Knowledge base · 02a-boundaries

02a - Boundaries

Status. Draft v0.1 · First draft: 17-03-2026 · Pre-discussion.

Why this matters. Carbon numbers are not comparable unless they are computed under the same boundary. A “low-carbon” steel coil at 0.8 tCO₂e/t and a “high-carbon” one at 2.2 tCO₂e/t may be the same physical steel measured against different boundaries - one cradle-to-gate-market-based, the other cradle-to-grave. Without a stated boundary, two numbers can disagree by a factor of three and both be “correct”; a buyer comparing them concludes one of them is dishonest, and is wrong. This doc gives stakeholders the vocabulary to spot the boundary in any published carbon number, and to avoid the most common mistake in carbon procurement: comparing incompatible numbers.

This is the doc to read before reading any third party’s published carbon claim - supplier datasheets, competitor sustainability reports, EPD documents, CBAM communications. Without it, those documents look like they’re saying something they’re not.

1. The five “cradle-to-*” families

Boundary language in carbon accounting is mostly framed as “cradle-to-X” - where the cradle is the raw material in the ground, and the X names where the accounting stops. The further along the life cycle the X is, the more emissions are included. Five families dominate; everyone working in steel, construction, or industrial procurement must read them on sight.

1.1 Gate-to-gate

What’s included. Only the direct emissions of a specific process step, from one factory gate to the next. No upstream materials. No transport. No purchased electricity if you’re being strict (or location-based grid factors only if you’re being slightly less strict).

Typical use. Internal operational benchmarking. “Is rolling line 2 more efficient than rolling line 1?” Gate-to-gate is the right cut for that question because both lines start with the same input (a slab) and produce the same output (a coil); only the rolling step’s emissions differ.

Failure mode. Useless for any product comparison across producers. A scrap-EAF mill’s gate-to-gate looks great, but if you’re buying scrap-EAF steel that was charged with high-carbon DRI, your supply chain’s actual carbon is much higher. Gate-to-gate hides upstream embodied carbon by definition.

1.2 Cradle-to-gate

What’s included. Everything from raw material extraction up to the moment the product leaves the producing factory’s gate. For steel: iron ore mining, ore beneficiation, coking, sintering, blast furnace, BOF (or EAF), casting, hot rolling. Plus the embodied carbon of every input that crossed the gate (alloying elements, refractories, electrodes for EAF, electricity at either location-based or market-based factor).

Typical use. The default boundary for industrial product carbon footprints. What buyers ask for in procurement. What CBAM uses (with its specific implementation details). What EPDs report under EN 15804 modules A1-A3 (see §2 below).

Two variants you must distinguish:

Cradle-to-gate, location-based Scope 2. Electricity is multiplied by the average grid intensity for the region.
Cradle-to-gate, market-based Scope 2. Electricity is multiplied by whatever contractual factor the producer can defensibly claim - a PPA factor, a REC-attributed factor.

The two can differ by an order of magnitude for the same physical kWh. See 02 §3. A claim labelled “cradle-to-gate” without specifying which Scope 2 method is incomplete; both ought to be reported per the GHG Protocol’s Scope 2 Guidance ¹.

Failure mode. Does not include emissions from transport from mill to construction site, installation, building use phase, or end-of-life. For a construction-product procurement decision that wants to optimise the whole building, cradle-to-gate is a starting point, not a finish line.

1.3 Cradle-to-site

What’s included. Everything in cradle-to-gate, plus transport from the producing factory’s gate to the project site. In EN 15804 terms, this is A1+A2+A3+A4.

Typical use. Sustainability-conscious construction projects optimising the delivered carbon of materials. A given grade of steel from Sweden may have a lower cradle-to-gate number than the same grade from India; once you add the 18,000 km transport to a Mumbai site, the Indian mill may win.

Failure mode. Frequently confused with cradle-to-gate in casual usage. A producer publishing a “cradle-to-site” number must declare the assumed transport distance and mode; otherwise the number is incomparable.

1.4 Cradle-to-grave

What’s included. The full life cycle. Cradle through gate, through transport to site, through installation, through use phase (operational energy, maintenance, replacements), through end-of-life (deconstruction, transport to waste processing, landfill or recycling). For construction products, this is EN 15804 modules A1+A2+A3+A4+A5+B1-B7+C1-C4.

