Question 1

How many litres of water are in a 100mm PVC pipe per metre?

Accepted Answer

A standard DN100 PVC pressure pipe has an internal diameter of about 102.2mm. That works out to roughly 8.2 litres per metre of pipe. So a 10-metre run of DN100 PVC holds around 82 litres of water sitting inside it.

Question 2

How do you calculate the volume of water inside a pipe?

Accepted Answer

Use the cylinder formula — pi times the radius squared times the length. The tricky part is using the internal diameter, not the nominal size or outside diameter. A DN20 copper pipe isn't actually 20mm inside — the bore is closer to 17mm or 19mm depending on whether it's Type A or Type B.

Question 3

What is the difference between Copper Type A and Type B in Australia?

Accepted Answer

Type A has a thinner wall and a larger internal bore. Type B has a thicker wall and a slightly smaller bore but handles higher pressure ratings. For volume calculations, the difference matters — DN25 Type A holds 0.496 litres per metre while DN25 Type B holds 0.417 litres per metre. That gap adds up over long runs.

Question 4

Why do I need to know the water volume in my pipes?

Accepted Answer

Mostly for dosing chemicals, sizing expansion vessels, and calculating dead-leg flush times. If you're commissioning a hydronic heating system, you can't dose the right amount of glycol inhibitor unless you know exactly how many litres the whole loop holds. Get it wrong and you either waste product or underprotect the system.

Question 5

How much does water inside a pipe weigh?

Accepted Answer

One litre of water weighs exactly one kilogram. So if your pipe run holds 45 litres, you're looking at 45 kilograms of water weight inside that pipe. On long overhead runs or pipe racks, this dead weight matters for bracket spacing and structural loading calculations.

Question 6

What is a dead-leg in plumbing and why does volume matter?

Accepted Answer

A dead-leg is the section of hot water pipe between the hot water unit and the point of use. The water sitting in that section cools down between uses, and you have to flush it before hot water arrives at the tap. Longer dead-legs with larger pipe diameters hold more cold water, which means more waiting time and more wasted water down the drain.

Question 7

How do I size an expansion vessel for a closed heating loop?

Accepted Answer

You need the total water volume of the system — pipe runs, boiler, manifold, and any buffer tank. The expansion vessel must be sized to absorb the volume increase when that water heats from ambient to operating temperature. Water expands about 4% when heated from 10°C to 80°C, so a 100-litre system needs at least 4 litres of expansion capacity, plus a safety margin.

Question 8

Does PEX pipe hold less water than copper for the same size?

Accepted Answer

Yes. PEX has thicker walls than copper to achieve its pressure rating, so the internal bore is smaller. A 20mm PEX pipe has a 16mm internal diameter compared to 17mm for DN20 Copper Type B. It's not a massive difference on a single fitting, but over a 50-metre hydronic loop the volume difference is noticeable.

Question 9

What is the internal diameter of DN50 copper pipe?

Accepted Answer

DN50 Copper Type A has an internal diameter of 50.64mm. Type B is slightly smaller at 48.31mm because of the thicker wall. Always check which type you're using before running volume calculations — the difference in litres per metre between Type A and Type B at DN50 is almost 0.2 litres per metre.

Question 10

How many litres per metre does 20mm PEX pipe hold?

Accepted Answer

Standard 20mm PEX has a 16mm internal bore and holds about 0.201 litres per metre. So a 100-metre underfloor heating loop of 20mm PEX contains roughly 20 litres of water. That's the number you'd use when dosing glycol for a hydronic slab system.

Question 11

Can I use outside diameter instead of internal diameter for the calculation?

Accepted Answer

No. The outside diameter includes the wall thickness, and no water flows through the pipe wall. Using the OD instead of the ID will overestimate the volume. For example, DN20 Copper Type B has a 19.05mm OD but only a 17.01mm ID — using the wrong one inflates the result by about 25%.

Question 12

How do I calculate pipe volume for a mixed system with different pipe sizes?

Accepted Answer

Calculate each section separately and add them together. Work out the volume per metre for each pipe size, multiply by the run length, then sum the lot. A typical hydronic system might have 150m of 16mm PEX, 20m of DN20 copper headers, and 5m of DN25 copper from the boiler — you need all three volumes added together.

Question 13

What is the volume of a DN25 galvanised pipe per metre?

Accepted Answer

DN25 galvanised iron pipe has an internal diameter of approximately 27.3mm, which gives a volume of about 0.585 litres per metre. Galvanised pipe has slightly different dimensions to copper for the same nominal size because of the thicker wall and the zinc coating on the inside surface.

Question 14

How does pipe volume relate to Legionella risk?

Accepted Answer

Stagnant water in long dead-legs is a breeding ground for Legionella bacteria, especially between 20°C and 45°C. Australian guidelines recommend keeping hot water dead-legs as short as possible. Knowing the pipe volume tells you how much stagnant water sits in the line and how long it takes to flush through — longer volumes mean higher risk in aged care, hospitals, and commercial buildings.

Question 15

Is there a maximum dead-leg length under Australian plumbing standards?

Accepted Answer

AS/NZS 3500.4 recommends minimising dead-leg distances but doesn't specify a hard maximum length in metres. However, the National Construction Code (NCC) and state health department guidelines for warm water systems (especially in hospitals and aged care) typically restrict dead-legs to limit stagnant water volume. Some health authorities recommend a maximum 1-litre dead-leg volume, which translates to about 6 metres of DN15 copper pipe.

Pipe Material	Nominal Size	Internal Ø (mm)	Volume per Metre (L/m)
Copper Type A	DN15	12.70 mm	0.1267 L/m
Copper Type A	DN20	18.92 mm	0.2812 L/m
Copper Type A	DN25	25.14 mm	0.4963 L/m
Copper Type B	DN15	13.06 mm	0.1339 L/m
Copper Type B	DN20	17.01 mm	0.2272 L/m
Copper Type B	DN25	23.03 mm	0.4165 L/m
PEX Pipe	16mm	12.00 mm	0.1131 L/m
PEX Pipe	20mm	16.00 mm	0.2011 L/m
PEX Pipe	25mm	20.00 mm	0.3142 L/m
PVC Pressure (Class 12/18)	DN50	53.60 mm	2.2562 L/m
PVC Pressure (Class 12/18)	DN100	102.20 mm	8.2047 L/m
PVC Pressure (Class 12/18)	DN150	152.40 mm	18.2415 L/m
Galvanised Iron	DN25	27.30 mm	0.5855 L/m
Galvanised Iron	DN50	53.10 mm	2.2141 L/m
Galvanised Iron	DN100	105.30 mm	8.7075 L/m

Pipe Volume Calculator (Litres)

Pipe Specifications & Length

Results Summary

Water Flow Rate Calculator

Water Pressure Drop Calculator

Why Pipe Volume Matters in Plumbing

The Formula — How Pipe Volume is Calculated

Quick Reference — Litres per Metre for Common Australian Pipes

Worked Example — Flushing a Hydronic Heating Loop

Internal Diameter vs Nominal Size — Why It Matters

Common Uses for Pipe Volume Calculations in Australia

Frequently Asked Questions