Embodied Carbon Tools Compared: Why Your Numbers Don’t Match Your Consultant’s

Picture this. You’re in a design team meeting, reviewing the structural package for a multi-story precast concrete frame — a mixed-use residential tower in a city center. Your structural engineer’s sustainability consultant has run the embodied carbon assessment and presents a figure of 320 kgCO₂e/m². Your architect’s team runs the same frame through their preferred tool and comes back with 410 kgCO₂e/m². Same building. Same concrete specification. Same floor-to-floor heights. Different tools — and both teams are absolutely certain they’re right.

The room goes quiet. The client looks between you both, wondering why two professional firms can’t agree on a number. And in that moment, neither team has a clean answer.

This isn’t an edge case. It is the current state of embodied carbon measurement — and it is creating real commercial problems as more clients begin writing carbon targets into contracts, procurement specifications, and planning submissions.

Who Needs to Understand This — and Why Now

This is a conversation aimed squarely at architects, structural engineers, sustainability consultants, and the clients commissioning them — particularly those working on structurally intensive building types where embodied carbon decisions carry significant financial and environmental weight. Precast concrete frames, composite steel structures, post-tensioned slabs, and concrete cores are where the numbers diverge most, because these are the material categories with the widest spread of EPD data quality and the most sensitivity to regional manufacturing assumptions.

If your practice is just beginning to use embodied carbon tools, or if you’re a client who has started asking for carbon assessments and is confused about why different consultants report different numbers — this is the explanation you’ve been looking for.

The Three Sources of Variance

The 90 kgCO₂e/m² gap in our opening example isn’t a mistake. It comes from three compounding sources of legitimate methodological difference:

1. Which life cycle modules are included

The standardized framework for life-cycle assessment of buildings under EN 15978 categorizes emissions into life-cycle stages — A1–A3 covers product manufacture (raw material extraction, transport to factory, manufacturing); A4–A5 covers transport to site and construction activities; B covers use-phase impacts; C covers end-of-life; and D captures credits and loads beyond the system boundary.

Cradle-to-gate or Product stage embodied carbon (A1–A3) is the minimum scope of life cycle data that can be included in an EPD — and many projects report only this scope, leaving out construction-phase emissions, waste, and temporary works entirely.

A1–A3 only versus A1–A5 is not a small difference on a concrete-intensive project. Construction waste, formwork, temporary propping, crane fuel, and concrete over-ordering regularly add 10–20% to the product-stage figure. If one firm reports A1–A3 and another reports A1–A5, the gap is structural — and entirely invisible unless both declare their scope explicitly.

2. Which EPD database is queried — and where it comes from

Tally relies on the 2017 GaBi background Life Cycle Inventory (LCI) database from Sphera, containing mostly industry average data with North American average values and no regionalisation. One Click LCA has the largest global LCI database, consisting of mostly publicly available manufacturer-specific and industry average EPDs, with regionalized assumptions for Canadian provinces and US states.

A key methodological difference between US and European tools is the characterization method used — TRACI in the US versus CML internationally. While some EPDs report in both methods, many do not. Including a European EPD in a LEED US Tool using the TRACI method requires specific verification with the manufacturer.

In plain terms: a precast concrete panel manufactured in Poland has a very different carbon intensity than the same panel manufactured in Texas. If your tool queries a North American industry average rather than a European manufacturer-specific EPD, the numbers will diverge — and neither is wrong, they’re just measuring different things.

3. How conservative the tool is when no specific EPD exists

EC3 is primarily an EPD-based procurement tool addressing cradle-to-gate data (A1–A3), with recently added options for A4 and A5 data. Tally generally does not allow product-specific EPDs to be applied directly — pushing data from Tally into EC3 is the primary way to model product-specific data, though the result would no longer be a whole-building LCA because EC3 lacks the later life cycle stages.

When a specific product EPD isn’t available, each tool falls back on a different default dataset. A conservative tool uses an industry-worst dataset; a less conservative tool uses an industry average. The difference on a major structural material like reinforced concrete or structural steel can be 30–40 kgCO₂e/m² on its own.

Tool Comparison Table

Is the Difference the Standard or the Location?

Both. And that’s what makes this genuinely complex rather than just a software preference question.

The European Commission mandated CEN to develop harmonised standards for assessing environmental impacts of construction products and buildings, resulting in EN 15804 (for EPDs) and EN 15978 (for building-level LCA). EPD schemes in the UK, France, Germany, the Netherlands, Sweden, Norway, Spain, Portugal, Italy and the United States have now adopted the EN 15804 standard — with the European schemes united through the ECO Platform organisation.

The major amendment EN 15804+A2 was approved in July 2019 and became mandatory in October 2022 for all new EPDs. In January 2025, the revised EU Construction Products Regulation entered into force, introducing additional requirements for EPDs aligned with the EN 15804 standard.