Typical use. Whole-life-cycle building assessments (LEED, BREEAM, IFC carbon-neutrality claims). The boundary the climate science community actually cares about because it captures the real climate impact of putting a product into service.

Failure mode. For steel specifically, the use-phase (B1-B7) is often near-zero (steel doesn’t emit during use) and end-of-life (C1-C4) is often near-zero or slightly negative depending on recycling assumptions. So a “cradle-to-grave” steel number is usually close to cradle-to-gate plus a small construction-phase addition. But the modelling assumptions about recycling at end-of-life can swing the answer materially, which is why EN 15804 separates Module D (see §2.5).

1.5 Cradle-to-cradle

What’s included. Cradle-to-grave plus the benefits from reuse and recycling at end-of-life - the carbon avoided because the steel is recycled into next-generation products rather than landfilled. This is an expanded system boundary that credits the producer for the downstream avoided emissions.

Typical use. Circular-economy positioning. Cement companies use cradle-to-cradle accounting when their slag is used as a clinker substitute downstream; steel companies use it when scrap inputs displace virgin BF-BOF.

Failure mode. Easy to abuse. The avoided-emission credits depend on assumptions about what would have happened otherwise - the counterfactual. Different assumptions give different credits. EN 15804 handles this rigorously by separating the credits into Module D (see §2.5), which is reported separately from the cradle-to-grave total. A cradle-to-cradle number that folds Module D into the headline figure is making strong implicit claims about counterfactuals and should be read with scrutiny.

2. EN 15804 modules - the construction-EPD language

Construction is the dominant end-market for steel; most steel sold into construction comes with an Environmental Product Declaration (EPD) under EN 15804+A2:2019 ². Reading EPDs fluently requires reading the standard’s modular life-cycle language. The modules are the granular building blocks; the cradle-to-X labels above are combinations of modules.

2.1 Product stage - A1, A2, A3

The “cradle-to-gate” portion of the life cycle.

Module	Stage	What’s included
A1	Raw material supply	Extraction and processing of all materials and energy at their origin (mining iron ore, drilling for natural gas, generating electricity at upstream power plants)
A2	Transport to manufacturer	Moving raw materials from their origin to the producing factory
A3	Manufacturing	Everything that happens inside the producing factory’s gates - blast furnace, coke oven, sinter plant, BOF, casting, rolling, finishing

A1+A2+A3 is the EN 15804 cradle-to-gate number. This is what is reported in most steel EPDs. The Carbon Engine’s EPD_A1_A3 boundary kind produces this directly.

2.2 Construction process stage - A4, A5

Module	Stage	What’s included
A4	Transport to construction site	From the producing factory’s gate to the construction site (distance + mode-specific factor)
A5	Installation / construction	On-site cutting, welding, machinery use, packaging waste, installation losses

A1-A5 together is “cradle-to-end-of-construction” - the carbon embodied in the building at the moment construction is complete.

2.3 Use stage - B1 through B7

The seven modules covering everything that happens during the operational life of the building.

Module	Stage	What’s included
B1	Use	Emissions arising from the product’s presence in the building (typically zero for steel; non-zero for some materials that off-gas or react)
B2	Maintenance	Cleaning, surface treatment, scheduled maintenance
B3	Repair	Unplanned repair work
B4	Replacement	Replacement of building components over the building’s life
B5	Refurbishment	Major refurbishment events
B6	Operational energy use	Heating, cooling, lighting, equipment - the energy the building consumes in operation
B7	Operational water use	Water consumed in operation

For steel structural products, B1-B5 are typically negligible. B6 dominates the use-stage carbon of most buildings but is properties of the building, not the product, and is generally not allocated to the steel.

2.4 End-of-life stage - C1 through C4

Module	Stage	What’s included
C1	Deconstruction / demolition	Energy and emissions from demolishing the building
C2	Transport	Moving the deconstructed material to waste-processing facilities
C3	Waste processing	Sorting, shredding, preparing for recycling or disposal
C4	Disposal	Final landfilling or incineration

C1-C4 must be reported in EPDs since EN 15804+A2 (the A1 version of the standard had them as optional; A2 made them mandatory). For steel, the C-stage emissions are typically small relative to the A-stage, but they depend on assumptions about recycling rates.

