Scope 3 is where most of a mid-market company's emissions live and where most of the data uncertainty lives too. Category 1 (purchased goods and services) alone can represent more than half of a company's total footprint, yet the suppliers who generate those emissions are often not disclosing their own carbon data. The result is that for most organizations, Scope 3 category 1 is estimated rather than directly measured. The right question is not whether to estimate, but how to estimate honestly and document the method with enough rigor to defend it.
What spend-based means
The spend-based approach converts procurement spending into an emissions estimate using an Environmentally Extended Input-Output model, commonly referred to as EEIO. An EEIO model maps economic sectors to emission intensities: for every dollar spent on a given sector (industrial chemicals, freight transportation, professional services, electronic components), it estimates the upstream greenhouse gas emissions embedded in that spending. The US EPA's USEEIO model is the most widely used source for US-based reporting. DEFRA's supply chain emission factors serve the equivalent function for UK-based procurement.
The approach works by multiplying the spend in a procurement category by the emission factor for the corresponding sector. As a worked example: $500,000 spent on industrial chemicals with an EEIO factor of 0.82 kg CO2e per USD produces an estimated 410 metric tonnes CO2e for that category. The number is an estimate, not a direct measurement. The GHG Protocol is explicit that spend-based methods are appropriate when supplier-specific data is unavailable, and the standard does not require a spend-based figure to be treated as more precise than it actually is.
Why it is acceptable and what its limits are
Many sustainability leads and finance executives feel uncomfortable disclosing a Scope 3 figure they know is an estimate. That discomfort is understandable, but it reflects a misreading of what the GHG Protocol expects. The standard asks for a reasonable, documented estimate using a recognized methodology. It does not ask organizations to skip categories where direct data is unavailable. Skipping them understates the footprint without disclosing the exclusion, which is the more serious problem from a reporting integrity standpoint.
The limitations of EEIO estimates are real and should be acknowledged in the report. EEIO factors are sector averages derived from national economic data. They do not account for variation between individual suppliers within a sector, they may not reflect recent changes in sector emission intensity, and they are sensitive to how procurement spend is categorized. A supplier that straddles two sectors can produce materially different estimates depending on which sector code is applied. None of these limitations make the method inappropriate. They make documentation and transparency essential.
EEIO vs. supplier-specific: the tradeoffs
- Spend-based (EEIO): Lower data burden. Uses sector averages from national economic accounts. Appropriate when supplier-specific emission factors are not available. Less precise at the individual supplier level but capable of covering the full category without gaps.
- Average data method: Uses average emission intensities per physical unit from databases such as ecoinvent or industry association publications. More granular than EEIO for specific materials but still not specific to a given supplier's actual operations.
- Supplier-specific: Uses emission data reported directly by suppliers via CDP or direct disclosure, or calculated from the supplier's own metered data. Highest precision, but requires active supplier engagement, data quality verification, and consistent scope definitions.
- Hybrid approach: Applies supplier-specific data where it has been collected and verified, and fills remaining categories with spend-based or average-data estimates. This is the target state for a mature Scope 3 program and usually takes two to four years to build meaningfully.
How to improve confidence over time
The path from a spend-based baseline to higher-confidence Scope 3 is a multi-year program. Year one is the baseline: apply EEIO to all procurement categories using a consistent spend classification, label the method clearly, and report a complete category 1 estimate with the factor source and vintage cited. The completeness of the baseline matters more than the precision of any individual line.
In year two, use the baseline to identify the three to five spend categories that contribute the most estimated emissions. Those categories are the candidates for supplier engagement. Request carbon disclosures via CDP or directly from key suppliers, and begin replacing EEIO sector averages with average-data or supplier-specific factors where verified data is available. The remaining categories continue on EEIO.
By year three, a hybrid approach is usually in place for the highest-priority categories, and the EEIO baseline remains for the long tail of smaller spend categories where the engagement cost exceeds the precision benefit. Each category should carry a confidence label in the report: high for supplier-specific metered data, medium for average-data or supplier CDP disclosure, low for spend-based EEIO. That labeling is what lets a sustainability lead report progress accurately without misrepresenting the precision of any given figure.
Disclosing the method honestly
A Scope 3 section that says "Category 1 emissions are estimated at 4,120 tCO2e, calculated using the US EPA USEEIO v2.0 supply chain emission factors, spend-based approach, 2023 spend data" is a credible disclosure. It names the method, the factor source, the model version, and the data year. A reviewer can understand what they are looking at, assess the limitations, and track changes in methodology between reporting periods.
A Scope 3 section that omits category 1 because the data is "not yet reliable enough to disclose" is a section with an unexplained gap in the footprint. The spend-based estimate, even with its limitations, is more honest than the omission, provided the limitations are stated clearly.
What Verdant does with your spend data
When you connect procurement data to Verdant, it maps your spend categories to EEIO sectors, applies the current EPA USEEIO or DEFRA factors depending on your reporting jurisdiction, and shows the factor, the spend amount, and the arithmetic for each line. Categories with the largest estimated emissions are flagged as candidates for supplier engagement planning. Each figure in the Scope 3 section of your draft report carries the confidence label and the methodology note.
As you collect supplier-specific data in subsequent years, those figures replace the EEIO estimates in the model and the confidence labels update accordingly. The result is a Scope 3 section where the method behind each figure is explicit and the progression toward higher-confidence data is visible in the report rather than described in narrative form.