Embodied Carbon · EN 15804 / EN 15978
Embodied Carbon Calculator — Concrete, Cement, GGBS, Fly Ash & LC3 (EN 15804 A1–A3)
Cradle-to-gate (A1–A3) embodied carbon for concrete mixes and cementitious binders. Model a specific mix design — CEM I against GGBS, fly ash, limestone fines, and LC3 — or look up market-average ready-mix by strength class. AR5-100 GWP basis throughout, the construction-EPD sector convention. Economic and by-product allocation both supported, with the clinker factor (k) shown for every mix.
What this calculator computes. Cradle-to-gate embodied carbon (EN 15978 modules A1–A3: raw material supply, transport to plant, and manufacturing) for either a user-defined concrete mix or a market-average ready-mix grade. All factors are EN 15804+A2 product-stage values on an AR5-100 GWP basis — the convention used by every construction EPD and the MPA fact sheet the binder model is built on.
Mix design mode (the binder model). Embodied carbon = Σ(constituent mass × constituent A1–A3 factor), summed over binder, coarse aggregate, fine aggregate, and batch water, per cubic metre. The binder is composed from CEM I Portland clinker (0.840 kg CO₂e/kg) and any combination of supplementary cementitious materials — GGBS, fly ash, limestone fines, or calcined clay. The displayed clinker factor (k) equals the CEM I fraction of the binder × 0.95 (the clinker content of CEM I).
Quick mode (ready-mix by grade). Market-average ready-mix factors by strength class (C8/10 to C55/67), sourced from ÖKOBAUDAT 2024. These already embed typical industry SCM use, which is why a Quick-mode C30/37 reads lower than a Mix-mode 100% CEM I C30/37. Quick mode answers “what does standard ready-mix of this grade emit?”; Mix design answers “what does my specified binder emit against a 100% CEM I baseline?”
Allocation basis. GGBS and fly ash are industrial by-products (of iron-making and coal combustion respectively), so their A1–A3 factor depends on how upstream burden is allocated. The calculator defaults to economic allocation (the MPA / RICS convention) and offers a by-product allocation toggle. The choice is disclosed in the result and materially affects the reported figure — see the allocation section below.
Excluded. Construction-stage (A4–A5), use-stage carbonation (B1), end-of-life (C1–C4) and reuse/recovery credit (module D) are out of the A1–A3 boundary. Where the ready-mix table shows C3 and D values, they are reported as separate EN 15804 lines and are never netted into A1–A3. Reinforcement steel is available as a separate constituent but is not part of the concrete factor itself.
Total cementitious (cement + SCMs). Typical 280–400 kg/m³.
From the admixture supplier’s EPD (optional).
Economic allocation is the MPA/RICS convention. By-product (zero-burden) shows SCMs in the most favourable light.
Set a binder content and SCM mix above to calculate
Results appear instantly. The component breakdown, the saving vs a 100% CEM I baseline, an SCM-strategy ladder, the clinker factor, and the full audit trail appear after calculation. Cradle-to-gate (A1–A3) only.
Results are indicative cradle-to-gate (EN 15804+A2 / EN 15978 Module A1–A3) embodied-carbon estimates for a concrete mix, reported as GHG Protocol Scope 3 Category 2 (Capital Goods) on an AR5-100 GWP basis. Binder and supplementary-cementitious-material factors are from the MPA Cement Fact Sheet 18 (UK cements, Rev 4, 2025); aggregate and water from the open ICE v2.0 dataset; ready-mix strength-class factors from ÖKOBAUDAT 2024. The result covers A1–A3 only — it excludes A4 transport to site, A5 construction/installation, B in-use effects (including any carbonation CO₂ re-uptake), and C end-of-life (except the C3/D modules shown in Quick whole-life mode). Reinforcement steel is a separate embodied-carbon line and is excluded from the concrete A1–A3 unless you toggle it in. GGBS and fly-ash factors depend materially on the allocation method (economic vs by-product) — the headline uses economic allocation by default. Any mix shown must still meet the specified strength class, early-age strength development, and durability/exposure requirements of BS 8500. Confirm the mix design, constituent quantities, and factors against your own records and supplier EPDs, and where material proceed to third-party verification.
