Plumbing: Waterflow

Water can flow only so fast through pipe.

There are many factors that determine how fast water can flow such as pipe diameter, length of pipe, the number of elbows and other fittings, etc. Is it free flowing? Is it being pumped?

It can be hard to determine exact flow rates but here are some very basic numbers to go by.

Pipe Sizes

Below is the cross area of different sizes of PVC pipe.

  • 1/2″ – .2 cubic in
  • 3/4″ – .4 cubic in
  • 1″ – .8 cubic in
  • 1 1/4″ – 1.2 cubic in
  • 1 1/2″ – 1.8 cubic in
  • 2″ – 3.1 cubic in

As you can see, a 3/4″ pipe can handle twice the water volume of a 1/2″ pipe, and a 1″ pipe about twice of a 3/4″ pipe.

Bulkhead Flow Rates

Below are some average, gravity fed bulkhead flow rates. Again, there are many factors that can alter them.

  • 1″ – 350 gph
  • 1 1/2″ – 1000 gph
  • 2″ – 1300 gph

Return Flow Rates

Below are some average flow rates for water being pumped back up to an aquarium from a sump. These can also vary because of height, pump pressure, etc.

  • .5″ – 850 – 1,250 gph
  • 3/4″ – 1,400 – 2,000 gph
  • 1″ – 2,000 – 3,500 gph

You can use these as a rough guide when designing overflow drains and return pipes. Aquarium manufacturers will usually have standard sizes that they recommend for their systems.