Pool & Spa plumbing sizes and names can be confusing, as plumbing sizes are referred to as "Nominal" sizes. "Nominal" means "by name only" and does not reflect the actual dimensions of the pipe. This is especially true with plastic fittings. For example, a 2"/50mm fitting refers to the pipe it's used with, which has an outside diameter of ~60.3mm.
The Nominal Pipe Size (NPS) system was established long ago to standardise pipe sizes for compatibility with fittings and other componends. This is something you're taught as a plumber and can be confusing for a layperson not familiar with it.
Important Measuring Information:
- For Pipe Sizes, measurements are always taken from the Outside Diameter (OD) of the Pipe itself, OR the Inside Diameter (ID) of the Socket of a fitting.
- For Pipe Classes or Schedules, meausrements are taken from the Internal Diameter of the pipe.
- The OD of a fitting or socket is completely irrelevent, as the thickness of fittings varies widely.
Below are some charts with names over the actual dimensions. In Pool & Spa water plumbing, for spas and pools, generally we only use "Pressure Pipe," not DWV or other plumbing sizes.
PVC Pressure Pipe ASNZ 1477 Dimensions¹:
| Imperial Name |
Metric |
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.
Hose & Tube Dimensions:
| Imperial Name | Metric Name | Inside Diameter |
| 3/8" | 9-10mm | 9mm |
| 3/4" | 19mm | 19mm |
BSP - British Standard Pipe Thread Sizes:
|
BSP Thread Size Naming |
Outside Diameter mm / inch |
TPI |
|---|---|---|
|
1/16 Inch BSP |
7.7 mm / 0.304" |
28 |
|
1/8 Inch BSP |
9.7 mm / 0.383" |
28 |
|
1/4 Inch BSP |
13.16 mm / 0.518" |
19 |
|
3/8 Inch BSP |
16.66 mm / 0.656" |
19 |
|
1/2 Inch BSP / 15mm |
20.99 mm / 0.825" |
14 |
|
5/8 Inch BSP |
22.99 mm / 0.902" |
14 |
|
3/4 Inch BSP / 20mm |
26.44 mm / 1.041" |
14 |
|
7/8 Inch BSP |
30.20 mm / 1.189" |
14 |
|
1 Inch BSP / 25mm |
33.25 mm / 1.309" |
11 |
|
1-1/4 Inch BSP |
41.91 mm / 1.650" |
11 |
|
1-1/2 Inch BSP / 40mm |
47.80 mm / 1.882" |
11 |
|
2 Inch BSP / 50mm |
59.61 mm / 2.347" |
11 |
|
2-1/4 Inch BSP |
65.71 mm / 2.587" |
11 |
|
2-1/2 Inch BSP |
75.18 mm / 2.96" |
11 |
|
3 Inch BSP |
87.88 mm / 3.46" |
11 |
|
4 Inch BSP |
113.03 mm / 4.45" |
11 |
|
5 Inch BSP |
138.43 mm / 5.45" |
11 |
|
6 Inch BSP |
163.83 mm / 6.45" |
11 |
