Material embodied-carbon comparison

What the embodied carbon comparison does

An embodied carbon calculator multiplies each material quantity by its global warming potential factor, in kg CO₂e per declared unit, and adds up the results. This tool does that for two options, A and B, so you can see which specification carries less embodied carbon for the lifecycle modules you choose, such as A1–A3.

The factors come from you, usually from Environmental Product Declarations (EPDs), or from a small offline table of illustrative presets (rounded ICE database v3 averages for concrete, steel, timber, brick, glass, plasterboard and insulation) that fills in the factor, its unit and a source name so you can start before EPDs are in hand. The tool enforces the checks that most often make comparisons wrong: every row must use the same lifecycle modules, each custom quantity unit must match its factor unit, and both options must contain at least one material.

It is a material comparison, not a whole-building life cycle assessment. Totals are reported per option and per unit of floor area, for A1–A3 only or for a simple whole-life scope that adds replacements (B4) and end of life (C1–C4) per row. The example factors are fictional placeholders and the presets are illustrative averages; replace them with values from real EPDs before you rely on a result.

How to use the material embodied-carbon comparison

1. Choose Imperial or Metric. Unit options switch between yd³, ft², lb, US ton and ft, and m³, m², kg, t and m; floor area switches between ft² and m².
2. Describe the comparison boundary: the function both options perform, and any service life or replacement assumptions. Choose the lifecycle scope: A1–A3 only, or whole life with B4 replacements and C1–C4. Enter the floor area to get kg CO₂e per ft² or m² (0 skips it).
3. Add a row for each material. Set the option (A or B), the material name, the quantity and the quantity unit. Pick a factor preset to use an illustrative factor, unit and source, or keep Custom and enter your own.
4. For custom rows enter the kg CO₂e per factor unit from the EPD, its factor unit, the lifecycle modules (for example A1-A3), the EPD source and its date; the quantity unit must match. For whole life, add the number of B4 replacements and the C1–C4 factor per unit for each row.
5. Read the totals for Option A, Option B and A − B, the per-floor-area figures, the bar chart by material and the table of factors used, modules, source and date. Download a PDF, CSV or SVG, copy the result, or save your inputs.

Worked example

Imperial: 13 yd³ at a user-supplied 190 kg CO₂e/yd³ gives 2,470 kg CO₂e for the declared modules, or 2.47 kg CO₂e/ft² over 1,000 ft².

Metric: 10 m³ at a user-supplied 250 kg CO₂e/m³ gives 2,500 kg CO₂e for the declared modules.

What do A1–A3 and the other lifecycle modules mean?

EPDs and building life cycle assessments divide a product's life into modules defined in the European standards EN 15804 (construction products) and EN 15978 (buildings). ISO 21930, used for many North American EPDs, follows the same structure. A fair comparison uses the same modules for every option.

Most construction EPDs report at least A1–A3, which is why it is the tool's default. A1–A3 comparisons are useful for choosing between similar products, but they miss differences in transport distance, installation waste, service life and end of life.

EPDs, declared units and functional units

An EPD is a verified report of a product's environmental impacts, prepared under ISO 14025 and a product category rule (PCR) for that type of product. The figure this tool needs is the global warming potential (GWP), in kg CO₂e, for the modules you are comparing.

Every EPD states a declared unit: 1 m³ of concrete, 1 metric ton of steel, 1 m² of insulation at a stated thermal resistance, and so on. Your quantity must be in the same unit. If an EPD is per metric ton and your quantity is in cubic yards, convert using the product's density first. The tool will not let a quantity in yd³ multiply a factor per m³.

A functional unit goes further and defines the job the material does, such as 1 m² of wall at a given U-value over a stated service life. Products with different declared units can only be compared once both are expressed per the same function. Use the comparison boundary field to record that function.

EPDs are generally only directly comparable when they follow the same PCR, cover the same modules and describe products that perform the same function. EPDs are commonly valid for five years, so note the date.

