Hot Tub Plumbing Pipe Sizes: The Complete Guide
Not sure which pipe or fitting you need for your hot tub? Metric and imperial plumbing can look almost identical, making it difficult to identify the correct replacement.
Learn how to measure your pipe in millimetres, identify its nominal size and find compatible plumbing components.
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Why getting the pipe size right matters
Hot tubs and swimming pools use different plumbing sizes depending on the equipment and installation. A pipe that looks the right size may not fit a replacement union or connector if the dimensions or standards differ.
The best starting point is to measure the outside diameter (OD) in millimetres. You can then compare your measurement with the references in this guide.
Important: Metric and imperial pipe sizes are not automatically interchangeable. Always check the actual measurement and fitting compatibility before ordering.
How to measure your pipe in 3 steps
Measure the outside diameter
Use a vernier calliper to measure straight across the outside of the pipe, from one outer edge to the other. Record the result in millimetres.
Identify the pipe type
Check whether your installation uses metric or imperial plumbing. Do not assume the size based on appearance alone.
Match the measurement
Compare your measurement against the tables below. Check the pipe and fitting specifications before purchasing a replacement.
No callipers available? Wrap a strip of paper around the pipe and mark where the ends meet. Measure the length in millimetres, then divide by 3.14 to estimate the outside diameter.
Formula: Outside diameter = circumference ÷ 3.14
Measure the pipe itself, not the outside of a union, socket or other fitting. If the pipe is difficult to access, take care not to confuse the fitting's larger dimensions with the pipe's diameter.
Metric hot tub and pool pipe sizes
Metric pipe is sized by its nominal outside diameter. For example, a 50 mm metric pipe has an outside diameter of approximately 50 mm.
Metric plumbing is often used in swimming pools and commercial spa installations, although the correct system depends on the equipment.
| Metric pipe size | Outside diameter |
|---|---|
| 20 mm | 20 mm |
| 32 mm | 32 mm |
| 40 mm | 40 mm |
| 50 mm | 50 mm |
| 63 mm | 63 mm |
| 75 mm | 75 mm |
Imperial hot tub and pool pipe sizes
Imperial pipe sizes use nominal measurements rather than the actual outside diameter. This means the stated size is not necessarily the measurement you will get with a calliper.
The following dimensions are common examples. Confirm the specific pipe standard before using them to select replacement fittings.
| Nominal imperial size | Outside diameter (mm) |
|---|---|
| 3/8 inch | 14.0 mm |
| 1/2 inch | 21.5 mm |
| 3/4 inch | 28.0 mm |
| 1 inch | 33.4 mm |
| 1 1/4 inch | 42.16 mm |
| 1 1/2 inch | 48.26 mm |
| 2 inch | 60.33 mm |
| 2 1/2 inch | 73.0 mm |
| 3 inch | 88.9 mm |
Remember: One inch equals 25.4 mm, but this does not mean that nominal 1-inch pipe has an outside diameter of 25.4 mm. Its actual outside diameter depends on the pipe standard.
Metric vs imperial: what is the difference?
Metric pipe
The nominal size normally corresponds to the outside diameter. A 50 mm metric pipe measures approximately 50 mm across its outer surface.
Imperial pipe
The nominal size relates to a defined pipe standard, rather than necessarily matching the outside diameter. A nominal 2-inch pipe, for example, may measure approximately 60.3 mm across.
These differences can cause problems when replacing pipework or connecting new equipment. Even when measurements appear close, the pipe and fitting standards may differ.
What are pipe schedules and classes?
Pipe schedule and pipe class describe standards relating to pipe wall thickness and pressure ratings. The terminology depends on the pipe system and manufacturing standard.
A thicker wall can affect the pressure the pipe can withstand, but it does not necessarily change the outside diameter within the same specified pipe size.
When selecting replacement plumbing, check the product specifications, pressure rating and compatibility with your existing installation.
Frequently asked questions
How do I know what size hot tub pipe I have?
Measure the pipe's outside diameter in millimetres using callipers, then compare your measurement with the pipe size references above.
Is 1-inch pipe the same as 25 mm pipe?
No. A nominal 1-inch pipe may have an outside diameter considerably larger than 25.4 mm. Check the applicable pipe standard and measure the pipe directly.
Can I connect metric pipe to imperial fittings?
Not necessarily. Their dimensions and connection standards can differ. Use a compatible transition fitting where required rather than forcing the connection.
Should I measure the inside or outside of the pipe?
Measure the outside diameter for the size references in this guide. Nominal imperial sizes do not always correspond to the outside measurement.
What if my measurement falls between two sizes?
Recheck the measurement and make sure you are measuring the pipe rather than a fitting. If the size remains unclear, contact Hydrospares with the measurement and a photograph of the pipework.
Need help finding the right plumbing?
Browse the Hydrospares plumbing range or contact our team for help identifying a suitable part for your hot tub or swimming pool.
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