> ## Documentation Index > Fetch the complete documentation index at: https://learn.sustainly.ai/llms.txt > Use this file to discover all available pages before exploring further. # Functional & Declared Units > Learn why defining a clear functional or declared unit is essential for credible Life Cycle Assessments (LCAs). Understand the difference between functional and declared units, when to use them, and how they impact sustainability comparisons. ## **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**. Life cycle assessment example comparing T-shirt production, showing that T-shirt A generates 20 kg CO2e while T-shirt B generates 10 kg CO2e. 👉Comparison of CO2e emissions between T-shirt A (20 kg) and T-shirt B (10 kg) in production. **Ready to make smarter sustainability decisions? Discover more with Sustainly.**