How do you compare two EPDs fairly

Compare environmental product declarations only after the products do the same job, the calculation uses compatible methods, and the figures refer to the same quantity and life-cycle stages. A smaller number on the first page is not enough. Converting the units can reverse the apparent result.

For UK construction buyers, a useful first output is a documented carbon screening calculation with an explicit boundary. It is not, by itself, permission to substitute a product or claim that a building has a lower whole-life footprint.

An environmental product declaration, or EPD, provides verified environmental information. It does not award the product an overall environmental ranking. EPD Australasia explains that the user must assess the information in the context of the project and intended use.

The immediate takeaways are:

• Read the unit beside the number, not just the number.
• Match the impact indicator, methodology and included stages.
• Preserve the assumptions that make the options functionally comparable.
• Describe the result as narrowly as the evidence allows.

An EPD can use a declared quantity such as a kilogram, square metre or cubic metre. H+H's UK explanation illustrates why that unit needs attention. Equal numerical values with different denominators are not equal product impacts.

Before calculation, identify the actual product, manufacturing coverage, EPD identifier, version and current validity. Check that the declaration covers the proposed specification; an industry average or a representative product may not describe the exact item being offered.

Then record the PCR—the product category rules—and relevant standard/version, impact method, indicator and stage coverage. EPD International describes PCRs as category-specific rules for consistent assessment. A shared acronym or a similar-looking table does not establish compatibility.

For example, the International EPD System's published limitations on use identify conditions including aligned PCRs, matching functions and technical performance, consistent life-cycle scope and impact methods, and current validity. Read the conditions in the actual declarations and applicable programme. This linked declaration is a source for that warning, not a product recommendation.

If you cannot establish compatibility, label the comparison not yet supported. Ask the supplier or competent life-cycle assessment practitioner for the missing basis. Do not repair a methodological difference by changing a spreadsheet label.

Everything in this example is fictional. These are invented inputs for arithmetic, not figures from real EPDs, tested products or a completed project.

A buyer is screening two panel options for 500 m² of specified panel coverage, not 500 m² of building floor area. For the exercise, assume both options have identical function, required technical performance and intended use; their valid declarations use compatible methods and the same GWP-total indicator. The compared values cover A1–A3, the product stage. Downstream stages are outside this calculation.

The tempting comparison is 1.80 versus 12.00. It is invalid because the units differ.

For A, multiply the emissions per kilogram by the product mass needed for one square metre:

1.80 kg CO₂e/kg × 8.00 kg/m² = 14.40 kg CO₂e/m².

B already uses the required area unit, so its result remains 12.00 kg CO₂e/m². The fictional mass and coverage factors are supplied assumptions; for a real purchase, retrieve them for the exact product configuration.

B is 1,200 kg CO₂e lower in this limited calculation. Relative to A, that is 1,200 ÷ 7,200 × 100 = 16.7%, rounded to one decimal. Using B as the denominator answers a different question: A is 20% higher than B.

This result concerns product-stage carbon for the assumed quantity. It excludes delivery, installation, waste allowances, supporting components, use, replacements and end-of-life impacts. Those exclusions are not assumed to be zero. No overall environmental superiority or real avoided emissions have been demonstrated.

A's break-even mass is 12.00 ÷ 1.80 = 6.6667 kg/m². Above that mass, A has the higher A1–A3 result; below it, A has the lower result—provided the same performance and methodological assumptions still hold.

If the verified requirement were instead 6.00 kg/m², A would produce 10.80 kg CO₂e/m², or 5,400 kg CO₂e for the package. That is 600 kg CO₂e below B, reversing the ranking.

This is a sensitivity test, not advice to make a panel thinner. A quantity change that reduces necessary performance invalidates the comparison. The practical next action is to verify the mass or coverage factor against the specified product and design, not to select the most favourable number.

Also keep boundaries separate. A1–A3 is not A1–A5, and neither is a whole-life result. RICS' whole-life carbon standard, second edition, version 3, treats functional requirements and service life as relevant to comparisons and reports module D separately. Do not subtract a possible recycling benefit from the A1–A3 figure and still label the result A1–A3.

A completed fictional screening record could read:

Record EPD-SCREEN-01 — 16 October 2026. Purpose: compare A1–A3 GWP-total for 500 m² of the specified panel coverage. Under the stated compatibility and performance assumptions, A totals 7,200 kg CO₂e and B totals 6,000 kg CO₂e. B is 16.7% lower relative to A within that boundary. These are fictional calculated results, not measured savings.

Decision: retain B as a candidate for further assessment; no substitution or whole-life claim approved. The buyer must obtain the exact EPD IDs and validity evidence, and the designer must confirm the product quantity and performance basis. A competent assessor must resolve method compatibility and the wider project assessment before any broader carbon conclusion.

In a real record, replace every assumption with a source reference or an explicit unresolved item. Retain the original figures, units, conversion, denominator, EPD version, retrieval date and decision owner. Our guide to keeping a factor's version and boundary explains the traceability principle; its South African grid factors are not inputs to this product calculation.

Finally, keep evidence checks separate from product acceptance. The same distinction underlies checking whether a supplier certificate covers your order. An authentic document can still leave the purchasing decision unresolved.

AI assistance was used in research and drafting. This article offers a screening example, not a product specification, engineering assessment or determination of legal obligations.

EPD Australasia — What is an EPD — declaration purpose and limits.

EPD International — Product Category Rules — category-specific assessment rules.

International EPD System — SEELA declaration — Ownership and limitation on use of EPD; cited only for comparability conditions.

H+H UK — Environmental Product Declarations — declared units and life-cycle scope; no manufacturer endorsement.

RICS — Whole life carbon assessment for the built environment — second edition, version 3, August 2024; sections 3.1 and 6.2.4. Primary sources checked 16 October 2026.

This article is general information. Verify current requirements and seek qualified advice for your specific circumstances.

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