What size water pipe do I need for a bathroom?

For a bathroom with a shower, a basin and a toilet, a bore around 20 mm is usually enough. Put your outlets in above to work it out for your own layout.

Do all the taps ever run at once?

No, and pipe sizing assumes they do not. In a single home a realistic figure is around a third to a half of the total outlets running together.

Is a bigger pipe always better?

No. An oversized pipe holds still water, makes you wait longer for hot water at the tap, and costs more in pipe, fittings and insulation.

Why does my shower go cold when the toilet fills?

The cold feed is undersized. The cistern takes cold water, the shower loses some of its cold, and the mixed temperature rises. A larger cold feed or a thermostatic mixer fixes it.

What is the difference between bore and pipe size?

Bore is the hole inside. Pipe size on the label is usually the outside diameter. PPR has thick walls, so a 25 mm pipe can have only about 16 mm of bore.

Will a pump fix low flow from a small pipe?

No. A pump adds pressure, not capacity. Pushing more water through a pipe that is too small just makes it fast, noisy and prone to hammer.

Pipe Size Calculator

Basin taps

About 6 litres a minute each.

Showers

About 9 litres a minute each.

Kitchen taps

About 12 litres a minute each.

Bath taps

About 20 litres a minute each.

Washing machines and dishwashers

About 10 litres a minute each.

How many run together

Outlets almost never all run together. This is the honest allowance.

Water speed

Your answer

Bore you need

22.8 mm

Nearest standard bore is 25 mm. PPR pipe for that will be labelled larger.

All outlets added up: 82 L/min
Realistic demand at once: 36.9 L/min
In litres a second: 0.62 L/s
Open area needed: 410 mm2
Standard bore to use: 25 mm

This gives the bore, which is the hole inside. PPR pipe is sold by its outside diameter, so the pipe you buy will have a larger number on it.

Pipe Size Calculator for Dubai and the UAE

This tool works out how big a water pipe needs to be. You tell it what outlets it feeds and how many of them are likely to run at once, and it gives you a bore in millimetres.

The number of outlets running together is the part that matters. A bathroom might have four taps, but four taps almost never run at the same moment.

Sizing for the worst case that never happens gives you pipes that are too big, and that has its own problems.

Why not everything runs at once

This is the idea that makes plumbing design possible, and it has a name: diversity.

A three bedroom flat might have two showers, three basins, a kitchen tap, a bath and two appliances. Added together that is over eighty litres a minute.

Nobody has ever used all of them at the same second. In a normal morning you might have one shower running and a basin tap. That is fifteen litres a minute.

So pipework is sized for a realistic simultaneous demand, not the theoretical total. In a single home that is usually somewhere between a third and half of the full list.

A hotel or an office is different, because a lot of people do the same thing at the same time. That is why commercial design uses different figures.

What happens if the pipe is too small

The symptoms are specific and they point clearly at the cause.

Flow drops when a second tap opens
The clearest sign. The pipe is delivering all it can, so opening another outlet shares the same amount.
The shower goes cold when a toilet fills
Classic undersized cold feed. The toilet takes cold water, the shower loses its cold, and the mixed temperature jumps.
Noise in the walls
Water forced through a small pipe moves fast, and fast water hisses and rushes.
Banging when a tap shuts
Water hammer. Fast moving water stopping suddenly. Undersized pipe makes it worse because the water was moving faster.
It was fine and got worse over years
That is not undersizing, that is scale narrowing the pipe from the inside. The effect is the same and the cause is different.

What happens if the pipe is too big

People assume oversizing is always safe. It is not, and the problems are less obvious.

Water sits in a big pipe. A long oversized run holds a lot of water that is not moving, and still water in a warm building is where things grow.

Hot water gets worse. A big hot pipe holds more cold water that has to be run off before hot arrives, so you wait longer and waste more at every tap.

It also costs more, in the pipe itself, the fittings, the insulation and the space it takes up in a wall or ceiling.

So the aim is right sized rather than generous: enough for realistic demand at a sensible speed, and no more.

Pipe in UAE buildings

What you will actually find behind the wall.

Green PPR
The standard in most modern buildings here. Heat welded joints, no threads, very reliable when welded properly. Sold by outside diameter with thick walls.
CPVC
Also common, solvent welded rather than heat welded. Rated for hot water and easy to work with.
Copper
Found in older and higher end work. Long lasting and expensive.
Galvanised steel
In older buildings. Rusts from the inside, narrowing the bore and eventually causing brown water and pinholes.
Flexible braided hose
The short connections to taps and appliances. Not pipework, and the part most likely to fail suddenly.

Bore against the size on the label

The number on a pipe is almost never the hole in it, and this catches people out constantly.

PPR is measured by its outside diameter and has thick walls. A pipe marked 25 mm may have a bore closer to 16 or 17 mm depending on its pressure rating.

So if this tool tells you that you need a 20 mm bore, you are not buying 20 mm PPR. You are buying whatever size of PPR gives you at least 20 mm of hole, which will be a larger number on the label.

The pressure rating matters here too. A higher rated pipe of the same outside size has thicker walls and less bore.

When you buy, ask for the bore, or check the specification table for the product. It is the only number that decides how much water gets through.

Questions people ask

What size water pipe do I need for a bathroom?: For a bathroom with a shower, a basin and a toilet, a bore around 20 mm is usually enough. Put your outlets in above to work it out for your own layout.
Do all the taps ever run at once?: No, and pipe sizing assumes they do not. In a single home a realistic figure is around a third to a half of the total outlets running together.
Is a bigger pipe always better?: No. An oversized pipe holds still water, makes you wait longer for hot water at the tap, and costs more in pipe, fittings and insulation.
Why does my shower go cold when the toilet fills?: The cold feed is undersized. The cistern takes cold water, the shower loses some of its cold, and the mixed temperature rises. A larger cold feed or a thermostatic mixer fixes it.
What is the difference between bore and pipe size?: Bore is the hole inside. Pipe size on the label is usually the outside diameter. PPR has thick walls, so a 25 mm pipe can have only about 16 mm of bore.
Will a pump fix low flow from a small pipe?: No. A pump adds pressure, not capacity. Pushing more water through a pipe that is too small just makes it fast, noisy and prone to hammer.

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