How to Improve Scope 3 Accuracy Without Overcomplicating It
Scope 3 has a reputation problem. It's the category everyone knows matters most — often 70-90% of a company's total footprint — and also the one most businesses quietly dread, because it can feel like it demands a level of data perfection that's simply out of reach for an SME with no dedicated sustainability team. The good news: you don't need perfect data to get meaningfully more accurate. You need a sensible sequence of improvements, applied where they matter most. Here's how to do that without the process spiralling.
1. Start by Ranking Your Categories, Not Calculating Them All Equally
The GHG Protocol lists 15 Scope 3 categories, and treating them all as equally urgent is the fastest way to overcomplicate things. Most businesses find that two or three categories — commonly purchased goods and services, upstream transportation, and use of sold products — dominate the total. Do a quick materiality pass first: a rough, spend-based estimate across all categories to see where the mass actually sits. Then focus your improvement effort on the categories that move the needle, and leave the small ones on simple estimates. Precision on a category that's 2% of your footprint is wasted effort.
2. Move From Spend-Based to Activity-Based Data — Category by Category
Spend-based estimates (multiplying £ spent by a generic emission factor) are the right starting point, but they're the least accurate method available, because they assume every pound spent in a category has the same carbon intensity. Once you know which categories matter, upgrade those specifically to activity-based data — actual quantities, weights, or units, matched to more specific emission factors. You don't need to do this everywhere. Upgrading your top two or three categories from spend-based to activity-based typically delivers most of the accuracy gain available.
3. Go After Primary Supplier Data Selectively, Not Universally
Asking every supplier for primary emissions data is unrealistic and will stall your project. Instead, identify your highest-emitting suppliers within your priority categories — often a small number driving a large share of the total — and approach them directly. A short, specific ask (their emissions per unit or per £ of what you buy from them) is far more likely to get a response than a generic sustainability questionnaire. Where you can't get primary data, a well-matched sector-average factor is a perfectly reasonable fallback.
4. Use Better-Matched Emission Factors
Before Chasing New Data Before asking anyone for anything, check whether you're using the most specific factor already available to you. Generic "average product" factors are often replaceable with sector-specific, region-specific, or product-specific ones from the same public databases you're already using. This is a low-effort, immediate accuracy improvement with no new data collection required.
5. Document Your Assumptions as You Go
Every estimate, proxy, and assumption should be written down as you make it — not reconstructed after the fact. This isn't extra work if it's built into the process from day one, and it means:
You can explain any figure if a customer or auditor asks
You can see exactly where your biggest assumptions sit, which tells you where future improvement effort should go
Next year's report builds on this year's, instead of starting from scratch
6. Improve Year on Year, Not All at Once
Scope 3 accuracy is a multi-year project for almost every business, not a one-off task. Trying to nail every category to activity-based, verified, primary data in year one is how Scope 3 projects stall out entirely. A more realistic approach: pick your top two or three priority categories, improve those this year, and expand the following year. Reporting frameworks generally expect and accommodate this kind of progressive improvement — what they don't forgive is silence or a static, unexplained estimate that never gets better.
The Underlying Principle
Better Scope 3 accuracy isn't about doing everything possible. It's about spending your limited time and data-collection effort on the categories and suppliers that actually move your total — and being transparent about where you're still estimating. That's both more achievable and, in practice, more credible to anyone reviewing your numbers than a report that looks precise everywhere but can't explain its own assumptions.
Combatting Carbon helps SMEs build Scope 3 accounts that improve year on year — prioritised, documented, and reviewed by people who understand the frameworks behind them.