Pipe Flow Calculator for Dubai and the UAE

Flow is the area of the hole multiplied by how fast the water is moving through it. That is the whole calculation.

The important part is that area goes up with the square of the diameter. Double the bore and you do not get double the flow, you get four times.

That is why a small increase in pipe size makes such a large difference. Going from a 15 mm bore to a 20 mm bore is not a third more water, it is nearly double.

It also explains why a partly blocked pipe fails so suddenly. Scale narrowing a 20 mm pipe to 14 mm has taken away more than half the flow, and the owner only notices when two taps run at once.

You could push water through a small pipe very fast and get the flow you want. Nobody does, for three reasons.

Noise. Fast water in a pipe hisses and rushes, and you hear it through the wall every time somebody opens a tap. Above about two metres per second it becomes obvious.

Wear. Water moving quickly scours the inside of the pipe and the fittings, particularly at bends. Over years this thins the material.

Water hammer. Fast moving water carries momentum. When a tap or a washing machine valve shuts quickly, that momentum has to go somewhere, and it arrives as a bang in the pipework. The faster the water was moving, the harder the bang.

That is why designers keep speed between about one and two metres per second. Quiet, long lasting, and no hammering.

This catches people out constantly, especially with the green PPR pipe used in most UAE buildings.

PPR is sold by its outside diameter. A pipe labelled 25 mm is 25 mm across the outside, and the wall is thick, so the actual hole is much smaller. Depending on the pressure rating it might be around 16 mm.

Copper is closer, but still measured differently depending on where it came from.

So when you use this tool, put in the bore, not the label. If you do not know it, look at a cut end, or check the pipe specification for its wall thickness and subtract twice that from the outside diameter.

Getting this wrong is why a pipe that should be fine on paper delivers half what was expected.

The number above is the best case for a straight clean pipe. Reality takes some away.

If one tap is weak and the rest are fine, it is that outlet: an aerator, a cartridge or a service valve. The pipe is not the problem.

If everything is weak all the time, it is the supply, the main pipe, or the pump. Work out what the pipe should deliver here, then measure what it actually does by filling a bucket for a timed minute.

A big gap between the two says the pipe is restricted somewhere, or the supply pressure is low.

If flow is fine alone but collapses when two things run, the pipe is undersized for the number of outlets it feeds. That is a pipe size problem, not a pressure problem, and adding a pump will not fix it.

That last one is worth being clear about. A pump adds pressure. It cannot push more water through a pipe that is too small without making it fast and noisy.