In a typical building, concrete is the single largest source of embodied carbon by mass — and almost all of it comes from one ingredient: the clinker in cement. The same cubic metre of structural concrete can carry anywhere from under 170 to nearly 300 kg CO₂e depending on a specification decision the structural engineer makes long before the pour.
Cut the clinker and you cut the carbon — supplementary cementitious materials are the highest-leverage decarbonisation lever in the built environment.
A standard 100% CEM I concrete at 340 kg/m³ binder emits about 295 kg CO₂e/m³ (cradle-to-gate, A1–A3). Replacing 50% of the cement with GGBS cuts that to roughly 165–179 kg CO₂e/m³, depending on allocation basis.
Aggregate and water together contribute a near-constant ~9.6 kg CO₂e/m³ regardless of mix. Everything else is the binder. That means specification — how much cement, and how much of it is replaced with SCMs — is where essentially all of a concrete’s embodied carbon is won or lost. Model your own mix in the calculator above before you specify.
What Concrete Embodied Carbon Is — A1–A3, the Declared Unit, and the Binder Model
Embodied carbon is the greenhouse gas burden of producing a material, as distinct from the operational emissions of using the building it goes into. For concrete, the standard reporting boundary is cradle-to-gate — EN 15978 modules A1 (raw material supply), A2 (transport to the plant), and A3 (manufacturing), reported together as A1–A3. This is the figure that dominates a structural frame’s embodied carbon and the one most specifications target first.
Cradle-to-gate (A1–A3): what’s in, what’s out
| Included in A1–A3 (this calculator) | Excluded — separate modules |
|---|---|
| Cement / binder production (the dominant term) | Transport to site — module A4 |
| Coarse and fine aggregate extraction and processing | Construction / installation — module A5 |
| Batch water | In-use carbonation (CO₂ reabsorption) — module B1 |
| SCM processing (GGBS grinding, fly ash handling, clay calcination) | End-of-life demolition and processing — modules C1–C4 |
| Mixing and batching at the ready-mix plant | Reuse / recovery credit — module D (reported separately, never netted into A1–A3) |
Why this calculator models the mix, not just the C-grade
A strength class alone — C30/37, C40/50 — does not fix a concrete’s carbon. Two C30/37 mixes that meet the same characteristic strength can differ by 40% or more in embodied carbon depending on how much clinker the binder contains. A grade-only lookup gives a market average; it cannot tell a structural engineer what their specified 50% GGBS mix emits. The binder model in this calculator composes the actual cementitious blend and sums it against the aggregate and water, so the answer reflects the specification rather than an industry average. Quick mode (the C-grade table later in this article) remains useful as a reference point, but the mix model is where specification decisions are tested.
The unit trap: per kg of binder vs per m³ of concrete vs per kg of good
Three different denominators circulate in concrete carbon accounting, and conflating them is the most common error on this topic.
- kg CO₂e per kg of binder or constituent — how the CEM I (0.840), GGBS, fly ash and aggregate factors are expressed. These are ingredient intensities, multiplied by mass to build a mix.
- kg CO₂e per m³ of concrete — the reportable figure for a poured volume, and how the ready-mix grade table reads. This is what a quantity surveyor multiplies by concrete volume.
- kg CO₂e per kg of good — the basis of EU CBAM default values for imported cement. These are deliberately conservative border-tax penalty defaults covering direct and indirect production emissions — a different metric and boundary from cradle-to-gate EPD embodied carbon, and not comparable to the values here. CBAM defaults belong in the CBAM workflow, not in an embodied-carbon comparison.
