What is the proper drain size for a lavatory?

The proper drain size for a lavatory is 1 1/4 inches. This size is commonly specified for bathroom sink drains, as it is adequate for handling the typical water flow and waste produced by lavatories. The 1 1/4-inch diameter provides sufficient capacity to prevent clogging while also allowing enough drainage speed to avoid standing water, which is critical for maintaining hygiene in a bathroom environment.

The choice of a larger drain size, such as 1 1/2 inches or 2 inches, may be more appropriate for different types of fixtures or applications, like bathtubs or kitchen sinks, which generate larger amounts of water and waste. Meanwhile, the option of 3 inches is generally used in more industrial or significant waste applications, such as sewage drains, far exceeding the requirements for a standard lavatory. Therefore, 1 1/4 inches is the most suitable and efficient drain size for a lavatory setting, aligning with plumbing codes and practices.