What is the difference between a declared unit and a functional unit, and which should my footprint use?

A declared unit is a plain quantity of product, such as 1 kg of yarn. A functional unit is the job the product does, such as painting 1 m2 of wall for ten years. Use a declared unit for materials and parts reported cradle to gate, and a functional unit to compare finished products by use.

The definitions

ISO 14067 and ISO 14044 define the functional unit as the quantified performance of a product system, used as the reference for the whole study. A declared unit is used instead when the product's function is not known or not stated, typically because it becomes part of something else. EN 15804 for construction products allows a declared unit for EPDs that do not cover the full life cycle.

When a declared unit is right

• You make an intermediate: yarn, fabric, castings, resin, cement, steel coil.
• Your buyer asks for kg CO2e per kg or per piece for their own Scope 3 or their product footprint.
• The boundary is cradle to gate, because you cannot know the use or end of life.

When a functional unit is needed

• You compare products that do the same job with different amounts of material or lifetimes.
• You publish a comparative claim, such as lower carbon than an alternative; both must use the same functional unit.
• The product category rules for your EPD set one.

How to write it

State the unit in full, with the product specification: for example 1 kg of 30s combed cotton yarn, or 1 m2 of interior wall covered for 10 years at a stated coverage. A footprint without a clear unit cannot be compared or checked, and buyers reject it.

Illustrative example: two paints, two answers

• Declared unit, per litre: paint A 5.0 kg CO2e; paint B 4.0 kg CO2e. B looks better
• Functional unit, 1 m2 of wall for 10 years, two coats each time:
• A covers 10 m2 per litre and lasts 5 years: 0.2 L per painting x 2 paintings = 0.4 L x 5.0 = 2.0 kg CO2e
• B covers 8 m2 per litre and lasts 3 years: 0.25 L per painting x 4 paintings = 1.0 L x 4.0 = 4.0 kg CO2e
• Per functional unit A emits half as much as B

All figures are invented to show how the unit changes the ranking.

What Senseible does here

Senseible reads your plant's electricity bills and fuel invoices into a Scope 1 and 2 record that you divide by your declared unit, such as tonnes or pieces produced. The unit and boundary are yours to state in the report.

• Cradle to gate or cradle to grave?
• Getting an EPD for an export product
• LCA vs product carbon footprint

Sources: ISO 14067:2018, Carbon footprint of products, and ISO 14044:2006 (iso.org)

Last reviewed 2026-10-19.