Clinker Is the Carbon — The CEM I Baseline and the Clinker Factor (k)
Portland cement clinker is made by heating limestone and clay to about 1,450°C. Roughly two-thirds of the resulting CO₂ comes from calcination — the chemical decomposition of limestone (CaCO₃ → CaO + CO₂) — and the remaining third from the fuel burned to reach kiln temperature. Because calcination CO₂ is intrinsic to the chemistry, it cannot be eliminated by switching fuels; the only way to reduce it is to use less clinker. That is the entire logic of supplementary cementitious materials. The producer-side process emissions are accounted separately in the Scope 1 Cement & Lime Process Calculator; this page accounts the same material from the purchaser’s embodied-carbon perspective.
The 0.840 CEM I factor and what k means
This calculator uses a CEM I (Portland) A1–A3 factor of 0.840 kg CO₂e/kg, from MPA Cement Fact Sheet 18 — the UK-relevant, internally consistent binder set. Every binder mix is described by its clinker factor (k): the fraction of the binder that is CEM I, multiplied by the 0.95 clinker content of CEM I. A 100% CEM I binder has k = 0.475 (after the calculator’s normalisation); a 55% CEM I binder — as in the LC3-50 preset — has k ≈ 0.523 on the same basis. Lower k means less clinker per tonne of binder, and clinker is where the carbon is. k is the single number that best predicts a mix’s embodied carbon.
If you compare EPDs from different suppliers, check the clinker factor before the headline number. A low A1–A3 figure with an undisclosed clinker factor usually means high SCM content — which is good, but the allocation method behind those SCMs determines how much of the saving is real on a consequential basis.
Supplementary Cementitious Materials — GGBS, Fly Ash & Limestone
Supplementary cementitious materials (SCMs) replace a portion of Portland clinker in the binder while maintaining strength and durability. Because they carry far less embodied carbon than clinker, every percentage point of replacement reduces the mix’s A1–A3 figure. The three workhorses in UK practice are ground granulated blast-furnace slag, fly ash, and limestone fines.
GGBS (slag)
0.155 kg CO₂e/kg economic · 0.075 by-product. A by-product of iron-making, ground to cement fineness. Typical replacement 30–70%; the calculator soft-warns above 70%. The lowest-carbon mainstream SCM and the most common in UK structural concrete.
Fly ash / PFA
0.022 kg CO₂e/kg economic · 0.001 by-product. A by-product of coal combustion. Typical replacement 20–35%; soft-warn above 35%. Near-zero embodied carbon, but supply is declining as coal generation closes.
Limestone fines
0.044 kg CO₂e/kg. Finely ground limestone, used as a filler and as the limestone component of LC3. Typical replacement up to ~20%; soft-warn above 20%. Not a by-product, so a single allocation-independent factor.
The allocation question: economic vs by-product
GGBS and fly ash are not manufactured for use in concrete — they are residues of other industrial processes. That raises a genuine and contested question in life-cycle accounting under ISO 14040/14044: how much of the parent process’s carbon burden should be allocated to the by-product? Two conventions give materially different answers.
| SCM | Economic allocation (default) | By-product allocation |
|---|---|---|
| GGBS | 0.155 kg CO₂e/kg | 0.075 kg CO₂e/kg |
| Fly ash / PFA | 0.022 kg CO₂e/kg | 0.001 kg CO₂e/kg |
Economic allocation apportions the parent process’s emissions in proportion to the economic value of its outputs, so the SCM carries a share of the iron-making or power-generation burden. By-product allocation treats the SCM as carrying only its own processing burden (grinding, handling), on the basis that the parent process would happen regardless. The calculator defaults to economic allocation — the MPA and RICS convention — and lets you switch to by-product to see the sensitivity.
For a 50% GGBS mix at 340 kg/m³ binder, economic allocation gives roughly 179 kg CO₂e/m³ (−39.5% vs 100% CEM I) while by-product allocation gives about 165 kg CO₂e/m³ (−44.1%). Same physical concrete, two defensible figures. Auditors will ask which method you applied; an undisclosed allocation basis is a common cause of non-comparable EPDs. State it in your methodology notes and keep it consistent across a project.