2.5 Beyond the system boundary - Module D

The most important module to read carefully. Module D reports benefits and loads beyond the system boundary - the carbon credits or debits arising from reuse, recycling, or energy recovery at end-of-life. For steel, Module D is dominated by the avoided-emissions credit when scrap recovered from the demolished building displaces virgin steel production elsewhere.

Module D is reported separately from A1–C4. It is never folded into the cradle-to-grave total. The reason is that Module D credits depend on a counterfactual (what would have been produced if this steel had not been recycled?) and the standard insists on transparency about that assumption. A producer claiming a low cradle-to-grave-including-Module-D number is making an implicit claim that the user should be able to challenge.

EN 15804+A2 expanded the impact indicators reported in each module from a single climate-change number to 13 environmental indicators, with climate change itself split into fossil, biogenic, land-use-change, and total ³. This means modern EPDs carry more nuance than the headline single-number summary suggests, and Module D’s complexity is correspondingly higher.

3. CBAM’s production process boundary

CBAM does not use any of the boundaries above directly. It defines its own boundary - the CBAM production process boundary - for each aggregated goods category, set in Annex I and elaborated in Implementing Regulation (EU) 2025/2547 ⁴. For most CBAM-covered goods, the boundary is functionally close to cradle-to-gate, but with specific rules:

Direct embedded emissions are the Scope-1-equivalent emissions of the production process for the aggregated goods category.
Indirect embedded emissions are the emissions from the electricity consumed by the production process - essentially a constrained location-based Scope 2.
Precursors (e.g., the ammonia used to make urea, the pig iron used to make steel) carry their own CBAM embedded emissions, which are added to the complex good’s emissions.
System boundaries are defined per aggregated goods category, so the same physical plant can have different boundaries when calculating CBAM SEE for different products it makes.

The Carbon Engine’s CBAM_PRODUCTION_PROCESS boundary kind produces this. It is not interchangeable with CRADLE_TO_GATE_LOCATION - the CBAM rules for indirect emissions, precursor handling, and aggregated-goods-category boundaries make the answers diverge.

See 03a - CBAM for the regulatory mechanics in full.

4. Why the same product has wildly different carbon numbers

A worked example. Consider one tonne of hot-rolled steel coil produced at a BF-BOF integrated mill in India, sold to a German construction project, used in the building for 50 years, and recycled at end of life. The mill is on a coal-heavy grid; the producer has signed a renewable PPA covering 60% of its grid electricity.

Boundary	Approximate value (kgCO₂e/t HRC)	Notes
Gate-to-gate (rolling step only)	80	Just the rolling line’s direct + electricity
Cradle-to-gate, market-based Scope 2	1,850	With PPA factor applied
Cradle-to-gate, location-based Scope 2	2,100	Indian grid average
Cradle-to-site (Mumbai → Hamburg, sea + rail)	2,180	Adds ~80 kgCO₂e/t for transport
Cradle-to-grave (A1–C4, EN 15804)	2,250	Adds ~70 kgCO₂e/t for installation, end-of-life
Cradle-to-grave + Module D (cradle-to-cradle)	1,650	Subtracts ~600 kgCO₂e/t recycling credit
CBAM production process boundary	~2,050	Roughly cradle-to-gate location-based with CBAM-specific precursor rules

The numbers are illustrative, not measured. The point is the spread - from 80 to 2,250 kgCO₂e/t, a 28× range for exactly the same physical steel. Every number above is defensibly correct under its own boundary. None of them are the right answer for every question.

A buyer who compares supplier A’s “1,650” (cradle-to-cradle, with Module D) against supplier B’s “2,100” (cradle-to-gate, location-based) concludes A is greener. Without normalising boundaries, they have learned nothing.

