How to Build a Scope 3 Emissions Inventory From Scratch

Most companies spend months getting Scope 1 and 2 right, then face a wall when they get to Scope 3. It's not a measurement problem. It's a data problem. Scope 3 covers every emission that happens outside your facility walls: your suppliers' energy use, the flights your employees take, the packaging your products ship in, what happens when customers use your product at home.

Under California SB 253, Scope 3 reporting becomes mandatory in 2027 for companies with $1B+ in California revenue. Under EU CSRD, Scope 3 is already part of double materiality assessments for qualifying companies. If you haven't started, you're not behind on compliance yet — but you are running out of time to build the data infrastructure you'll need. See the full SB 253 timeline breakdown →

What is Scope 3? The 15 categories, briefly

The GHG Protocol Corporate Standard organizes Scope 3 into 15 categories split between upstream (things you buy) and downstream (things your customers do with what you sell).

You don't have to measure all 15. The GHG Protocol's "minimum boundary" approach asks you to identify which categories are material — meaning they represent a meaningful portion of your total footprint. For most mid-market companies, 5-8 categories cover 90%+ of Scope 3 emissions. Identifying them first saves enormous effort. See how CSRD and SB 253 handle Scope 3 differently →

Step 1: Conduct a materiality assessment

Before collecting any data, map your business activities to the 15 categories and rank them by likely contribution. A simple spend-based screening is enough for a first pass: for each category, estimate annual spend or activity volume and multiply by an EEIO (environmentally extended input-output) emission factor to get a rough tCO2e estimate.

This screening takes a day or two with spend data from your ERP or procurement system. It's rough, but it's the right starting point. Try CarbonPilot's Scope 3 calculator with your actual spend data →

Step 2: Choose your data collection method

Once you know which categories to focus on, pick a data method for each. Three main approaches exist, each with a precision/ effort tradeoff:

• Spend-based: Multiply supplier spend by an EEIO emission factor. Fast, cheap, works for any supplier category. Accuracy is ~±30-50%. Good for initial inventory and Categories 1 and 2.
• Activity-based: Collect physical activity data (tonnes of materials, kilometers traveled, kWh consumed) and apply granular emission factors. Accurate, but requires supplier data or direct measurement. Best for Categories 6 (travel), 7 (commuting), and major purchased goods where you have supplier cooperation.
• Hybrid: Use spend-based for the long tail of suppliers you can't get data from, activity-based for your top 10-20 vendors by spend. This is the practical standard for most first-time Scope 3 inventories.

Step 3: Select emission factors

For spend-based Scope 3 (Categories 1 and 2), the standard source is EXIOBASE — a free, academically maintained database maintained by academic consortiums, mapping 163 industry sectors across 49 countries. For travel (Category 6), DEFRA's annual emission factors cover flights by distance band, rail, car, and hotel stays. See the full guide to choosing the right emission factors →

Make sure you use factors that match the geography and year of your data. An emission factor for "professional services" in the US will differ from the same category in India. If you're mixing global suppliers under one spend-based estimate, use region-specific factors where possible or apply a global average with a documented caveat.

Step 4: Calculate, then validate

For each material category:

1. Pull your activity data (spend, quantity, or both)
2. Apply the relevant emission factor
3. Sum into a Scope 3 total
4. Check for obvious outliers — a $500K software spend producing 50,000 tCO2e is a factor error, not a real number

For SB 253's limited assurance requirement, auditors will look at your methodology documentation, not just the final number. Keep records of: which emission factor database you used, what version, what year, and how you applied it to your data.

Common pitfalls to avoid

Double counting Scope 2 in Scope 3

Category 3 (fuel- and energy-related activities) captures upstream emissions from purchased electricity generation — the emissions that occur at the power plant before the electricity reaches your facility. Do not double count this with your Scope 2 figure. Scope 2 is the emissions from generating the electricity you actually consume. Category 3 is the upstream supply chain of that electricity.

Stale or mismatched emission factors

EXIOBASE updates every 3-5 years. DEFRA updates annually. Using a 2019 factor for 2025 data introduces error. Document your factor version and publication date. If you're doing year-over-year comparisons, use the same factor version for both years so the trend is real, not a factor update artifact.

Incomplete organizational boundaries

Scope 3 must cover all entities you control or have significant influence over — which often includes joint ventures, minority investments, and leased assets. If you have operations in multiple entities or countries, define your boundary clearly and document the exclusion rationale for any material category you skip.

Ignoring the long tail of suppliers

Your top 20 suppliers by spend might represent 70-80% of your Category 1 emissions. The remaining hundreds of suppliers are the long tail. It's acceptable to use spend-based estimation for the long tail, but don't exclude it entirely — it adds up.

What a first Scope 3 inventory looks like in practice

Most first-time Scope 3 inventories land between 60-90% of total company emissions. For a $200M revenue company with a meaningful supply chain, 5,000-30,000 tCO2e in Scope 3 is common. The number is large, but it's not the first number that matters — it's the methodology. A defensible estimate based on spend-based methods is more useful than a precise-looking number built on guesswork.

Once you have your first inventory, the job isn't done: year-over-year, you should be collecting better supplier data and replacing spend-based estimates with activity-based data for your top categories. That's how Scope 3 quality improves over time.

Try CarbonPilot's Scope 3 calculator to build your first inventory →