TL;DR: For many companies, 70–95% of the total carbon footprint sits in Scope 3. These emissions are not under direct control; they belong to suppliers, customers and product usage. Hard to measure — but you cannot reduce what you do not measure.

Global supply chain visualisation with container ships, trucks and factories
Scope 3 is the carbon born outside your boundaries — and written into your account.

Definitions: Scope 1, 2, 3

  • Scope 1 — Direct: emissions from owned/controlled sources (boilers, gensets, vehicle fleet).
  • Scope 2 — Indirect, energy: generation of purchased electricity, heat or steam.
  • Scope 3 — Other indirect: the rest of the value chain, organised into 15 categories.

The 15 Categories

Upstream

  1. Purchased goods and services — typically the largest category.
  2. Capital goods (machinery, equipment, buildings).
  3. Fuel- and energy-related activities (not in Scope 1/2).
  4. Upstream transportation and distribution.
  5. Waste generated in operations.
  6. Business travel.
  7. Employee commuting.
  8. Upstream leased assets.

Downstream

  1. Downstream transportation and distribution.
  2. Processing of sold products.
  3. Use of sold products — dominant in energy-using products.
  4. End-of-life treatment of sold products.
  5. Downstream leased assets.
  6. Franchises.
  7. Investments — the largest category for financial institutions.

Calculation Methods

The GHG Protocol Scope 3 Standard recognises four core methods:

  1. Supplier-specific: verified product carbon footprints (PCF) from suppliers. Highest accuracy.
  2. Hybrid: supplier data where available, average data for the rest.
  3. Average data: industry average factor × physical quantity (kg, m³).
  4. Spend-based: currency spent × emission factor. Easiest, lowest accuracy.

Practical strategy: start supplier-specific for the top 20 suppliers and spend-based for the long tail; raise the supplier-specific share every year.

Where Does the Data Come From?

  • Purchasing / ERP: spend and physical quantities for Categories 1, 2, 4.
  • HR / travel: Categories 6 and 7.
  • Logistics providers: tonne-km for Categories 4 and 9.
  • Life-cycle assessment (LCA): Categories 11 and 12.
  • Supplier surveys / CDP Supply Chain: for supplier-specific data.

Common Mistakes

  • "My suppliers don''t measure, so I can''t." Build a spend-based baseline first, then improve.
  • Double counting. Draw a clear line between Cat-1 and Cat-4 to avoid duplicating the same emissions.
  • Ignoring the use phase. For appliances, heating and electronics, Category 11 can be the bulk of the total.
  • Running it in Excel. 15 categories, thousands of SKUs, hundreds of suppliers — not sustainable.

The Role of ETTA

ETTA maps ERP purchasing data to an emission-factor library, collects supplier surveys in a single interface and combines them with consumption/production data to produce CDP/GHG-aligned reports for all 15 categories. Every number carries a source and method trace — a defensible footprint at verification time.

Bottom Line

The uncertainty of Scope 3 is not an excuse — it''s a starting point. Measure roughly first, focus on the largest category, raise accuracy every year. In three years you''re ahead of competitors.