Converting EPD factors between metric and US units

Many EPDs report per metric unit. To use a metric factor with US quantities, convert the factor as well as the quantity, and keep each pair consistent.

For example, a factor of 250 kg CO₂e/m³ becomes 250 × 0.7646 = 191.1 kg CO₂e/yd³. That is why the tool's fictional imperial example uses about 190 per yd³ alongside 250 per m³ in metric. Factors per metric ton convert to factors per US ton with a multiplier of 0.9072, because a US ton is the smaller unit.

Worked embodied carbon comparison (illustrative factors)

All factors in this example are illustrative placeholders, not typical or recommended values. A foundation needs 40 yd³ of concrete and 3,000 lb of reinforcing steel. Option A uses one concrete mix and Option B a lower-carbon mix. The steel is the same in both.

Option A totals 9,100 kg CO₂e and Option B 7,100 kg CO₂e, so A − B is 2,000 kg CO₂e, a 22% reduction for the declared modules. The steel rows cancel out: listing identical items in both options does not change the difference, but it makes each total a fuller picture of the element.

Before relying on a result like this, confirm that the lower-carbon mix meets the same strength, curing and durability requirements. That is the functional equivalence check the tool cannot do for you. Quantities can come from the concrete quantity calculator and the steel and rebar weight calculator.

Illustrative preset factors and the whole-life option

The factor presets are rounded A1–A3 averages from the ICE database v3.0 (Circular Ecology, 2019). They are illustrative starting points for early comparisons, not product data: a specific mix, mill or forest can be well above or below the average, and timber values here exclude sequestered biogenic carbon. When you pick a preset the tool converts your quantity to the preset's unit (lb, kg, t or US ton to kg; yd³ to m³) and records the source in the table.

The whole-life scope is deliberately simple. Each row becomes quantity × (A1–A3 factor + C1–C4 factor) × (1 + replacements), so a finish replaced twice over the study period counts three times. Transport and installation (A4–A5), use-stage modules other than replacement, and module D benefits are not included; record them in the boundary field if they matter to your comparison.

Where to find EPDs and carbon factors

Start with the manufacturer you expect to buy from, since a product-specific EPD describes that product more closely than an average. Record the EPD name, program operator, date and validity in the source and date columns so others can check the comparison.

• Manufacturer EPDs, usually on product pages or available on request.
• Industry-wide EPDs published by trade associations, which average several producers and suit early design before a supplier is chosen.
• EPD program operators, which verify, register and publish EPDs.
• EPD databases such as Building Transparency's EC3, which collect EPDs and let you filter by material category and region.

Who it is for and when to use it

Method, formulas & assumptions

Each row impact = quantity × carbon factor (kg CO₂e per declared unit); whole life adds the C1–C4 factor per unit and multiplies by 1 + B4 replacements. Preset factors are illustrative ICE v3 values and quantities are converted to the preset unit; custom rows must match quantity and factor units and all rows must declare the same modules. Totals for A and B are also divided by the floor area when one is entered, and charted by material.

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Comparing A1–A3 for one option with A1–A5 for another. The tool rejects mixed modules for exactly this reason.
• Multiplying a per-ton factor by a volume. Convert with the product's density, from the EPD or data sheet, before you enter the row.
• Comparing declared units that are not functionally equivalent, such as 1 m² of two insulation boards with different thermal resistance.
• Assuming EPDs made under different product category rules line up. Differences in method can outweigh differences between products.
• Leaving the fictional example factors in place. They are placeholders and do not describe any real product.
• Mixing biogenic carbon conventions. Some EPDs report carbon stored in wood as a negative value; the tool accepts negative factors, but both options must treat biogenic carbon the same way.

Scope & limits

Enter factor source, date and modules. Example factors are fictional placeholders and presets are illustrative database averages, not product EPDs. A4–A5, B1–B3, B5–B7 and module D are excluded. This is a selected-material comparison, not a full LCA; functional equivalence and service life need independent review.

Frequently asked questions

Further reading

• Typical exterior wall assemblies
• All building performance tools
• All free architecture & construction tools
• Harth resource library

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