What is the minimum size of the water supply inlet for a lavatory?

The minimum size of the water supply inlet for a lavatory is determined primarily by the flow rates required for effective operation. A 3/8 inch inlet is suitable for lavatories because it allows for adequate flow while maintaining sufficient water pressure for typical use cases, such as filling the basin and activating the faucet without delays.

Using a smaller inlet, such as a 1/4 inch size, could lead to insufficient water flow, which would not meet user demands and might result in poor performance. In contrast, while a 1/2 inch size could also be used and is commonly found, it exceeds the minimum requirements, indicating that 3/8 inch is the minimum sufficient size that balances practicality with water supply needs effectively. Larger sizes, like 3/4 inch or 1 inch, would be unnecessary for a standard lavatory installation, leading to increased material costs without providing proportional benefits in typical residential applications.