What counts as Scope 3?

Scope 3 is every indirect emission outside your own operations and purchased energy — split by the GHG Protocol into 15 categories across upstream activities (what you buy) and downstream activities (what happens to what you sell). It is usually the largest part of a company's footprint, often the hardest to measure, and the part stakeholders, customers and frameworks such as CSRD, VSME, CDP and PCAF scrutinise most. The good news: you do not need perfect data to begin. A defensible first inventory built on screening and spend-based estimates is far more useful than waiting for supplier-specific data that may take years to gather.

How to calculate Scope 3 emissions step by step

Follow these seven steps in order. Each one builds on the last, taking you from a blank page to an audit-ready Scope 3 inventory you can report and improve.

1. 1

   Map which of the 15 Scope 3 categories are relevant to you

   Start with the GHG Protocol's 15 categories and decide which apply to your business. Walk through each one — purchased goods and services, capital goods, fuel- and energy-related activities, transport, waste, business travel, commuting, leased assets, use and end-of-life of sold products, franchises and investments. Mark each as relevant, not relevant or uncertain, and write a one-line reason. This boundary-setting step prevents both gaps and double-counting later.

2. 2

   Run a quick screening to size each category

   Before investing in precise data, do a rough screening using readily available figures — total spend by category, headcount, floor area or production volume multiplied by broad average emission factors. The goal is not accuracy but proportion: which categories are likely large and which are negligible. This tells you where to focus effort and lets you set a sensible materiality threshold.

3. 3

   Choose a method per category: spend-based, average-data, or supplier-specific

   You can mix methods across categories. Use the spend-based method (financial spend multiplied by an emission factor per currency unit) where you only have invoices; the average-data method (physical quantities multiplied by average factors per kg, km or kWh) where you have activity data; and the supplier-specific method (actual data from suppliers) for your largest, most material categories. Pick the best method each category's data supports — accuracy follows the data you can realistically get.

4. 4

   Collect activity and spend data from finance and suppliers

   Gather the inputs your chosen methods need. Pull spend by category from finance and procurement systems (general ledger, accounts payable, ERP), and physical activity data — litres of fuel, tonne-kilometres freighted, kWh, units sold — from operations. For material categories, request primary data from suppliers via a survey or supplier portal. Keep raw sources attached to each figure so the inventory stays traceable.

5. 5

   Apply the right emission factors and calculate tCO2e

   Multiply each piece of activity or spend data by the matching emission factor to get tonnes of CO2e. Use a recognised, dated factor database (for example DEFRA, EPA, ecoinvent or EXIOBASE-derived spend factors) and record the factor, its source and its year against every line. Make sure units line up — spend factors expect currency, activity factors expect physical units — and convert all greenhouse gases to CO2-equivalent using current global warming potentials.

6. 6

   Aggregate, identify hotspots, and document assumptions

   Sum the results by category and across all of Scope 3, then rank categories by emissions to find your hotspots — usually a small number of categories drive most of the total. Document every assumption, method choice, exclusion and data-quality rating in a methodology note. This is what makes the inventory auditable and what reviewers under CSRD, VSME, CDP or PCAF will expect to see.

7. 7

   Improve data quality each year toward a hybrid method

   Treat your first inventory as a baseline, not a final answer. Each year, replace spend-based estimates in your largest categories with average-data or supplier-specific figures, expand supplier data collection, and refresh emission factors. Over time you converge on a hybrid method — primary data where it matters, estimates where it does not — improving accuracy while keeping the effort proportionate.

Choosing a Scope 3 calculation method

The GHG Protocol lets you mix methods by category. Start with whichever method you can actually source data for, then upgrade the highest-impact categories first. A blend of methods — a hybrid approach — is the realistic end state, not a single perfect method.

The 15 Scope 3 categories at a glance

Use this as your screening checklist in step 1. Mark each category as relevant, not relevant or uncertain, then size the relevant ones. For most companies, a handful of categories drive the majority of the footprint.

Categories vary by sector: a retailer's hotspot is usually purchased goods (category 1), a manufacturer may be dominated by use of sold products (category 11), and a bank by investments (category 15). Screen honestly rather than assuming.

Making Scope 3 calculation manageable

The maths is simple — activity data multiplied by an emission factor. The hard part is doing it consistently across 15 categories, keeping factors current, and proving every number when an auditor asks. That is where CarbonTool removes the spreadsheet pain:

Want to go deeper? Try the free Scope 3 calculator, see how the supply chain platform runs supplier surveys, compare your options in our guide to the best Scope 3 emissions software, or read how to choose supplier emissions data collection software.