and 2 was independently assessed by Globus Thenken in August 2025.

How do you keep a Scope 3 baseline auditable as the data improves?

Document the source of every figure as you go. An auditable Scope 3 baseline is one where a third-party auditor can see which emission factor set produced each number, which lines are estimated and which are actual, and what changed since the previous version. Building that record afterwards is the expensive way to do it.

Three questions separate a tool that will scale from one that will not. Which emission factor databases produced this number? Which lines are estimated and which are measured? Can the calculation be re-run when better data arrives? A tool that cannot answer the third question produces a baseline you will throw away.

Treat audit-ready as a claim to check rather than accept. Ask any vendor what a named third party has verified, and against which standard. The DitchCarbon Portal calculator is verified to ISO 14064-3 at limited assurance by UL Solutions, renewed annually, and the report can be downloaded from the trust centre. Our review of which carbon software is actually verified lists every platform we checked, with the verifier and the evidence for each.

Do shared industry platforms bring the cost down?

Sometimes, where an industry shares suppliers and a methodology. Sector platforms cut duplicate data requests, which the suppliers on the receiving end notice, and they make figures more comparable between companies in the same sector. They also spread the cost of the infrastructure across the members rather than each company buying its own.

The trade-off is control. A shared platform sets the methodology and the disclosure rules for everyone in it, so read those terms before joining rather than after.

What is DitchCarbon, and how does it help you start?

DitchCarbon provides verified emissions data for over 2 million organisations, so procurement, sustainability and finance teams can measure and act on supply chain and portfolio emissions from one source.

In practice that means a first Scope 3 picture without a survey round. Spend or supplier lists go in and are matched against DUNS, LEI and ISIN identifiers, and every figure carries its source and change history, so the weak parts of the inventory are visible rather than hidden. A recent deployment reached about 60% of a large supplier base within 2 weeks.

Numbers you can defend within 2 weeks is a reasonable target for a first baseline, and the categories that matter most to your business are the ones to improve after that. How it works sets out the sequence.

Two ways to start

To see the coverage on your own supplier or portfolio list, request a walkthrough and bring a spend file.

If you are on the other side of the request, and a customer or an investor has asked you for emissions data, claim your company profile and answer once instead of filling in the same survey again. Claiming a profile is free.

Reviewed 29 July 2026.