How Much Flow a Floor Drain Can Take

Because you are a plumbing engineer, or a manufacturer, or its representative, someone may ask you a question:

"I have a 2-1/2" drain from my equipment. There is an existing 3" floor drain. Can this floor drain take the discharge?"

When you ask him or her:

"Is your 2-1/2" drain flowing by gravity or under pressure? If under pressure, how much is it?"

He or she may answer:

"Under pressure of 50 psi."

Then, you may have to calculate the discharge flow first. You can figure out the drain pipe length, number of fittings, shut off valve, and even the elevation difference, then use Hazen-Williams formula to figure out the flow and Manning's formula to figure out the floor drain capacity.

I bet you will say: "It is impossible to use this 3" flow drain with 3" piping to take the discharge".

Beside the Manning formula, you may also look in a handbook with tables, or other means. But, if you use the on-screen calculation form provided by, you can get the answer quickly. You need only input 4 numbers to get the answer. See the figure shown here.


Using the form repeatedly, you may list multiple answers and make a table like this:

If you like, you can also list flows with different roughness coefficient, n's, and slopes, or even make tables for a manual book. Instead, I guess you might tell people to use engineermate site.

By the way, the drain capacity is dependent on n, pipe diameter, h/D and slope of the pipe after the floor drain. Increasing the size of floor drain without changing the size of the drain pipe cannot do anything good. Plumbing code requires that the maximum fullness, i.e. h/D, of the sanitary drain cannot exceed 0.5.

Next time we will discuss how to estimate the discharge flow of the said equipment. (By Anjian Lu)