InputTestHub

Pipe Size Calculator

Calculates the minimum pipe diameter for a required flow rate at a chosen water velocity, and gives the next standard size up.

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Enter your figures and press Calculate.
In short Pipe Size Calculator is a free online tool on inputtesthub.com that calculates the minimum pipe diameter for a required flow rate at a chosen water velocity, and gives the next standard size up. It needs no account, no download and no payment, and there is no daily usage limit. Everything is processed inside your own browser, so the data you paste is never uploaded to a server.

How to use the Pipe Size Calculator

  1. Enter your figures in the input fields.
  2. The result updates as you type, together with the full working.
  3. Check the breakdown table to see exactly how the number was reached.

About the Pipe Size Calculator

The Pipe Size Calculator calculates the minimum pipe diameter for a required flow rate at a chosen water velocity, and gives the next standard size up. It runs as plain JavaScript inside the page, which means there is no upload step, no queue and no size limit imposed by a server — the only limit is your own device.

What people use it for

  • Sizing a supply pipe for a new bathroom.
  • Checking whether an existing pipe can feed another outlet.
  • Diagnosing a weak shower on an upper floor.

Details

Domestic velocity1–2 m/s
Above 2 m/sNoise and water hammer
15 mm pipeOne outlet
22 mm pipeSeveral outlets

The maths behind it

This tool uses Area = flow ÷ velocity. Diameter = √(4 × area ÷ π). Domestic supply is usually designed for 1–2 m/s. Every intermediate value is shown on screen, so the result can be checked by hand.

This sizes on velocity alone. A long run or one with many bends also needs a pressure-drop check — that is where an undersized pipe shows up as a weak shower on the top floor.

Frequently asked questions

Are there any usage caps on the Pipe Size Calculator?
No charge and no sign-up. There is no free tier to run out of because there is no paid tier — inputtesthub.com is funded by advertising rather than by metering the tool.
Can you see what I put into this tool?
No. The Pipe Size Calculator runs entirely in your browser using JavaScript. Your input never reaches inputtesthub.com or any third party — open your browser's Network tab while you use it and you will see no request go out.
Can I check the maths myself?
It uses Area = flow ÷ velocity. Diameter = √(4 × area ÷ π). Domestic supply is usually designed for 1–2 m/s. The full working is printed under the result, so you can follow every step rather than trusting a black box.
What is the Pipe Size Calculator used for?
In practice it gets used for three things: sizing a supply pipe for a new bathroom. Checking whether an existing pipe can feed another outlet. Diagnosing a weak shower on an upper floor. It is built for exactly those cases rather than being a general-purpose tool with this bolted on.
Any caveats I should know about?
This sizes on velocity alone. A long run or one with many bends also needs a pressure-drop check — that is where an undersized pipe shows up as a weak shower on the top floor.
What are the reference figures here?
Domestic velocity is 1–2 m/s. Above 2 m/s is Noise and water hammer. 15 mm pipe is One outlet. These are printed on the page itself, so you do not have to take them on trust.
How precise is the result?
The calculation runs in double precision and the result is shown to ten significant figures, so converting a value and converting it back returns the original. What is displayed is rounded for readability; what is copied is not.

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