What water velocity is recommended?

Under 5–8 ft/s (1.5–2.4 m/s) to limit noise and erosion.

How much flow does a shower need?

1.8–2.5 gpm with modern heads.

How do I get inside diameter?

Outside diameter minus twice the wall thickness, or the pipe schedule chart.

How many gpm through ½ in PEX?

About 2–3 gpm at reasonable velocities.

How do I measure faucet flow?

Fill a bucket of known volume and time it.

What is water hammer?

A pressure spike when a valve shuts fast; it grows with velocity.

Pipe flow rate calculator

Inside diameter and water velocity: you get flow rate in gallons or litres per minute, litres per second and m³/h.

Flow rate

7.468 gpm

Flow rate0.47 l/s
Hourly flow1.001 CFM
Inside cross-section314.16 mm²

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How it's calculated (visible formula)

Q = π/4 × D² × v

Net geometric measurement, no waste.
Adjustable waste % for cuts and breakage.
Purchase rounded up to sellable units (boxes, bags, buckets, boards).

See methodology and formula versions

How to calculate it, step by step

Measure or look up the pipe's inside diameter.
Set the water velocity you want to check.
Inside area = π/4 × D².
Multiply by velocity for flow in m³/s.
Convert to gpm, l/min or m³/h.

Worked example: ¾ in (20 mm ID) pipe at 5 ft/s (1.5 m/s)

Area: π/4 × 0.02² = 0.000314 m²
× 1.5 = 0.000471 m³/s

→ 7.5 gpm (28.3 l/min).

Common mistakes

Using the outside diameter.
Designing for high velocity — noise and water hammer.
Mixing up gpm and gph.

Pro tips

Keep velocity low on pipes near bedrooms.
Check friction loss on long runs too.
PEX inside diameter is much smaller than nominal.
Stay within your code's maximum velocity for the pipe material.

From calculation to quote

Add labour, overhead and your margin, and generate the professional PDF.

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FAQs

What water velocity is recommended?: Under 5–8 ft/s (1.5–2.4 m/s) to limit noise and erosion.
How much flow does a shower need?: 1.8–2.5 gpm with modern heads.
How do I get inside diameter?: Outside diameter minus twice the wall thickness, or the pipe schedule chart.
How many gpm through ½ in PEX?: About 2–3 gpm at reasonable velocities.
How do I measure faucet flow?: Fill a bucket of known volume and time it.
What is water hammer?: A pressure spike when a valve shuts fast; it grows with velocity.

Sources and standards

Maintained by Srikanta M · Formula v1.0.0 · Verified 2026-09-10 · Updated 2026-09-10 · Limits: quantity estimation, not structural design or site supervision.