How is a product life cycle (LCA) calculated? In six steps

Calculating a product life cycle means carrying out a life cycle assessment (LCA) as described by ISO 14040 and ISO 14044. The arithmetic is not the hard part — flow quantities are multiplied by characterisation factors and summed. The hard part is deciding what to measure and finding the data.

Step 1 — Define the functional unit

Everything starts here, and if this is defined wrongly everything after it is invalid. LCA measures a function, not a product. ‘One litre of paint’ is a quantity; ‘protecting one m² of surface for ten years’ is a function. If you will compare two products you must show both deliver the same function at the same performance.

Step 2 — Draw the system boundary

Decide which stages are included: cradle-to-gate (A1-A3), cradle-to-grave (A1-C4), or a module selection. If you sell an intermediate product, up to the gate suffices; if you sell a final product, excluding use and end-of-life distorts what the number means.

In the same step, write down your cut-off rules: which flows you will omit and why. A cut-off decision is not an error; an undocumented cut-off is.

Step 3 — Collect the inventory

This step consumes most of the total effort. For processes under your control — your own site, your own recipe — you collect primary data.

• Recipe — raw material and additive quantities per product, including scrap rate
• Energy — the production line’s electricity and fuel consumption, split by product where possible
• Process emissions — emissions from chemical reactions, independent of fuel
• Water — withdrawn and consumed volumes, separately
• Waste — type, quantity and treatment destination
• Packaging — all packaging materials delivered with the product
• Transport — inbound raw materials and outbound product; distance and vehicle type

Step 4 — Match to background data

Every input you buy has its own production chain, and you do not model it yourself. Those chains come from a life cycle inventory database such as ecoinvent. The critical point is match accuracy: pick the activity closest to your actual supply in geography, technology and time.

If a supplier gives you a verified EPD for their own product, use it instead of the generic database record — this is the single intervention that most raises the quality of the result.

Step 5 — Run the impact assessment

Hundreds of inventory flows are multiplied by characterisation factors and become meaningful indicators: climate change in kg CO₂e, acidification in mol H⁺ equivalent, water use in scarcity-weighted m³. Which method set is used changes the result and must be stated in the report — if you are preparing an EPD, the EN 15804+A2 indicator set is mandatory.

Step 6 — Interpret

When the first result arrives, the thing to do is not report the number but look at the distribution. Which stage, which material does the impact concentrate in? Almost always a small share of the lines accounts for most of the total. Then run a sensitivity analysis: which assumption moves the result, and by how much?