How does the water hammer calculator work?

It uses the Joukowsky equation for a sudden valve closure: surge pressure equals water density (1000 kilograms per cubic metre) times the pressure-wave speed times the flow velocity before closure. That result is converted to bar, added to your static line pressure to give the total peak the pipe sees, and also shown as metres of water head. It models the worst-case instantaneous slam, so it is a conservative upper bound.

What inputs do I need and what values should I use?

Three inputs: flow velocity before closure, pressure-wave speed, and static line pressure. Domestic supply velocity usually sits around 1 to 2 metres per second. Wave speed depends on pipe material: rigid metal like GI, copper and DI runs roughly 1200 to 1450 metres per second, while CPVC and PVC are much softer at about 350 to 600, so plastic pipe tames the surge. Static pressure is your normal working line pressure in bar.

How do I reduce water hammer and is my pipe safe?

Slowing the valve closure, fitting a water-hammer arrestor or air chamber near the fixture, and using softer plastic pipe all cut the surge, because a slow closure spreads the pressure change over time. Check that the pipe and fitting pressure rating comfortably exceeds the total peak this tool shows. Figures are indicative for planning only; confirm with a qualified consultant, manufacturer data and the relevant NBC 2016 Part 9 provisions before procurement.

Interactive Calculator · 2026

Water Hammer Calculator

Estimate the water-hammer surge pressure from a sudden valve closure using the Joukowsky equation. Enter the flow velocity before closure, the pressure-wave speed of the pipe and the static line pressure — get the surge pressure in bar, the total peak pressure and the surge head instantly.

Surge pressure (sudden closure)0.00 bartotal peak ≈ 0.00 bar · 0.0 m of head

Static line pressure → surge → total peak the pipe must survive

Your flow & pipe

Flow velocity before closure1.5 m/s

The steady velocity in the pipe just before the valve slams shut. Domestic supply usually sits around 1–2 m/s.

Pressure-wave speed (a)1,250 m/s

Speed the pressure wave travels back up the pipe. Rigid metal (GI, copper, DI) sits at ~1200–1450 m/s; CPVC/PVC is far softer at ~350–600 m/s, so plastic pipe measurably tames the surge.

Static line pressure (bar)

The normal working pressure in the line — added to the surge to give the total peak the pipe sees.

Surge pressure

0.00 bar

Total peak pressure

0.00 bar

Surge head

0.0 m

Static vs surge vs total peak

The surge stacks on top of the static line pressure — the pipe and fittings must be rated for the total.

A sudden valve closure converts the moving water's momentum into a pressure spike of 18.75 bar — landing on top of the static line pressure for a total peak of 21.75 bar.

Drop the wave speed toward the plastic-pipe range, or slow the closure, and the surge falls in proportion. Always confirm the pipe and fitting pressure rating covers the peak.

How this is calculated

Surge pressure (Joukowsky, sudden closure) = ρ × a × v = 1000 × 1,250 × 1.5 = 18,75,000 Pa = a × v ÷ 100 = 18.75 bar.
Total peak pressure = static + surge = 3 + 18.75 = 21.75 bar.
Surge head = surge (bar) × 10.2 = 18.75 × 10.2 = 191.25 m of water.

This is the worst-case instantaneous ("sudden closure") surge — slow-closing valves, water-hammer arrestors and plastic pipe all reduce it. Indicative figures for concept planning; confirm with a qualified consultant, manufacturer data and the pipe/fitting pressure rating before procurement.

Understand the number you just got

Noisy Pipes & Water HammerWhy pipes bang, and how arrestors and slow-close valves stop it.The Plumbing Valves GuideValve types, how they close and which ones tame surge.

Frequently asked questions

How does the water hammer calculator work?: It uses the Joukowsky equation for a sudden valve closure: surge pressure equals water density (1000 kilograms per cubic metre) times the pressure-wave speed times the flow velocity before closure. That result is converted to bar, added to your static line pressure to give the total peak the pipe sees, and also shown as metres of water head. It models the worst-case instantaneous slam, so it is a conservative upper bound.
What inputs do I need and what values should I use?: Three inputs: flow velocity before closure, pressure-wave speed, and static line pressure. Domestic supply velocity usually sits around 1 to 2 metres per second. Wave speed depends on pipe material: rigid metal like GI, copper and DI runs roughly 1200 to 1450 metres per second, while CPVC and PVC are much softer at about 350 to 600, so plastic pipe tames the surge. Static pressure is your normal working line pressure in bar.
How do I reduce water hammer and is my pipe safe?: Slowing the valve closure, fitting a water-hammer arrestor or air chamber near the fixture, and using softer plastic pipe all cut the surge, because a slow closure spreads the pressure change over time. Check that the pipe and fitting pressure rating comfortably exceeds the total peak this tool shows. Figures are indicative for planning only; confirm with a qualified consultant, manufacturer data and the relevant NBC 2016 Part 9 provisions before procurement.

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