Polyethylene Pipe Flow and Size Calculator

What is the flow of a polyethylene pipe?

The flow (discharge) of a pipe is the volume of water that passes through its bore every second or every hour. It is the water velocity multiplied by the bore area: Q = v × A. The area comes from the inside diameter, not the outside one, and the inside diameter is the outside diameter minus twice the wall thickness.

That is why “the flow of a 4 inch pipe” is not one fixed number. The market’s 4 inch pipe is the 110 mm outside diameter, but a lower SDR means a thicker wall and a smaller bore, so the same velocity carries less water. On this page you also pick the SDR, so the flow is worked out for a row that we actually supply.

Water pipe size calculation for a required flow

To size a water pipe, enter the flow you need in cubic metres per hour and then type the lowest and highest velocity you accept for your project. The tool lists every published size of that SDR whose velocity falls inside the band. A high velocity means noise, water hammer and large pressure loss; a very low one means a pipe larger and dearer than needed.

We do not suggest a velocity band, because it depends on the use, the pressure and the designing engineer, and no approved value is recorded on this site yet. Enter the limits you work with. If you are unsure, send us the project details for technical advice.

Pressure loss in polyethylene pipes

Pressure loss in polyethylene pipes depends on the length of the run, the flow and the inside diameter. The loss is proportional to length, rises faster than linearly with flow, and falls steeply as the bore gets larger. Moving up one size can therefore cut the loss several times over.

A common method is the Hazen-Williams formula, which needs a roughness coefficient for the pipe material. An engineer must approve that coefficient, and until then the pressure-loss section of this tool stays hidden so that we do not print a guessed number. For long transmission lines, always check the loss with the design engineer.

Notes on the results

Results rest on the nominal inside diameter of the published rows. Manufacturing tolerance, scale and the fittings on a line change the real bore and the real loss. The table lower on the page shows the flow of every size at the velocity you entered above. To order, add the size and SDR to your inquiry list or contact us.