What is the smallest plumbing fixture used as a basis for determining the fixtures unit value?

The lavatory or wash basin is considered the smallest plumbing fixture used as a basis for determining the fixture unit value. This fixture typically has a lower water demand compared to others like bathtubs, toilets, and showers, which helps establish a baseline for calculating plumbing system requirements.

In plumbing design, fixture unit values are critical for understanding the overall water supply and drainage needs of a building. The lavatory or wash basin is generally assigned a fixture unit value that reflects its typical usage, contributing to calculations of peak demand and ensuring that the plumbing system can handle the water flow efficiently. Its designation as the smallest fixture highlights its foundational role in plumbing calculations, allowing for appropriate sizing of piping and drainage systems throughout a structure.