Scope 3 Emissions: Why They're Hardest — How to Start

Scope 3 emissions are the indirect emissions across your value chain — everything from the goods and services you buy, to how your staff travel, to what customers do with the products you sell. They are the hardest of the three scopes to measure because they happen at sources you don't own or control, so the data sits with other people. The honest way to start is not to measure everything at once: screen a first year with rough estimates, find the handful of categories that dominate, then improve the numbers that matter.

Key takeaways

• Scope 3 covers all other indirect emissions in your value chain — 15 categories defined by the GHG Protocol — and for most businesses it is the largest slice of the footprint by far.
• It is hard because the data belongs to suppliers, hauliers, employees and customers, not to you, and they measure it inconsistently if at all.
• Start with a spend-based screening year to find the hotspots, then replace estimates with activity and supplier data where it actually moves the total.
• Scope 3 is largely voluntary under UK SECR today, but UK SRS S2 (February 2026) and proposed GHG Protocol revisions are pushing it into the mainstream.
• The 2026 UK emission factors landed on 11 June 2026; the factors you apply to scope 3 fell sharply, so flag it in any report to avoid a factor-driven drop being mistaken for real reduction.

What are scope 3 emissions?

Scope 3 is one part of a complete carbon footprint, so if you're building yours from scratch, our pillar guide on how to calculate your business carbon footprint covers the whole picture. Under the GHG Protocol Corporate Standard, scope 3 is the catch-all for indirect emissions that are a consequence of your activities but occur at sources you neither own nor control — the extraction and production of materials you buy, the fuel burned delivering your goods, the electricity a customer uses running a product you sold. Scope 1 (direct) and scope 2 (purchased energy) sit inside your walls and your bills; scope 3 sits everywhere else. If you want the full breakdown of how the three fit together, our companion guide, Scope 1, 2 and 3 emissions explained, works through each with examples.

The reason scope 3 matters is proportion. For a service business, an office footprint of scope 1 and 2 might be a rounding error next to the emissions embedded in the software, professional services and equipment it buys. For a product business, the use and disposal of what you sell can dwarf everything that happens in your own factory. You cannot manage a footprint you have not measured, and for most companies the part worth managing lives in scope 3.

Why is scope 3 the hardest to measure?

Three things make it hard.

First, the data isn't yours. Scope 1 comes off your meters and fuel invoices; scope 2 comes off your electricity bills. Scope 3 lives with third parties — your suppliers' production processes, a courier's fleet, an employee's commute, a customer's usage patterns. Getting a primary figure means asking someone else to measure and share it, and many of your suppliers haven't measured their own footprint yet.

Second, it's broad. The GHG Protocol splits scope 3 into 15 distinct categories, deliberately defined to be mutually exclusive so nothing is double-counted between them. Few organisations are material in all 15, but working out which ones apply — and drawing a defensible boundary — takes judgement.

Third, estimates need caveats. Early scope 3 numbers are built from averages and spend data, which means they carry real uncertainty. That's fine, and expected, but it has to be communicated honestly rather than presented as a precise measurement.

None of this is a reason to avoid scope 3. It's a reason to sequence it: screen wide and rough first, then go deep only where the numbers are large.

The 15 scope 3 categories

The Corporate Value Chain (Scope 3) Standard defines exactly 15 categories — eight upstream (things you buy and use) and seven downstream (what happens to what you sell).

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

Reporting in conformance with the standard means reporting emissions by category, disclosing and justifying any category you exclude. You don't need a big number in every row — you need an honest account of which categories are relevant to your business and why.

How to start: screen, then deepen

The practical route is the same one we walk clients through, and it deliberately avoids the trap of trying to be complete on day one. It sits on top of a GHG-Protocol-aligned footprint of your scope 1 and 2 first, so build that baseline before you layer scope 3 on top.

