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

The minimum size of a fixture water supply pipe for a lavatory is indeed 3/8 inches. This size is typically specified in plumbing codes and standards because it provides sufficient water flow to meet the demands of lavatory fixtures under normal operating conditions. A 3/8-inch pipe can deliver an adequate volume of water for lavatory faucets, ensuring proper performance without excessive pressure loss.

Using a pipe smaller than 3/8 inches, such as 1/4 inch, might result in insufficient water flow, leading to problems with fixture usage, including weak streams or limited functionality. On the other hand, larger pipe sizes, like 1/2 inch or 3/4 inch, while capable of delivering greater amounts of water, are not required for the standard lavatory fixture and may lead to unnecessary costs and complications in installation without providing additional benefits for such a small fixture.