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

The minimum size of the water supply inlet for a flush tank is 3/8 inch. This size is typically sufficient for most residential flush tanks, ensuring adequate flow rates for refilling the tank promptly after each flush. A 3/8 inch inlet allows for a balance between efficient water delivery to the tank and compatibility with standard plumbing fittings.

Using a smaller size, such as 1/4 inch, might not provide enough water to refill the tank quickly enough, leading to longer wait times between flushes. On the other hand, larger sizes, while capable of delivering more water, could lead to unnecessary complexity in plumbing design, increased costs, and might not be necessary for the typical demands of a flush tank system. Therefore, 3/8 inch strikes the right balance for optimal operation in most scenarios.