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IMPORTANT: The flow ratings in the charts below are for Rigid PVC Pipe. Reduce flow by 3% (Multiply by .97) for flow going through Flexible PVC Pipe.

Water Flow Chart #2
Here is a set of data predicting the amount of flow through an orifice based on pressure on one side of the orifice. Note: This is through an orifice, not a pipe. Adding pipe and fittings will drop this flow significantly. In other words, this would be the theoretical maximum amount of water through a hole based on the pressure above it. The table above is more "real world" information.

Water Flow Chart #3
This chart predicts how much flow you will get across a stainless metal ball valve of the diameter & length specified with a 1PSI pressure drop from one side of the valve assuming about 100psi on one side of the valve.

GPM/GPH Flow based on PVC Pipe Size

Pipe Size vs Flow Nomograph

The nomograph (link above) allows you visually see the effect of pipe size and flow rates. You can click on the link and print it out to make it more usable to you. You should size your pipe so that your flow velocity stays in the green or yellow range. The green range is safest, most efficient and will produce little to no noise. Flow velocities in the yellow range may be noisy and have additional back pressure. Flow velocities in the red are not recommended because of the risk of hydraulic shock and pipe/fitting/joint & pump failure.

Note: Back pressure (restriction) is exponentially dependent on flow velocity. For example in a 1" pipe going from a flow velocity of 2 ft/sect (about 5gpm) to a flow velocity of 3.86 ft/sec (about 10gpm) will increase back pressure by 300%. Going to a flow velocity of 7.71ft/sec (about 20gpm) will increase back pressure by 1300%!

These figures are for straight pipe only! The effect of putting direction changes in will compound the back pressure even more and could even result in failure of the system or burning up the pump. You will never be hurt by going to a bigger pipe and will gain by using less electricity due to a more efficient system which may offset the initial price difference for the larger pipe.

Find your flow in the first column (GPM) and then select the pipe size you want in the second column (pipe, ID in inches.) Draw a straight line between them all the way to the last column. If the line ends up in the green you are good. If it ends in the yellow or red, increase the pipe size until your line ends in the green (best) or yellow (just okay) area.

Friction Loss Further Detailed Information

If you really want to get technical and calculate the exact friction loss through PVC and CPVC pipe you can use the Hazen-Williams equation as expressed below for water:

f = 0.2083 (100/c)^1.852 q^1.852 / d_h^4.8655        

where

f = friction head loss in feet of water per 100 feet of pipe (ft_h20/100 ft pipe)

q = volume flow (gal/min)

d_h = inside diameter (inches)

c = a constant for internal pipe roughness. 150 is the commonly accepted value for PVC and CPVC pipe.

You can also print out and use the Nomograph courtesy of Plastics Pipe Institute, a division of The Society of The Plastics Industry. (Note: You normally want to keep your flow velocity under 12 feet per second for 4" and under and 5 feet/second for 5" and above to avoid hydraulic shock.)

What about fittings? How do they affect flow? See our Friction loss due to pvc pipe fittings chart.

Compared to other materials on construction for pipe, thermo-plastic pipe smoothness remains relatively constant throughout its service life.

If you are flowing something other than water, you'll have to adjust the formula for the viscosity of the liquid you are flowing.

Note: One of the benefits of using Flexible PVC pipe is being able to make long gradual bends instead of using fittings which will allow more flow with less noise, less back pressure, and less load on the pump. In other words, a more efficient system!


^*"High Pressure" is a general and non-specific figure. What might be "high pressure" for 1/2" pipe (600psi) may not be "high pressure" for 2" pipe (280psi). There are just too many variables to consider to give a real world number. The fact of the matter is, on a pressurized system, the pump will dictate the flow and pressure as much as the pipe used. To achieve the flow figures in the peak column, it's assuming there are no bends and a short straight flow path. If your system has bends and T's, Wyes, etc, you should go to a larger pipe to achieve the flow desired. Also feed pressure effects the system. If the feed pressure is too low, you can get cavitation and you'll damage the pump and flow very little.