Scope 3 emissions: where to start and what verifiers check

What scope 3 emissions cover

Scope 1 is what your own boilers, vehicles and processes release. Scope 2 is the electricity, heat, steam and cooling you purchase. Scope 3 is everything else your activity causes but someone else emits: the materials in what you purchase, the lorries that carry your goods, your staff's flights, and the energy and disposal of your products after sale.

The GHG Protocol's Corporate Value Chain (Scope 3) Standard, released in 2011, organises this into 15 categories. Under the 2004 Corporate Standard, scope 3 is an optional reporting category; scopes 1 and 2 are the minimum. Once a company reports against the Scope 3 Standard, the expectation changes: it has to account for all scope 3 emissions and disclose and justify anything it leaves out.

ISO 14064-1:2018 takes a different route to the same ground. It reports direct emissions and five categories of indirect emissions, and asks you to decide which indirect emissions are significant. Our guide to mapping ISO 14064-1 categories to GHG Protocol scopes reconciles the two; this page uses the scope 3 vocabulary buyers and disclosure platforms ask for.

Where to start: screen before you calculate

A common first move is to pick the category that seems obvious, often business travel, and calculate it precisely, only to find later that it is a small share of the footprint while the largest category has no figure at all. A screening avoids that.

Step 1: get scopes 1 and 2 in order first

The Scope 3 Standard assumes a scope 1 and scope 2 report prepared to the Corporate Standard sits alongside it, and the organisational boundary you set for scopes 1 and 2 decides what counts as your value chain. Our guide to organisational and reporting boundaries covers that decision.

Step 2: a first-pass estimate for every category

Take each of the 15 categories and ask two questions: does it apply to us at all, and roughly how large could it be? A first pass usually needs only data the organisation already holds, such as ledger spend by supplier group, freight invoices, travel bookings, headcount by site and sales volumes, multiplied by average factors to give an order of magnitude. Record every source; the screening becomes evidence later.

Step 3: rank and decide

Rank the categories on four things: estimated size, the quality of data you could realistically obtain, how much influence you have over the emissions, and what your customers, lenders or target scheme expect to see. ISO 14064-1 requires significance criteria to be set and applied to indirect emissions, with an informative annex giving guidance on identifying significant ones; the GHG Protocol expects the same discipline through its disclosure and justification rule. Our guide to indirect emissions and significance goes further into the criteria themselves.

Step 4: check what your target scheme demands

Version 2.0 of the SBTi Corporate Net-Zero Standard, published in June 2026 and effective from 1 February 2027, expects the largest companies to cover at least every scope 3 category of five per cent or more of emissions. The screening shows which categories cross that line.

Data for scope 3: estimate first, then improve where it matters

Most first scope 3 inventories lean on estimates. A verifier needs to see which figures are estimates, how they were produced and why the method suits the category. Sterling's recommended approach: broad estimates for small categories, data effort on the two or three that dominate.

For those categories, improve in steps: replace spend with physical quantities where your records hold them, then request emissions data from your largest suppliers, then build that request into purchasing so it arrives every year. The GHG Protocol's technical guidance on calculating scope 3 emissions, with a chapter per category, is the working reference for method choice.

Two rules make any method defensible. Every activity figure has an owner and a source record that reconciles to it, and every factor has a reference and a year. Our guide to emission factors, data quality and uncertainty covers factor selection and the uncertainty assessment ISO 14064-1 requires.

Exclusions: allowed, but never silent

Leaving a category out is not a problem; an exclusion that appears only as an absence in the report is.

ISO 14064-1 defines three levels of documentation, and the strictest, "justify", means explaining why the alternatives were not chosen and giving supporting data. Treat every scope 3 exclusion at that level. Record the category, the reason, the screening estimate behind it and the date of the next review.

What verifiers check in a scope 3 inventory

Verification of an ISO 14064-1 inventory is carried out under ISO 14064-3 by an independent verification body accredited to ISO/IEC 17029 with ISO 14065. The body agrees the objectives, criteria, level of assurance (limited or reasonable) and materiality with you before it starts, reviews the documentation, visits sites as its risk assessment requires, completes an independent review and issues a statement with its opinion. Scope 3 draws attention because estimates concentrate there.

Two points shape the result. Materiality applies to the inventory as a whole, so a large, poorly evidenced category can put the opinion at risk even when scopes 1 and 2 are clean. ISO 14064-3 also names conservativeness among its principles, so a convenient-looking estimate invites questions. Where scope 3 includes a base year for a science-based target, SBTi version 2.0 requires the largest companies to obtain independent third-party assurance of that base-year inventory, scope 3 included, at a minimum of limited assurance. For the full sequence of the engagement, see our guide to the ISO 14064-1 verification process.

Where scope 3 inventories go wrong

• Precision in the wrong place. Careful work on a small category, and nothing on the large one, because the small one had easy data.
• Spend figures that do not reconcile. The spend used in the calculation differs from the ledger for the reporting year, and nobody can explain the gap.
• Boundary drift. Scope 3 calculated for a different set of entities or a different year from scopes 1 and 2.
• A base year with no scope 3 in it. Targets set against a base year that omitted the categories now being reported, so the trend means nothing.

Scope 3 emissions: questions buyers ask