If you’ve started mapping your company’s scope 3 emissions, you’ve probably hit a fork in the road pretty quickly: do you use spend-based or activity-based data? It’s one of the most practical decisions in any scope 3 calculation, and it shapes everything from how much time you’ll spend gathering data to how confident you can be in the final numbers. Neither method is universally better, but one will almost certainly fit your situation more naturally than the other.
Understanding the difference between these two approaches, and knowing when to use each, is what separates a scope 3 inventory that actually drives decisions from one that just fills a reporting box. Let’s walk through both methods honestly, including where they shine and where they fall short.
How each calculation method works in practice
The spend-based method estimates emissions by taking what your company spent with a supplier or on a category, then multiplying that figure by an emissions factor expressed in units of greenhouse gas per unit of currency. In practice, this means pulling procurement data from your finance systems and applying published average emission intensities, often sourced from databases like Exiobase or the EPA’s supply chain factors. It’s a top-down approach: you’re working from financial flows rather than physical ones.
The activity-based method works from the ground up. Instead of asking “how much did we spend?”, it asks “how much of something did we actually buy or use?” You collect physical data, such as kilograms of raw material purchased, kilometres travelled by a freight carrier, or litres of fuel consumed by a supplier, and multiply that by an emissions factor tied to the specific activity. The result is a figure grounded in what actually happened rather than what it cost.
Accuracy and data quality trade-offs
Spend-based calculations are faster to complete because financial data is almost always already available. Most companies can run a spend-based scope 3 inventory relatively quickly once they’ve identified the right emissions factors. The trade-off is accuracy: currency-based emissions factors are averages derived from economic input-output models, which means they represent a broad sector rather than your specific suppliers. Price fluctuations, geography, and supplier-specific practices all introduce noise into the result.
Activity-based data is harder to collect but significantly more precise. When you know that a specific supplier shipped 4,000 tonnes of goods by road over a known distance, you can calculate emissions far more accurately than any spend-based proxy allows. The challenge is that this data often sits with suppliers, logistics providers, or other partners who may not track it consistently or be willing to share it. For companies with complex supply chains spanning dozens of countries and hundreds of suppliers, collecting activity-based data across every category quickly becomes a resource-intensive exercise.
Which scope 3 categories suit each approach
Not every scope 3 category responds equally well to each method, and this is where being strategic really pays off.
Spend-based calculation tends to work reasonably well for categories where the connection between spending and emissions is relatively stable and where collecting physical data would be disproportionately difficult. Purchased goods and services (Category 1) is the classic example, particularly for companies with fragmented supplier bases. Capital goods (Category 2) and upstream transportation and distribution (Category 4) are also frequently handled with spend-based data in early-stage inventories.
Activity-based calculation is the stronger choice for categories where physical data is more accessible and where the emissions impact is significant enough to justify the effort. Business travel (Category 6) is a natural fit because flight and hotel data are usually already tracked internally. Employee commuting (Category 7), use of sold products (Category 11), and end-of-life treatment of sold products (Category 12) are also well-suited to activity-based approaches because the underlying physical data, distances travelled, energy consumed, and waste volumes, is either available or estimable with reasonable precision.
The practical takeaway is that most companies end up using a hybrid: spend-based for the long tail of lower-impact categories, and activity-based where emissions are concentrated and the data is within reach. This isn’t a compromise; it’s actually the approach recommended by the GHG Protocol’s Corporate Value Chain Standard.
Key factors that determine the right fit for your company
Choosing the right starting point comes down to a few practical realities about your company’s situation.
- Data availability: If your procurement team tracks spending in detail but has little visibility into physical quantities, spend-based is the realistic starting point. If you have strong supplier relationships and can request activity data, you have more options.
- Materiality of categories: Where emissions are largest, accuracy matters most. It’s worth investing in activity-based data for your top two or three emission hotspots even if you use spend-based everywhere else.
- Reporting requirements: If you’re reporting under CSRD or disclosing through CDP, the quality of your scope 3 data will be scrutinised. Spend-based figures are acceptable, but you’ll need to be transparent about their limitations and show a credible path toward improvement.
- Internal capacity: Activity-based data collection requires time, supplier engagement, and often a specialist who understands what data to ask for and how to validate it. A scope 3 emissions reduction consultant, for example, will approach this very differently from a generalist sustainability consultant, so the right support matters.
Taken together, these factors point toward a simple principle: start with what’s realistic, be honest about uncertainty, and focus your energy where it actually moves the needle. A spend-based inventory that’s completed and used is more valuable than an activity-based one that’s perpetually in progress.
Moving from spend-based to activity-based over time
Most companies that take scope 3 seriously don’t stay purely spend-based forever. The natural progression is to use spend-based data to build a first inventory, identify which categories drive the most emissions, and then gradually shift those hotspot categories to activity-based data in subsequent reporting cycles.
This transition typically starts with supplier engagement. Reaching out to key suppliers to request emissions data, or at least physical quantity data, is often the first step. Over time, some companies embed data requests into procurement processes or supplier onboarding, making activity-based data collection a routine part of how they work with their supply chain rather than a one-off project.
It’s worth noting that the transition doesn’t need to be all-or-nothing. Many mature scope 3 inventories maintain spend-based estimates for categories that are low-impact or where supplier data remains inaccessible, while applying activity-based methods to the categories that matter most. The GHG Protocol explicitly allows this mixed approach, and frameworks like SBTi and CDP both recognise that improving data quality is an ongoing process rather than a prerequisite for getting started.
Ready to get your scope 3 calculation right?
Scope 3 accounting is genuinely complex, and getting the method right from the start saves a lot of rework down the line. Whether you’re building your first inventory or trying to improve the quality of an existing one, having the right expertise on your side makes a real difference.
At Dazzle, we match organisations with pre-screened sustainability freelancers who specialise in exactly this kind of work, from scope 3 emissions specialists to sustainability reporting experts who know their way around CSRD and CDP requirements. You can be working with the right expert within 48 hours, without the lengthy processes that come with traditional consultancies. If you’d like to find the right match for your project, our team is ready to help.
