Why a Unit Matters in LCA
An LCA is only meaningful if impacts are reported per a clearly defined unit. It answers: Impact per what?- Makes LCAs comparable and credible.
- Prevents misleading claims by ensuring every result has a defined basis.
- Without a unit, numbers like “100 kg CO₂e” are meaningless.
| Aspect | Functional unit | Declared unit |
|---|---|---|
| What it defines | The service provided by the product or system | A reference quantity when the full function is not defined |
| When to use | Comparing products that deliver the same function | Materials or products where final use is unknown |
| Basis | Function, performance, lifespan | Mass, volume, piece, area |
| Example | 200 wears of a t-shirt | 1 kg of product, 1 litre of paint |
Simple example 👕
Imagine two T-shirts. T-shirt A has 20 kg CO2e from production, T-shirt B has 10 kg CO2e. If you only look at the production phase, B looks better. The difference is durability and end of life: A lasts for 200 washes and can be recycled; B lasts for 50 washes and goes to landfill. Washing emissions are the same, so the use phase is comparable. If you compare per function instead of per item and set the functional unit to 200 wears, you need four of B to match the service of one A. That changes the comparison result. This is why comparative LCAs use a functional unit, while EPD and PEF reporting uses a declared unit such as 1 kg, 1 liter, or 1 piece.