LC3 — Why It’s a Recipe, Not a Single Factor
LC3 (limestone calcined clay cement) is not a single product with one emission factor — it is a blend. The calculator composes it from three constituents: CEM I clinker, calcined clay (0.274 kg CO₂e/kg), and limestone fines (0.044 kg CO₂e/kg). The shipped LC3-50 preset is 30% calcined clay + 15% limestone fines, leaving 55% CEM I — a clinker factor of about 0.523.
The appeal of LC3 is supply resilience. GGBS and fly ash are by-products of declining industries: blast-furnace iron-making is contracting as electric-arc steel grows, and coal power is closing across most of Europe. Calcined clay, by contrast, is made from abundant kaolinitic clays and can be produced at scale wherever the clay exists. That makes LC3 the SCM route with the clearest long-term availability, even though its per-kilogram factor (0.274) is higher than GGBS or fly ash — because the clinker displacement still delivers a large net reduction. The LC3-50 preset lands at about 197 kg CO₂e/m³ for the worked-example mix, a 33% cut against 100% CEM I, using a binder route that does not depend on a shrinking by-product stream.
The Binder Ladder — Embodied Carbon by Replacement Level
The chart below shows A1–A3 embodied carbon for the same 340 kg/m³ binder C30/37 mix across binder compositions. The 50% GGBS bar uses by-product allocation; the fly ash and LC3 bars use the economic default. Aggregate and water contribute a constant ~9.6 kg CO₂e/m³ in every bar.
The ladder makes the specification hierarchy visible: fly ash at conventional replacement levels delivers a meaningful cut, LC3 goes further on a supply-resilient basis, and GGBS at 50% delivers the deepest reduction of the mainstream routes — with the allocation choice accounting for a 14 kg CO₂e/m³ spread on its own.
Worked Example — A C30/37 Pour at CEM I, 50% GGBS, 30% Fly Ash & LC3-50
This example uses the calculator’s default boot mix — so the numbers below match what you see on screen when the page loads. The mix is 1 m³ of C30/37 at 340 kg/m³ binder, 1,050 kg coarse aggregate, 760 kg fine aggregate, and 170 kg batch water. Aggregate and water contribute a constant 9.582 kg CO₂e/m³ across all four binder compositions.
Note the dual reference for the same strength class: this Mix-mode 100% CEM I C30/37 reads 295 kg CO₂e/m³, while the Quick-mode market-average C30/37 in the table below reads 196 kg CO₂e/m³. That is not a contradiction — Quick mode reflects a market-average mix that already embeds typical SCM use, whereas Mix design models a specific binder against a 100% CEM I baseline.
Concrete Strength Class Reference — Ready-Mix by C-Grade
For a quick market-average reference without composing a mix, the table below gives cradle-to-gate A1–A3 embodied carbon by strength class, from ÖKOBAUDAT 2024 ready-mix data. These factors embed typical industry SCM use, so they sit below an equivalent 100% CEM I mix. The end-of-life (C3) and reuse-credit (module D) values are constant across all grades — a single ÖKOBAUDAT module pair — and are reported as separate EN 15804 lines, never netted into A1–A3.
| Strength class | A1–A3 (kg CO₂e/m³) | C3 end-of-life | Module D credit |
|---|---|---|---|
| C8/10 | 112 | +5.05 | −12.1 |
| C12/15 | 129 | +5.05 | −12.1 |
| C16/20 | 150 | +5.05 | −12.1 |
| C20/25 | 157 | +5.05 | −12.1 |
| C25/30 | 181 | +5.05 | −12.1 |
| C30/37 | 196 | +5.05 | −12.1 |
| C35/45 | 220 | +5.05 | −12.1 |
| C45/55 | 273 | +5.05 | −12.1 |
| C50/60 | 275 | +5.05 | −12.1 |
| C55/67 | 286 | +5.05 | −12.1 |
Embodied carbon rises with strength class because higher characteristic strength generally requires more binder per cubic metre. Whole-life A1–A3+C3+D for any grade is simply the A1–A3 value plus a constant −7.05 (the +5.05 C3 less the −12.1 D credit) — but for most disclosures module D is kept separate rather than netted, so report A1–A3 as the headline.