Three rules for reading any published carbon number:

If the boundary is not stated, the number is not a claim. It is a marketing artefact. Push back and ask for the boundary specification.
If two numbers under different boundaries are compared without normalisation, the comparison is invalid. Either re-state both to a common boundary, or compare module-by-module under EN 15804.
If Module D is folded into the headline number without separate reporting, the producer is making a counterfactual claim you should be able to challenge. Insist on seeing Module D reported separately.

5. How the platform handles boundaries

Three operational realities the Carbon Engine handles directly:

Every CalculationResult carries its full boundary descriptor. Kind (closed enum: GATE_TO_GATE, CRADLE_TO_GATE_LOCATION, CRADLE_TO_GATE_MARKET, CBAM_PRODUCTION_PROCESS, EPD_A1_A3, etc.), Scope 2 method, Scope 3 inclusion list, geography, reporting date. A result without a boundary descriptor cannot exist. See 06 §5.

The same subject under different boundaries produces different CalculationResults, all retained. Asking the engine “what’s the cradle-to-gate-location-based PCF of this coil?” and “what’s the EPD A1-A3 PCF?” produces two distinct, queryable results. Neither overwrites the other. This is what makes the platform’s claim of “one canonical inventory, many regime projections” technically possible.

Boundary-specific rules are applied automatically. Allocation defaults (mass for GHG Product Standard; CBAM-specific by-product treatments; EN 15804 recycling-credit rules) follow the boundary kind. Scope 2 method follows the boundary’s scope2Method field. Geographic factors follow the boundary’s geography. The engine does not require the operator to remember which rules apply where; the boundary kind drives the rule set.

This is the technical answer to the boundary-confusion problem stakeholders face when comparing numbers across producers. The platform can produce any of the listed boundaries from the same underlying data, label them correctly, and surface them with the right rules applied. What it cannot do - and what no platform can do - is make a buyer’s incompatible cross-supplier comparison valid retrospectively. That requires the buyer to ask for the right boundary upfront.

6. Quick reference card

For carrying into stakeholder conversations:

Boundary	EN 15804 modules	What it answers
Gate-to-gate	(subset of A3)	“How efficient is our process step?”
Cradle-to-gate	A1+A2+A3	”What’s the embodied carbon of this product as it leaves our factory?”
Cradle-to-site	A1+A2+A3+A4	”What’s the delivered embodied carbon at the construction site?”
Cradle-to-grave	A1+A2+A3+A4+A5+B1-B7+C1-C4	”What’s the lifetime climate impact of this product, ignoring recycling?”
Cradle-to-cradle	A1+A2+A3+A4+A5+B1-B7+C1-C4 + Module D	”What’s the lifetime climate impact, crediting recycling at end-of-life?”
CBAM production process	(CBAM-specific)	“What does this product owe in CBAM certificates?”
EPD A1-A3	A1+A2+A3	”What carbon do we declare on the EPD?”

The first thing to ask of any number is: which row of this table is it?

The next doc, 03 - Regulations: the landscape, shows which boundaries each regulation requires.

References & further reading

Primary sources (cited inline)

Authoritative secondary sources

Circular Ecology - EN 15804 Modules Explained. Clean per-module walkthrough with practitioner-side context on how each module is computed in real EPDs. https://circularecology.com/en15804-modules-explained.html
One Click LCA - EN 15804+A2: What the changes mean for EPDs. Software-vendor explainer covering the practical implications of the A1 → A2 transition for producers issuing EPDs. https://oneclicklca.com/en/resources/articles/en-15804-changes-epds
EPD Guide - EN 15804 Core Rules for Construction Products, Explained. Reader-friendly overview useful for non-LCA-specialist stakeholders. https://epd.guide/standards-and-schemas/en-15804-core-rules-for-construction-products-explained
Decerna - Complete Guide to EN Construction EPD Standards (EN 15804 and ISO 21930). Comparison between the European (EN 15804) and ISO (ISO 21930) construction-product EPD standards. https://www.decerna.co.uk/knowledge-base/life-cycle-assessment/iso-standards/en-15804-and-iso-21930/
Irish Green Building Council - From EN15804+A1 to EN15804+A2: what is going to change?. Concrete examples of how the A2 changes affect specific construction-product EPDs. https://www.igbc.ie/en15804-a2-epd