1. Set the boundary. Decide which of the 15 categories are relevant. A B2B service firm may only meaningfully touch categories 1, 2, 3, 5, 6 and 7; a manufacturer will care about the use and end-of-life of its products (11 and 12).
2. Screen with spend. For a first year, estimate the relevant categories from what you spent, using average emissions-per-pound factors. It's crude, but it's fast, and it tells you where the mass sits.
3. Find the hotspots. Almost always, two or three categories account for the bulk of scope 3. That's where the effort belongs.
4. Deepen the hotspots. Replace spend estimates in those categories with activity data (physical quantities) and, where suppliers can provide it, their own product-level figures.
5. Document and repeat. Record your methods, assumptions and exclusions so next year is a refinement rather than a fresh start.

This is a multi-year improvement path, not a one-off exercise — and it is the honest way to produce a scope 3 number you can stand behind.

Spend-based vs activity-based data

The GHG Protocol Technical Guidance for Calculating Scope 3 Emissions sets out four methods for category 1, in decreasing order of specificity, and the same logic applies across the value chain:

• Supplier-specific — cradle-to-gate data collected from the actual supplier. Most accurate, hardest to get.
• Hybrid — supplier data where you have it, average data to fill the gaps.
• Average-data (activity-based) — physical quantities (kWh, litres, kilometres, tonnes) multiplied by average per-unit emission factors.
• Spend-based — the financial value of what you bought multiplied by an emissions-per-currency factor. Quick, useful for screening, but the least accurate and distorted whenever prices move rather than volumes.

The direction of travel is to start spend-based and improve. That matters because the draft revision to the Scope 3 Standard would make you disclose how much of your inventory is spend-based versus activity- or supplier-specific — so building the habit of tracking data quality now is worth it.

What's changing in 2026 — and why it matters

Being current is the point here, because two 2026 developments change how a scope 3 footprint is built and read.

New emission factors (11 June 2026). DESNZ published the 2026 UK Government GHG conversion factors, and the numbers moved a lot. The headline electricity factor fell 26% to 0.13096 kgCO2e/kWh (from 0.17700), partly through genuine grid decarbonisation and partly through a methodology change that cut the data lag from two years to one — absorbing two years of grid change in a single update. That flows straight into scope 3 category 3 (fuel- and energy-related activities): the well-to-tank factor for UK electricity fell about 20% and transmission-and-distribution losses about 30%. If your electricity-related emissions drop this year, part of it is the factor, not your behaviour — say so in the report, or a like-for-like comparison will mislead.

Proposed GHG Protocol revisions (draft). On 31 March 2026 the GHG Protocol published a Scope 3 Standard Revisions Phase 1 Progress Update. It is explicitly a working draft — not a standard, and not yet open for consultation — but it signals the direction: a prescriptive 95% minimum inclusion boundary (report at least 95% of your scope 3, with justified exclusions capped at 5%), a new Category 16 "other value chain activities", category 15 narrowed to financed emissions, and mandatory disaggregation of emissions by data type with a verification-status disclosure. All of this now sits under a broader update process run jointly with ISO, whose experts joined the technical working groups in early 2026. Final standards are expected around 2027; until then, today's Scope 3 Standard applies and nothing above is a rule yet.

Where scope 3 fits with UK reporting

For most UK companies, scope 3 is still voluntary. SECR requires quoted companies, and "large" unquoted companies and LLPs (meeting two of: over £36m turnover, over £18m balance sheet, more than 250 employees), to report energy use and scope 1 and 2 emissions — not scope 3. But the pull is growing: UK SRS S2 asks for scope 1, 2 and 3, customers increasingly request supply-chain data, and tenders reward it. A robust scope 3 baseline is becoming a commercial asset rather than a compliance chore.

Depth costs more than a screening exercise, and how far you take scope 3 is the main driver of price. If you're budgeting, our sibling guide breaks down how much a carbon footprint assessment costs in the UK.

The bottom line

Scope 3 is the hardest scope because the data isn't yours and the surface area is large — but "hardest" is not "start last". Screen a first year with spend data, concentrate on the two or three categories that dominate, and improve those with activity and supplier data over time. Get the method and the caveats right and you'll have a scope 3 number that holds up to scrutiny.

Every credible scope 3 inventory rests on a solid, GHG-Protocol-aligned footprint underneath it. That baseline — scope 1, scope 2 and a screened, hotspot-driven scope 3 — is exactly the work our carbon reporting service delivers, and it's where we'd start with you.