How to build a scope 3 carbon footprint baseline for the first time

Building a scope 3 carbon footprint baseline for the first time can feel like staring at a very large, very complicated puzzle. You know the pieces exist somewhere, but figuring out where to start, which ones matter most, and how they all fit together is genuinely tricky. The good news is that it’s far more manageable than it looks once you break it down into clear steps. This guide walks you through exactly that, from understanding what scope 3 actually covers to turning your completed baseline into a plan for real emissions reductions.

Getting this right matters more than ever in 2026. With frameworks like CSRD placing stricter reporting expectations on companies across Europe, a well-constructed scope 3 carbon footprint baseline isn’t just a nice-to-have. It’s quickly becoming the foundation of credible climate action.

What scope 3 emissions actually cover

Scope 3 is the category that catches most companies off guard. Unlike scope 1 (direct emissions from sources you own or control) and scope 2 (emissions from purchased energy), scope 3 covers everything else across your value chain. That means emissions generated by your suppliers, your customers, the products you sell, and the services you buy.

The Greenhouse Gas Protocol divides scope 3 into 15 categories, split between upstream activities (things happening before your product reaches you) and downstream activities (what happens after it leaves). Upstream categories include purchased goods and services, business travel, and employee commuting. Downstream categories cover the use of sold products, end-of-life treatment, and investments, among others. For most companies, scope 3 represents the vast majority of their total carbon footprint, which is exactly why it’s both the hardest and most important part to measure.

Choosing which scope 3 categories to prioritize

You don’t have to tackle all 15 categories at once, and trying to do so is one of the fastest ways to get overwhelmed and stall. The smarter approach is to identify which categories are most material to your business first.

Materiality here means relevance and scale. A manufacturing company will likely find that purchased goods and services dominate its footprint. A professional services firm might find that business travel and employee commuting are far more significant. To figure out where your emissions are concentrated, start with a high-level screening using spend data or industry benchmarks to estimate which categories are likely to be largest. From there, you can decide where to invest effort in detailed data collection and where a reasonable estimate will do for now. SBTi guidance also provides useful direction on which categories companies in different sectors should prioritize when setting science-based targets.

Data sources and collection methods for each category

Once you know which categories to focus on, the next challenge is actually gathering the data. This is where many first-time scope 3 projects hit friction, because the data lives in a lot of different places and in a lot of different formats.

There are several approaches you can use depending on the category and what’s available:

• Spend-based methods: You use financial data and emissions factors to estimate the carbon intensity of your purchases. This is often the easiest starting point when supplier-specific data isn’t available yet, though it’s less precise than other approaches.
• Activity-based methods: You collect actual activity data, such as kilometers traveled or tonnes of materials purchased, and apply relevant emissions factors. This gives more accurate results but requires more detailed data collection.
• Supplier-specific data: You request actual emissions data directly from suppliers. This is the gold standard for accuracy, but it takes time and depends on your suppliers’ own measurement capabilities.
• Life cycle assessment (LCA) data: For product-related categories, LCA data can provide detailed, product-level emissions information. LCA specialists are a distinct type of sustainability expert, and their work can be invaluable here.

The right method depends on the category, the data you have access to, and how much precision you need at this stage. In practice, most first-time baselines use a combination of all of these, starting with spend-based estimates where necessary and improving data quality over time. What matters most at this stage is being consistent and transparent about your methodology so your baseline is defensible and comparable year on year.

Common pitfalls when building a scope 3 baseline

Even with the best intentions, there are a few places where scope 3 projects tend to go sideways. Knowing them in advance saves a lot of backtracking.

One of the most common mistakes is double-counting. Because scope 3 spans your entire value chain, it’s possible to count the same emissions more than once, particularly when emissions from one company’s scope 3 overlap with another’s scope 1 or 2. Clear boundary-setting at the start of your project helps prevent this. Another frequent issue is inconsistent base years. If you’re building a baseline to track progress against, you need to be rigorous about using a consistent reference year and documenting any recalculations when your business structure changes significantly. Finally, many companies underestimate the importance of internal alignment. Scope 3 data collection touches procurement, finance, operations, and HR, among other teams. Getting buy-in and clear ownership across those functions early makes the whole process much smoother.

How to document and validate your baseline

A scope 3 baseline is only as useful as its documentation. If the methodology, assumptions, and data sources aren’t clearly recorded, it becomes very difficult to update, audit, or use as the basis for target-setting.

Good documentation covers your chosen calculation methodology for each category, the data sources used, any estimation approaches applied, and the emissions factors you relied on (including their source and year). It also captures your organizational and operational boundaries, so it’s clear what’s in and what’s out. Once documented, many organizations choose to have their baseline externally verified. Frameworks like CDP encourage or require third-party assurance, and for companies reporting under CSRD, the expectations around data quality and verification are becoming more formal. Validation doesn’t have to be a huge undertaking at the baseline stage, but having someone independent review your methodology and spot-check your calculations adds real credibility to the numbers.

Turning your baseline into a reduction roadmap

A baseline on its own is just a number. Its real value comes from what you do with it. Once you have a clear picture of where your scope 3 emissions are coming from, you can start identifying where meaningful reductions are actually possible.

The most impactful lever for most companies is supplier engagement. Working with key suppliers to reduce their own emissions, switch to lower-carbon materials, or share better data creates improvements that flow directly into your scope 3 footprint over time. Product design is another high-impact area, since decisions made at the design stage often determine the majority of a product’s lifetime emissions. For companies setting science-based targets through SBTi, the baseline also becomes the reference point for calculating required reduction trajectories across your value chain. The important thing is to treat the baseline as a living document rather than a one-time exercise. As your data quality improves and your reduction efforts take effect, your scope 3 picture will sharpen, and your roadmap should evolve with it.

Ready to get started?

Building a scope 3 carbon footprint baseline for the first time is a significant undertaking, but it’s entirely achievable with the right expertise in your corner. Whether you need a scope 3 emissions specialist to lead the full process or someone to validate the methodology you’ve already developed, the kind of help you need depends on where you are in your journey.

At Dazzle, we match organizations with pre-screened sustainability freelancers who specialize in exactly this kind of work. Our network includes scope 3 emissions experts, LCA specialists, and sustainability reporting professionals who can step in quickly and flexibly, whether you need project-based support or an interim team member. You can be working with the right expert within 48 hours. If you’re ready to move forward, get in touch with our team and we’ll find the right match for your specific challenge.

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