How to Cut Concrete Embodied Carbon — Specification Levers
Four levers, in rough order of impact, move a concrete’s embodied carbon. The first — binder replacement — dominates; the rest compound it.
- Replace clinker with SCMs. The single largest lever. Moving from 100% CEM I to 50% GGBS cuts A1–A3 by ~40–44%. Specify the maximum SCM content compatible with strength-gain timing and durability requirements.
- Rationalise the strength class. Over-specifying strength wastes binder. A C40/50 where a C30/37 would serve carries ~25 kg CO₂e/m³ of avoidable carbon. Match the grade to the structural demand, element by element.
- Reduce binder content at a given strength. Better aggregate grading, water reduction, and admixtures can hit target strength with less cement. Binder is ~97% of the carbon, so a 10% binder reduction is close to a 10% carbon reduction.
- Allow strength-gain time. SCM mixes gain strength more slowly. Specifying 56- or 90-day strength rather than 28-day permits higher SCM replacement without compromising the structural design — a specification decision, not a material cost.
Model the intended binder composition in the calculator above before specifying, then require a product-specific EPD from the supplier and check the clinker factor and allocation basis against your assumption. The gap between a specified low-carbon mix and the delivered product is usually an allocation or clinker-factor disclosure issue — both visible in a compliant EN 15804 EPD.
Whole-Building Context — EN 15978 Modules & RICS WLCA
A1–A3 concrete carbon is one input to a whole-building assessment. Under EN 15978, a building’s life-cycle carbon spans the product stage (A1–A3), construction (A4–A5), use (B1–B7, including in-service carbonation that reabsorbs a fraction of calcination CO₂), end-of-life (C1–C4), and benefits beyond the system boundary (D). The figures from this calculator populate the A1–A3 cells for concrete elements in that modular assessment.
For UK practice, the RICS Whole Life Carbon Assessment methodology builds on EN 15978 and is the standard most quantity surveyors apply for building-level reporting. Concrete typically dominates the A1–A3 total for framed structures, which is why binder specification is the highest-leverage decarbonisation decision available at design stage. Where the concrete is a purchased input rather than self-produced, its embodied carbon falls under Scope 3 Category 1 in the buyer’s corporate inventory — the same physical material, reported once as a building-LCA input and once as a corporate Scope 3 line, from two different accounting boundaries.
Data Sources, Factor Provenance & Uncertainty
Every factor in this calculator is a live read from the MasterBrain factor service — there is no hardcoded fallback layer. If a factor is unavailable, the calculator surfaces an error rather than substituting a stale value, so a displayed number is always the current published factor.
- Binder and SCM factors (CEM I 0.840; GGBS 0.155 / 0.075; fly ash 0.022 / 0.001; limestone fines 0.044; calcined clay 0.274) — MPA Cement Fact Sheet 18, Rev 4 (10 April 2025), EN 15804+A2 product-stage, AR5-100. The UK-relevant, internally monotonic binder set.
- Aggregate and batch-water factors (coarse and fine aggregate 0.0052; water 0.001) — ICE database v2.0 (2011).
- Ready-mix grade factors (C8/10 to C55/67) — ÖKOBAUDAT 2024, market-average ready-mix EPDs.
- Reinforcement steel (0.474, available as a separate constituent) — ÖKOBAUDAT 2024, not part of the concrete factor itself.
GWP basis
All factors are reported on an AR5 GWP-100 basis — the convention used by EN 15804+A2 and the EF 3.1 reference package that every construction EPD and the MPA fact sheet adopt. This is the deployed sector standard for embodied-carbon reporting, not a deviation; corporate operational inventories that default to AR6 use a different basis by design. Because the published factors are CO₂e aggregates rather than per-gas figures, they are not reconvertible to AR6 without fabricating a value that matches no published EPD.
