# How to choose the pipe size for household plumbing

> How to choose pipe sizes for the supply, risers and tap connections, the difference between outer and inner diameter and why a thin pipe “eats” pressure.

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*July 15, 2026*

![Diagram: cross-sections of three pipe sizes with water flow and a dripping tap](https://watertrade.com.ua/uploads/articles/pipe-diameter.jpg)

The pipe diameter decides whether the shower keeps its pressure when someone starts the dishwasher. A pipe that is too thin has high resistance and the water runs fast and noisily. One that is too thick costs more and takes longer to deliver hot water.

## What the diagram shows

1 — a thin pipe for a single tap connection, 2 — a medium pipe for distribution in the house, 3 — a thick pipe for the supply and the main from the pump.

## Outer and inner diameter

Plastic pipes are labelled by their outer diameter. A 20 mm polypropylene pipe is only about 13–15 mm inside, depending on the wall thickness. That is why 20 mm PP-R carries roughly as much water as a 1/2" steel pipe.

## A simple table for a private house

| Section | Polypropylene | Multilayer | HDPE (outdoor) |
| --- | --- | --- | --- |
| Supply from the borehole or well | 32 mm | 32 mm | 32 mm |
| Main inside the house | 25–32 mm | 26 mm | — |
| Connection to one fixture | 20 mm | 16 mm | — |

In a large house with three or more bathrooms, make the main 32 mm and the supply 40 mm.

## How to check yourself

- Water velocity in household pipes should not exceed 1.5–2 m/s. Faster flow brings noise and large pressure losses.
- A long thin pipe loses more pressure than a short one. On long branches go one size up.
- Never reduce the pump’s suction pipe: keep it the size of the pump inlet or larger.

> Saving on the main pipe size is the most common reason for “weak pressure” in a house with a powerful pump.

If you are still not happy with the pressure, check the [pressure switch](https://watertrade.com.ua/en/catalogue/avtomatika-dla-nasosov) settings and the size of the [pressure tank](https://watertrade.com.ua/en/catalogue/gidroakkumulatory).

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