Water capacity of pipe

Answer 1

The answer posted below in bold is NOT correct. The edited answer is technically correct if your definition of capacity is how much water can the pipe hold. This answer is a simple cylindrical volume calculation with a conversion to gallons.

If your definition of capacity is how much water can flow through the pipe then the answer becomes much more complex. Typically these pipe capacities are given in units of cubic feet per second (cfs) or gallons per minute (gpm). To calculate this capacity you must use Manning's equation.

Q=vA=(1.49/n)*R^(2/3)*S^(0.5)*A

where Q is flow capacity in cfs, v is velocity in feet per second, A is area in square feet, n is the Manning coefficient, R is the hydraulic radius in ft (equals area divided by wetted perimeter), and S is the pipe slope in ft/ft.

If the pipe is pressurized which is the case with municipal water supply then the pipe will always flow full and R=D/4 (D is diameter in feet).

If the pipe is not pressurized which is the case with most drain lines then the hydraulic radius and area become a function of water depth in the pipe.

Refer to a Hydraulics text book for more information on any of this. Piping designs should always be calculated by a professional engineer who is trained in these types of applications and is aware of local codes and regulations.

Pi (3.14159265....) X r (radius inches)sq X L (length inches) divided by 231 = gallons of fluid.