How we count carbon
in the things we build.
From the cement in a foundation to the kilowatt-hour that lights a corridor — every engineering decision has a measurable climate cost. This is a working introduction to how we calculate it.
Six modules. One framework.
Each module builds on the last. Move through in order, or jump to what you need.
Fundamentals
What carbon emissions actually are, the gases that matter, and the difference between embodied and operational carbon.
Methods
Process-based, input-output, and hybrid LCA. The four scopes. When to use which method.
Calculator
Build a real building. Pick materials, set the energy mix, and watch the carbon footprint update live.
Simulation
An animated 60-year building lifecycle showing where the emissions come from, and when.
Engineering Workflow
The 8-step LCA procedure as used in practice — from goal definition to interpretation.
Knowledge Check
Ten questions to test your understanding before you sit a real exam.
What we mean by carbon.
Engineers rarely measure pure carbon. We measure CO₂-equivalent — a common currency for several greenhouse gases of different strengths and lifetimes.
The Seven Greenhouse Gases (Kyoto Basket)
Each gas is converted to "CO₂-equivalent" using its Global Warming Potential (GWP) over 100 years.
| Gas | Formula | GWP-100 | Main Source in Engineering |
|---|---|---|---|
| Carbon dioxide | CO₂ | 1 | Combustion, cement calcination, electricity |
| Methane | CH₄ | 27 – 30 | Natural gas leakage, landfills, construction waste decay |
| Nitrous oxide | N₂O | 273 | Fertilisers, diesel engines, industrial processes |
| Hydrofluorocarbons | HFCs | 4 – 12 400 | Refrigerants in HVAC, foam insulation blowing agents |
| Perfluorocarbons | PFCs | 6 630 – 11 100 | Aluminium smelting, electronics manufacturing |
| Sulphur hexafluoride | SF₆ | 23 500 | Electrical switchgear insulation |
| Nitrogen trifluoride | NF₃ | 17 400 | Semiconductor manufacturing |
Source: IPCC AR6 Working Group I (2021). One tonne of N₂O warms the planet 273 times more than one tonne of CO₂ over a century.
E = emissions in kgCO₂e · A = activity data (kg of cement, kWh of electricity, km driven) · EF = emission factor (kgCO₂ per unit of A) · GWP = global warming potential (often baked into EF)
Two carbon families
In the built environment, every kgCO₂e falls into one of two buckets. Knowing which is which determines how — and when — you have to count it.
Carbon locked in materials & construction
All emissions released before the building is occupied — from quarrying limestone for cement, to firing bricks, to trucking steel to site, to the cranes themselves.
- Raw material extraction (A1)
- Transport to factory (A2)
- Manufacturing (A3)
- Transport to site (A4)
- Construction process (A5)
- Maintenance & replacement (B1–B5)
- End-of-life demolition (C1–C4)
Carbon from running the building
Emissions during the building's daily life — heating, cooling, lighting, lifts, hot water, plug loads. Calculated annually and multiplied by the building's expected lifespan.
- Heating energy (B6)
- Cooling & ventilation (B6)
- Lighting (B6)
- Equipment / plug loads (B6)
- Domestic hot water (B6)
- Lifts & auxiliary systems (B6)
- Water supply & treatment (B7)
The Three Scopes (GHG Protocol)
When an organisation — a construction firm, a university, a city — reports its emissions, it uses three "scopes" to avoid double-counting.
Direct emissions
Sources the organisation owns or controls.
- On-site boilers
- Company-owned vehicles
- Fugitive refrigerant leaks
- Backup generators
- Process emissions (e.g., cement)
Indirect — energy
Purchased electricity, steam, heat, cooling.
- Grid electricity
- District heating networks
- District cooling
- Purchased steam
Indirect — value chain
Everything else, upstream and downstream.
- Purchased goods (materials)
- Employee commuting
- Business travel
- Use of sold products
- Waste disposal
- Investments
For most construction projects, Scope 3 is the largest category — often 70% or more — because it captures the embodied carbon of every material brought to site.
Four ways to measure a footprint.
Different problems call for different methods. The right choice depends on data availability, required accuracy, the system boundary, and how much time you have.
| Method | How it works | Best for | Limitations |
|---|---|---|---|
| Process-based LCA ISO 14040EN 15978 |
Tracks every input and output of a product through each life-cycle stage using physical measurements (kg, kWh, km). | Single products, specific materials, EPDs, building-level studies. | Truncation error — boundary cut-offs miss diffuse emissions in supply chains. |
| Economic Input-Output LCA EIO-LCA |
Maps emissions to sectors of the economy using national input-output tables. Spending data → emissions. | Whole organisations, broad estimates, completeness. | Sector-average data — can't distinguish a "green" supplier from a polluting one in the same sector. |
| Hybrid LCA | Combines detailed process data for foreground processes with IO data for background supply chain. | Comprehensive building or infrastructure assessments. | Data-intensive, expertise-heavy, risk of double-counting at the boundary. |
| Tier 1 / 2 / 3 (IPCC) national |
Tiered emission factors of increasing accuracy: Tier 1 default factors → Tier 3 country & technology-specific. | National GHG inventories, sector-wide reporting. | Tier 1 has wide uncertainty (±50%); Tier 3 requires extensive measurement. |
Standards engineers actually use
ISO 14040 / 14044
The international foundation for any LCA. Defines goal, scope, life-cycle inventory (LCI), impact assessment (LCIA), and interpretation. Any robust carbon study cites this.