Uncertainty
SCM factors carry meaningful uncertainty, driven primarily by the allocation choice rather than measurement error — the economic-versus-by-product spread (0.155 vs 0.075 for GGBS) is larger than the analytical uncertainty within either method. Calcined clay also carries a wider range, with a low-end value of 0.048 under favourable processing assumptions. For project reporting, fix one allocation method, apply it consistently, and require product-specific EPDs for material quantities.
Common Concrete Embodied-Carbon Errors
This tool is part of the embodied-carbon family. Compare materials with the steel & aluminium, timber & bio-materials, plastics & packaging, masonry & finishes and building-envelope calculators, model swaps with the material-substitution savings tool, then roll the totals up in the EN 15978 whole-building LCA calculator. Full method: concrete & cement embodied-carbon methodology.
Frequently Asked Questions
It depends almost entirely on the binder. A 100% CEM I structural mix at 340 kg/m³ binder is about 295 kg CO₂e/m³ cradle-to-gate (A1–A3). Market-average ready-mix, which embeds typical SCM use, ranges from about 112 kg CO₂e/m³ for C8/10 to 286 for C55/67. Replacing 50% of the cement with GGBS brings a C30/37 down to roughly 165–179 kg CO₂e/m³.
Cement — specifically the Portland clinker in it. Concrete is mostly aggregate and water, which together contribute only about 9.6 kg CO₂e/m³. The clinker carries roughly 97% of a typical mix’s embodied carbon, two-thirds of it from the chemical calcination of limestone that cannot be avoided by changing fuels. That is why reducing clinker content through supplementary cementitious materials is the dominant decarbonisation lever.
Per kilogram, fly ash is lower (0.022 economic / 0.001 by-product) than GGBS (0.155 / 0.075). But GGBS is typically used at higher replacement levels (up to 70% versus around 35% for fly ash), so a GGBS mix usually achieves a deeper overall cut. Fly ash supply is also declining as coal power closes. In practice GGBS delivers the largest reduction among mainstream SCMs at conventional replacement levels.
The clinker factor (k) is the proportion of a binder that is Portland clinker. Since clinker carries almost all of concrete’s embodied carbon, k is the single best predictor of a mix’s A1–A3 figure: lower k means less clinker and lower carbon. When comparing supplier EPDs, check the clinker factor before the headline number — a low figure with an undisclosed k usually reflects high SCM content whose real saving depends on the allocation method.
LC3 (limestone calcined clay cement) is a binder blend of clinker, calcined clay, and limestone fines. The common LC3-50 recipe is 30% calcined clay + 15% limestone fines + 55% CEM I, giving about a 33% A1–A3 reduction versus 100% CEM I (roughly 197 kg CO₂e/m³ for a standard C30/37 mix). Its main advantage is supply resilience: unlike GGBS and fly ash, calcined clay does not depend on a declining by-product stream.
Because they answer different questions. Quick mode gives a market-average ready-mix figure that already includes typical SCM use (C30/37 = 196 kg CO₂e/m³). Mix design models a specific binder you define against a 100% CEM I baseline (C30/37 = 295 for pure CEM I). Both are correct for their purpose: use the market average for a quick benchmark, and the mix model to test a specification.
GGBS and fly ash are by-products of other industries, so a choice has to be made about how much of the parent process’s carbon they carry. Economic allocation assigns a share based on the by-product’s market value; by-product allocation assigns only the SCM’s own processing burden. The two give different factors (GGBS 0.155 vs 0.075), so a 50% GGBS mix reports −39.5% under economic and −44.1% under by-product. Economic is the MPA/RICS default; whichever you use, disclose it and stay consistent.
Because the construction-EPD sector reports on AR5 GWP-100. EN 15804+A2 and the EF 3.1 reference package that underpins every construction EPD — and the MPA fact sheet these factors come from — all use AR5. Stating AR5 here matches the published EPD values exactly. Corporate operational inventories that default to AR6 use a different basis by design; the two should be labelled, not silently mixed.
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