What size is required for a lavatory drain?

The correct size for a lavatory drain is 1 1/4 inches. This size is specifically chosen for lavatory sinks as it provides an appropriate balance between adequate drainage capacity and space efficiency. Using a 1 1/4 inch drain allows for sufficient water flow from a typical lavatory while minimizing the risk of clogs.

In plumbing codes, this specification ensures that the drainage system can handle the volume of water that could potentially be discharged during use without creating excessive back pressure or slow drainage conditions. This sizing is widely accepted and helps maintain a functional and compliant plumbing system.

Larger sizes, such as 1 1/2 inches or 2 inches, are more suitable for fixtures that require higher drainage capacity, like showers and bathtubs, which typically handle larger volumes of water. Conversely, a 1-inch drain is inadequate for a lavatory, as it would be prone to frequent clogs and slow drainage due to the higher flow rates associated with sink usage. Therefore, 1 1/4 inches is the standard and most effective size for lavatory drains.