How do I calculate the volume of water in a pipe?

Measure the pipe's inner diameter and the length of the run, enter them above, and read off the volume in litres or gallons. The formula is pi x (inner radius)^2 x length, the same cylinder-volume formula used for a tank.

Does pipe wall thickness affect the volume calculation?

No — only the inner diameter matters for fluid volume, since that's the space the liquid actually occupies. Wall thickness and outer diameter matter for the pipe's weight, which the separate pipe weight calculator handles.

What pipe sizes are common in Australian plumbing?

Australian domestic plumbing commonly uses 15 mm and 20 mm copper or PEX pipe for hot and cold water supply, sized to AS/NZS 3500 — these are nominal sizes, so use the pipe's actual inner diameter in this calculator for an accurate volume.

Pipe Volume Calculator

How much liquid does a pipe hold?

A pipe's internal volume — the amount of water or other fluid it can actually hold — matters whenever you need to know how long a water heater takes to deliver hot water to a distant tap, how much chemical solution a dosing line needs to prime, or how much water you'll drain out when you cut into a pipe run. This calculator works out that internal fluid capacity from a pipe's inner diameter and length, plus the liquid's mass if you fill it, using the straight cylinder-volume formula so you get an exact figure rather than a rough guess.

How to use the pipe volume calculator

Enter the pipe's inner diameter (the actual bore the liquid flows through, not the nominal or outer size) and pick its unit — mm, cm, inches, or feet.
Enter the length of the pipe run and its unit.
Choose a liquid — water is the default, or pick diesel, petrol/gasoline, or milk from the density preset, or enter a custom density — then read off the volume in litres, US or UK gallons, or cubic metres/feet, along with the liquid mass.

The pipe volume calculation

A pipe run is a simple cylinder, so its internal volume is V = pi x (ID/2)^2 x L, where ID is the inner diameter and L is the length. Multiply that volume by the liquid's density to get its mass: mass = V x density.

Example: a 10 m run of DN25 (1 in) pipe with a 26.6 mm inner diameter, filled with water (density 997 kg/m3). Radius = 13.3 mm = 0.0133 m. Volume = pi x 0.0133^2 x 10 = 0.00556 m3, which is 5.56 litres. Mass = 0.00556 x 997 = 5.54 kg of water.

Pro tips

An Australian licensed plumber assessing a pipe run's capacity or flow — for example before dosing a hydronic heating loop or estimating drain-down volume before cutting a line — typically charges around AUD 90-340 as a callout, regardless of the pipe's actual size.

Always use the inner diameter (bore), not the nominal pipe size or outer diameter — a nominal 1 in (DN25) pipe typically has an actual inner diameter of around 26.6 mm, not 25.4 mm, so using nominal size alone will under- or overstate volume.
Changing the liquid preset only changes the mass output, not the volume — volume depends purely on the pipe's geometry, so switching from water to diesel or milk gives the same litres but a different weight.