So the underlying standard framework is increasingly converging. The divergence that remains is largely driven by location: the regional carbon intensity of electricity grids, the SCM (supplementary cementitious material) content of local concrete mixes, the transport distances assumed between factory and site, and the manufacturing profile of local steel production. A concrete frame assessed in Germany using German-average EPDs will report lower embodied carbon than the same frame assessed in the US using North American averages — not because Germany is cheating, but because German cement production is genuinely lower-carbon on average, and the European grid is cleaner.

RICS Whole Life Carbon Assessment 2nd edition (effective July 2024) sits on top of BS EN 15978 and provides a practical global standard — applying across buildings and infrastructure worldwide, with a clear data hierarchy requiring practitioners to use manufacturer-specific EPDs for high-impact elements such as facade systems and structural systems, falling back to industry-average or generic data only when product-specific data is unavailable.

What Can We Actually Do About It? A Proposed Framework

This is the part of the conversation that usually gets skipped. Most discussions of embodied carbon tool variance stop at “the numbers don’t match” and leave practitioners no closer to resolving the problem. So here is a practical framework for unifying results across firms and tools:

Step 1 — Declare the methodology at appointment. Before any model is opened, the project’s Embodied Carbon Assessment Methodology should be documented in the EIR (Employer’s Information Requirements) or BEP (BIM Execution Plan). This document should specify: which tool is to be used (or which tools are acceptable), which life cycle modules are in scope (at minimum A1–A5 for new build), which EPD database hierarchy applies, whether product-specific or industry-average EPDs are required for structural elements, and which standard governs the assessment (EN 15978 / RICS WLCA 2nd edition for UK; ASHRAE / LEED pathway for US).

Step 2 — Align on a single reference database for structural materials. For concrete-intensive projects, require that all structural concrete assessments reference the same regional concrete mix EPDs — NRMCA regional benchmarks for US projects, or UK Concrete Industry EPDs for UK projects. This single alignment decision eliminates a significant portion of inter-tool variance on concrete frames.

Step 3 — Use EC3 for material procurement comparison, not whole-building reporting. EC3 enables carbon-smart choices during material specification and procurement — it is the only free and open-access global embodied carbon accounting tool, and using it to select and procure low-carbon materials has enabled firms including Microsoft to reduce embodied carbon by at least 30% compared to baseline on major campus projects. But EC3’s A1–A3 scope makes it unsuitable as a whole-building reporting tool. Use EC3 for product-level procurement decisions; use One Click LCA or Tally for whole-building compliance reporting.

Step 4 — Run a cross-tool calibration exercise at Stage 2. On any project with a contractual carbon target, both the design team’s tool and the client’s verification tool should be run on the same Stage 2 structural model, with results compared and variance sources documented. This is a 2–3 hour exercise that prevents a costly dispute at Stage 4.

Step 5 — Require manufacturer-specific EPDs for the top three carbon hotspots. EPDs are available for thousands of products, but some major data gaps remain for certain building materials and technologies — and EPDs are not designed to be directly comparable between products due to inconsistencies in LCA methodologies and background data. For structural concrete, structural steel, and facade systems — which typically account for 70–80% of upfront embodied carbon on commercial buildings — insisting on manufacturer-specific EPDs rather than generic defaults is the single most effective way to improve accuracy and comparability.

A Lesson the Industry Needs to Learn Collectively

The embodied carbon tool problem isn’t a technology problem. The tools are good enough. It’s a process and communication problem — and it starts at the client brief.

A common industry approach that’s worth adopting widely: at the very first sustainability workshop, before any carbon assessment is commissioned, work through the methodology questions explicitly with the client and all design team members together. Which tool? Which scope? Which database? These aren’t technical decisions to be made quietly by a sustainability consultant — they’re project governance decisions that affect whether a contractual carbon target can ever be verified consistently.

The firms getting this right are documenting the methodology at appointment, the same way they document the fire strategy or the acoustic specification. The firms getting it wrong are discovering the variance problem at planning submission — or worse, at handover, when a client asks why the as-built figure is 30% higher than what was promised.

Collaborative efforts between standard-setting bodies towards harmonizing definitions and coverage of emission scopes across different levels of standards would contribute to a more consistent and comparable carbon assessment — and a unified data scheme with standard nomenclature and data format would ensure interoperability across the tools the industry relies on. That harmonization is coming. But we don’t have to wait for it: declaring methodology clearly and early is something every project can do today.

The Bottom Line

Two firms reporting 320 and 410 kgCO₂e/m² for the same building are not making a mistake — they’re making different legitimate methodological choices that have never been aligned. The commercial and environmental stakes are now high enough that “different tools, different answers” is no longer an acceptable project outcome.

Specify the methodology at appointment. Align on the database. Calibrate at Stage 2. And when a client asks why the numbers don’t match, have the answer ready — because increasingly, they will ask.