A floor drain is an important piece of the plumbing puzzle. It needs to be big enough to drain effectively, but not so big that it drains too quickly and overpowers the pipes connecting to it.
Commercial plumbing drains are measured in inches. They range in size from 2 to 6 inches, and are dependant upon the size of the hole in the floor, shower, or tub, as well as the pipes that fit into the space underneath it. For example, a 2 inch drain requires 3.14 inches of immediate connecting pipe.
When choosing a drain, it is important to consider how large the connecting pipes are, and how much use the drain is going to get. Other pertinent aspects to decide are mainly cosmetic: a round or square drain and the color of the metal.
Gallons Per Minute
The demand on a drain can be measured in GPM, or gallons per minute. This is essential to consider when deciding on a drain size. A standard bathroom faucet uses 6 gallons per minute when running at full blast, so you would need a larger sized drain for it than for a drinking fountain, which only uses .75 of a gallon per minute.