Water Pipe Size

Free Water Pipe Size calculator with clear step-by-step results.

About Water Pipe Size

Sizes a pipe by solving the continuity equation for diameter at a maximum design velocity, then rounds up to the next standard size and reports the actual velocity that results. The head loss budget shows how much friction the run can afford once static lift is deducted from supply pressure.

Formulas

Minimum bore

Diameter = 2 x square root of (flow / (velocity x pi))

Head budget

Available head = supply bar x 10.2 - static lift, divided by run length

Worked example

With the default values below, Water Pipe Size returns Next standard pipe size of 20. Change any input above and every figure here is recalculated in your browser.

Inputs used

Required flow rateminimum 0.1 L/min: 30 L/min
Maximum design velocityaccepts 0.2–5 m/s: 2 m/s
Available supply pressureminimum 0.1 bar: 3 bar
Pipe run lengthminimum 0.1 m: 20 m
Static lift to the outletminimum 0 m: 4 m

Results

Next standard pipe size: 20 mm
Minimum bore required: 17.84 mm
Velocity in the chosen pipe: 1.592 m/s
Head loss budget per metre: 1.3300 m/m

Step by step

1Values used
2Minimum bore
3Head budget
4Next standard pipe size= 20 mm
5Minimum bore required= 17.84 mm
6Velocity in the chosen pipe= 1.592 m/s
7Head loss budget per metre= 1.3300 m/m

Assumptions and limits

The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Pipe sizing, materials, backflow prevention and pressure limits are governed by plumbing and water regulations that vary by jurisdiction. Have designs checked by a qualified plumber or engineer.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Pipe sizing, materials, backflow prevention and pressure limits are governed by plumbing and water regulations that vary by jurisdiction. Have designs checked by a qualified plumber or engineer.

Frequently Asked Questions

Why cap velocity at 2 m/s?

Above that, flow noise becomes audible, erosion-corrosion accelerates in copper, and water hammer forces on valve closure rise with the square of velocity.

Is one bar really 10.2 metres of head?

Yes, for water at ordinary temperatures - one bar lifts a water column about 10.2 metres. That conversion is what links pressure and head in pipe sizing.