EN 15978
European standard for the environmental performance of buildings. Defines the A–B–C–D modular framework you'll see used throughout this manual (A1–A5 for embodied, B6–B7 for operational, etc.).
EN 15804
Sets the rules for Environmental Product Declarations (EPDs) — the documents that publish a product's footprint. As of 2026, almost all major construction products in the EU have an EPD.
GHG Protocol Corporate Standard
The world's most-used standard for organisational accounting. Defines the three scopes you saw in Module 01 and how to draw organisational boundaries (equity share, financial control, operational control).
IPCC Guidelines (2019 Refinement)
The methodology national governments use to report under the Paris Agreement. Defines the Tier 1/2/3 system and publishes default emission factors.
RICS Whole-Life Carbon (2nd ed.)
UK industry standard for assessing whole-life carbon of buildings and infrastructure. Required for major public projects in the UK and increasingly used worldwide as a practical companion to EN 15978.
The Life-Cycle Stages — visualised
EN 15978 splits a building's life into four phases and seventeen modules. This is the spine of modern building carbon assessment.
Calculate a real building.
Use real emission factors from the ICE database (Inventory of Carbon and Energy, Bath) and DEFRA grid factors. Change anything; the results update live.
① Building geometry
② Materials (embodied)
Quantities are auto-suggested from floor area. Adjust to match your design.
③ Energy & grid
Cradle-to-grave estimate
Watch a building emit, 60 years in 60 seconds.
Press play. Each second of animation is one year of the building's life. The yellow plumes are operational carbon (heating, lighting, running). The brown puffs are embodied carbon spikes from construction and renovation events.
Sim parameters
The eight-step workflow.
In professional practice, a building carbon assessment follows the same ISO 14040 sequence. Skip a step and the result is not defensible.
Goal & Scope
Why are we measuring? What boundary (cradle-to-gate? cradle-to-grave?). Set the functional unit — usually "1 m² of GIA over 60 years".
System Boundary
Which life-cycle modules (A1–D) are included? Cut-off rule: typically include processes that contribute > 1% of total mass or energy.
Inventory (LCI)
Quantity take-off from the design model — every kg of steel, m³ of concrete, m² of insulation. Use BIM where possible.
Emission Factors
Match each material to an EPD or a generic database (ICE, Ecoinvent, OneClick LCA, EC3). Note geographic and temporal validity.
Calculation
Apply E = A × EF for every line item. Sum within each life-cycle module. Most teams use specialist software, but a spreadsheet works for early-stage studies.
Impact Assessment
Aggregate to a single indicator (kgCO₂e). Some studies also report other impact categories (acidification, eutrophication, ozone depletion).
Interpretation
Sensitivity analysis — which assumptions matter? Hot-spot analysis — where are the biggest emissions? Identify reduction opportunities.
Reporting
Deliver per RICS / EN 15978 / client requirement. Include uncertainty range, data quality assessment, and recommendations.
A worked example: 1 m³ of concrete
Walk through the calculation by hand. Every entry comes from real industry data.
| Step | Activity data | Emission factor | Emissions |
|---|---|---|---|
| Cement (CEM I) | 320 kg | 0.912 kgCO₂e/kg | 291.8 kgCO₂e |
| Fine aggregate | 800 kg | 0.0048 kgCO₂e/kg | 3.8 kgCO₂e |
| Coarse aggregate | 1050 kg | 0.0048 kgCO₂e/kg | 5.0 kgCO₂e |
| Water | 165 kg | 0.00034 kgCO₂e/kg | 0.1 kgCO₂e |
| Batching energy | 2 kWh | 0.207 kgCO₂e/kWh | 0.4 kgCO₂e |
| Transport (50 km) | 2.4 tonne·km | 0.107 kgCO₂e/t·km | 0.3 kgCO₂e |
| TOTAL — 1 m³ concrete (A1–A4) | ~ 301 kgCO₂e | ||
Insight: 97% of the carbon in concrete comes from cement alone. This is why low-carbon cements (using GGBS or fly-ash replacement) are the single biggest lever in structural design.
Ten questions. One try each.
Cover everything from the previous modules. At the end you'll see a score and an explanation of any wrong answers.