---
term: "Functional unit and declared unit"
aliases: ["functional unit", "declared unit", "reference flow"]
category: Concept
updated: 2026-09-08
canonical: https://verdatir.com/glossary/functional-unit
publisher: Verdatir (https://verdatir.com)
---

# Functional unit and declared unit

> The quantified reference to which all inputs, outputs and impacts in an LCA are related: a functional unit describes the function delivered, while a declared unit describes a quantity of product when the function is not yet known.

Every LCA result is a ratio: impacts per something. The functional unit defines that something in terms of the function the product provides, so that alternatives can be compared on what they do rather than on what they weigh.

## Functional unit

ISO 14040 defines the functional unit as the quantified performance of a product system for use as a reference unit. For insulation, a functional unit might be one square metre providing a stated thermal resistance over a stated service life. The reference flow is the amount of product needed to deliver that function.

## Declared unit

When the function of a product depends on how it is used in a building or assembly, EN 15804 allows a declared unit instead, for example one kilogram or one cubic metre of material. Most cradle-to-gate EPDs use a declared unit; the function is defined later, at building level.

## Why it matters

Comparing results calculated on different units, or on the same unit with different service lives, is meaningless. Unit definitions are one of the most common sources of misinterpretation of EPD and PCF data.

## Where Verdatir fits

Verdatir checks that the unit is defined as the PCR requires and that all results, conversions and comparisons in a declaration refer to it consistently.

## Sources

- [ISO 14040:2006 — Life cycle assessment — Principles and framework](https://www.iso.org/standard/37456.html)
- [CEN — EN 15804:2012+A2:2019 Sustainability of construction works — Environmental product declarations](https://standards.cencenelec.eu/)