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Pipe Size Calculator

Pipe diameter needed for a required flow rate at a sensible velocity, with the next standard size.

Enter values

Domestic supply 1–2 m/s, waste 0.7–1.5 m/s

Results update as you type.

Result

Recommended pipe size

22 mm

Exact diameter needed
20.6 mm
Actual velocity in that pipe
1.315 m/s
Required bore area
333.33 mm²
In inches
0.866
Capacity of that pipe at 2 m/s
45.6 L/min

About the Pipe Size Calculator

Pipe diameter needed for a required flow rate at a sensible velocity, with the next standard size.

Also known as: pipe diameter calculator, what size pipe do i need, plumbing pipe sizing, pipe sizing chart.

How to use this calculator

  1. Enter required flow rate, target velocity, pipe standard.
  2. The result updates instantly as you type — press Calculate (or Enter) at any time.
  3. Review the breakdown shown under the main result.
  4. Use Copy result or Share to save the answer or send a link with your inputs.

Formula

A = Q / v · d = √(4A / π)

Worked example

Using the values pre-filled in the calculator above:

  • Required flow rate = 30 L/min
  • Target velocity = 1.5 m/s
  • Pipe standard: Metric copper / PEX

the calculator returns:

  • Recommended pipe size: 22 mm
  • Exact diameter needed: 20.6 mm
  • Actual velocity in that pipe: 1.315 m/s
  • Required bore area: 333.33 mm²