What is a pressure reducing valve (PRV) and when do I need one?

A PRV (also called a pressure limiting valve or PLV) reduces the supply pressure to a preset level -- typically 300-350 kPa for residential properties. Under AS/NZS 3500, a PRV is required wherever the supply pressure exceeds 500 kPa at the meter. High pressure causes tap and fixture damage, burst flexible hose connections (a leading cause of water damage in homes) and water hammer. PRVs are inexpensive insurance.

Why is minimum pressure 100 kPa at the highest fixture?

At pressures below 100 kPa, flow rates from fixtures are insufficient for normal use -- showers feel weak and taps barely dribble. Some flow control devices (thermostatic mixing valves, certain shower heads) have minimum pressure requirements of 150-200 kPa to function correctly. Always check the minimum pressure rating of specific fixtures against the calculated available pressure.

Does water pressure change with temperature?

Yes -- hot water systems create thermal expansion that increases pressure in closed systems. AS/NZS 3500 requires an expansion control valve (or return water allowed back to the main) on hot water systems to manage this. Thermal expansion pressure increases can exceed the 500 kPa limit and damage the hot water unit.

Water Pressure Calculator — Free Tradies Calculator

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A plumber needs to confirm that a shower head on the second floor of a building will receive adequate pressure. The street pressure is 350 kPa. The shower is 7.5m above the water main connection point.

Water Pressure Calculator

Plumbing

Pipe height (m)

Display unit

Pressure (kPa) = Height (m) × 9.81 Residential: 100–500 kPa · 1 bar = 100 kPa = 14.5 psi

Reference: AS/NZS 3500 — Plumbing and Drainage

ℹ️ Results are estimates for planning purposes. Verify with current standards and a qualified professional.

1 What this calculator does

Calculates static water pressure from a head of water (pipe height above reference point). Converts between kPa, psi and bar. Checks the result against AS/NZS 3500 minimum and maximum service pressures for plumbing installations.

2 Formula & professional reasoning

Pressure (kPa) = Height (m) x 9.81
Pressure (psi) = Height (m) x 1.450
Pressure (bar) = kPa / 100
AS/NZS 3500 range: 100 kPa minimum (at highest fixture) to 500 kPa maximum (at meter)
Pressure at fixture = Supply pressure - (Height x 9.81 kPa/m)

Hydrostatic pressure converts height (head) of water to pressure. Every metre of water height generates 9.81 kPa of pressure. For a supply system, pressure at any point equals the supply pressure minus the pressure lost in lifting the water to that height. A second-floor fixture 7.5m above the main loses 7.5 x 9.81 = 73.6 kPa from the supply pressure.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Pressure at second floor shower

Given: Street pressure: 350 kPa | Height above main: 7.5m

Working: Pressure loss from height: 7.5 x 9.81 = 73.6 kPa | Pressure at shower: 350 - 73.6 = 276.4 kPa

Answer: Pressure at shower: 276.4 kPa -- within AS/NZS 3500 range (100-500 kPa)

💡 276 kPa at the shower head is good pressure. Minimum for a shower is typically 100 kPa -- there is 176 kPa of margin. Friction losses in pipe fittings will reduce the actual delivered pressure slightly.

Standard

Water tower head calculation

Given: Water tower height: 35m above service connection

Working: Pressure: 35 x 9.81 = 343.4 kPa

Answer: Service pressure from tower: 343 kPa (approx 50 psi)

💡 A 35m water tower provides approximately 343 kPa or 50 psi to connected services -- this is a moderate pressure suitable for residential distribution.

Advanced

Check pressure at ground floor with high street pressure

Given: Street pressure: 650 kPa | Ground level connection (no height difference)

Working: Static pressure: 650 kPa -- above the 500 kPa AS 3500 maximum | Pressure reducing valve (PRV) required

Answer: 650 kPa exceeds maximum 500 kPa -- PRV required to set 300-350 kPa

💡 High street pressures (above 500 kPa) require a PRV at the service connection. Without it, fixtures fail prematurely and water hammer is a problem.

4 Sanity check

AS/NZS 3500 pressure limits

Minimum at highest fixture: 100 kPa | Maximum at meter: 500 kPa | Recommended service: 200-350 kPa

Pressure conversion reference

100 kPa = 14.5 psi = 1 bar | 350 kPa = 50.8 psi | 500 kPa = 72.5 psi

Pressure loss from height

Every 1m of height loses 9.81 kPa | A second floor 3m above ground loses 29.4 kPa

Dynamic pressure vs static pressure

This calculator gives static pressure (no flow). Dynamic pressure at the fixture is lower due to pipe friction and flow velocity losses

5 Common errors

Error	Cause	Consequence	Fix
Confusing static and dynamic pressure	Using the calculated static pressure as the delivered pressure during flow	Overestimating delivered pressure at fixtures -- inadequate flow may result	Static pressure is pressure with no flow. Dynamic pressure (actual pressure during use) is lower due to friction losses in pipes and fittings. For detailed flow calculations, add pipe friction loss using the Darcy-Weisbach or Hazen-Williams method.
Not installing a PRV where street pressure exceeds 500 kPa	Not checking the water authority's supply pressure before connection	Fixture failure, tap seat damage, burst flexible hoses and potential water damage	Always check the water authority's supply pressure and confirm it is within the 500 kPa maximum. Where pressure exceeds this, install a pressure reducing valve set to 300-350 kPa.
Not allowing for minimum pressure at the highest fixture	Calculating street-level pressure without subtracting height losses	Top floor fixtures below the 100 kPa minimum -- inadequate flow from taps and showers	Always calculate the pressure at the highest fixture: supply pressure - (height above main x 9.81 kPa). This must be at least 100 kPa (AS/NZS 3500).
Using psi when AS/NZS 3500 specifies kPa	Working in imperial pressure units on an Australian installation	Non-compliance with Australian standards -- inspection failure	Australian plumbing standards use kPa. US and UK commonly use psi. Confirm which unit system applies to the relevant standard and convert as needed.