Plumbing Sizes - PVC Pipe, Hoses, Tubing - Dimension and Names

Understanding Plumbing Sizes & Nominal Measurements

Plumbing sizes can be confusing, especially for those unfamiliar with how PVC and other pipe fittings are labeled. Unlike standard measurements, plumbing sizes are referred to as "Nominal" sizes, meaning they are named rather than based on actual dimensions. This applies to many types of plumbing fittings, including those used in pool and spa systems.

For example, a 2"/50mm fitting refers to the pipe it connects to, but the pipe’s actual outside diameter is approximately 60.3mm.

Why Don’t Pipe Sizes Match Their Names?

The Nominal Pipe Size (NPS) system was developed to standardise pipe dimensions, ensuring compatibility between pipes, fittings, and other components. While this system is common knowledge for plumbers, it can be confusing for those outside the trade.

Key Measuring Guidelines:

• Pipe Size Measurements are taken from:
  • The Outside Diameter (OD) of the pipe itself.
  • The Inside Diameter (ID) of the socket (fitting opening).
• Pipe Class or Schedule Measurements refer to the Internal Diameter (ID) of the pipe.
• The Outside Diameter of a fitting or socket is irrelevant, as fitting thickness varies widely.

Below are charts showing nominal sizes alongside actual dimensions. In pool and spa plumbing, as well as many other plumbing applications, Pressure Pipe is commonly used—not DWV or other plumbing types.

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PVC Pressure Pipe ASNZ 1477 Dimensions¹:

Imperial Name	Metric Name	Outside Diameter	Class 9 IDº	Class 12 IDº	Class 18 IDº
1/2"	15mm	~ 21.2-21.5mm		18.3mm	17.8mm
3/4"	20mm	~ 26.6-26.9mm		23.7mm	22.4mm
1"	25mm	~ 33.4-33.7mm	30.5mm	29.8mm	28.1mm
	32mm	~ 42.1-42.4mm	38.5mm	37.5mm	35.4mm
1.5"	40mm	~ 48.1-48.4mm	45.2mm	44.1mm	42.8mm
2"	50mm	~ 60.2-60.5mm	55.2mm	53.7mm	50.5mm
	65mm	~ 75.2-75.5mm	68.9mm	67.0mm	63.2mm
2.5"		~ 73mm

• º Some pipe manufactures may have different Internal Dimensions
• ¹ Not all pool or spa fittings are exactly these measurements, as they do not necessarily adhere to Australian or International Standards. Chinese spas will sometimes have completely proprietary fittings.

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Hose & Tube Dimensions:

Imperial Name	Metric Name	Inside Diameter
3/8"	9-10mm	9mm
3/4"	19mm	19mm

 

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BSP - British Standard Pipe Thread Sizes: