Check plumbing pipe size by reading its markings first. If none are legible, measure the outside diameter (OD) of a straight, exposed section of rigid pipe, then match the measurement, material and pipe system to the manufacturer’s current dimensional data. The labelled nominal size often differs from the measured diameter.
Steps to identify the size
Read the markings. Record the nominal size, material, manufacturer and any class, series or schedule. Do not rely on colour alone.
Identify the pipe system. Check whether the markings refer to pressure pipe or a sanitary or DWV (drain, waste and vent) system, as dimensions can vary between products and specifications.
Measure the outside diameter. Use callipers across the widest point of a straight, undamaged section, away from fitting sockets. A ruler can provide a rough measurement. Do not use the inside diameter of the pipe for initial identification.
Calculate diameter from circumference if needed. Wrap a flexible tape squarely around the pipe, then use:
Diameter = circumference ÷ 3.142
Keep all measurements in the same unit, such as millimetres.
Check the correct size chart. Compare the OD with current manufacturer or standards-based dimensions for that material and pipe system. Include the class, series or schedule where shown; avoid generic conversion charts.
For a loose fitting, measure the socket. Measure the inside diameter of the socket that receives the pipe, not the fitting’s outside wall, then compare it with the manufacturer’s fitting dimensions.
Why the measurement may differ from the label
Nominal pipe size, sometimes called nominal bore in particular systems, is a compatibility designation. It is not necessarily the exact OD or inside diameter (ID). Wall thickness may vary by class, series or schedule, so pipes with the same OD can have different IDs.
Only inspect safely accessible pipe. Do not cut, disconnect or uncover it for identification. Stop if access could disturb water, gas or electrical services, or suspected asbestos-containing material. If markings and measurements do not give a confident match, contact our licensed Sydney plumbing team before selecting or altering fittings.