How to measure a replacement hot tub filter correctly

how to measure a hot tub filter
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How to replace a hot tub filter

Your hot tub filter is responsible for keeping the hot tub water clean and safe by catching dirt, debris, and contaminants in the water. Therefore, it naturally gets dirty and needs to be cleaned and replaced regularly.

We usually recommend cleaning your hot tub filter weekly, with a deep clean every month.

And we recommend you replace the hot tub filter every 6months to a year depending on condition and use.

PRO TIP: Have a second filter that you can swap out when you are cleaning and drying the other one.

There are hundreds of different filter makes and sizes. So, it can be hard to find the one that suits you.

We look at how to measure the hot tub filter in order to get the correct filter for your hot tub.


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The easiest way to measure your hot tub filter

Most hot tub filters have a part number printed on the end cap eg. PRB50. And the label will be on the wrapper of a new hot tub filter, so try and keep a note of this for the next time you need this to make it easier. 

How to measure a hot tub filter

There are three main sections you need to measure on the filter:

A) Outer diameter of the filter

B) Overall length of the filter (not including handles or thread)

C) Inside diameter of the opening/thread.

D) Thickness of the thread (if it has one)

A. Outer diameter of the filter

Measure the entire diameter of the filter. From the left outer edge to the right outer edge.

B. Overall length of filter

Measure the overall length of the filter from top to bottom. Do not include and handles of threads. 

C. Inside diameter of the opening/thread

Measure the inside diameter of the opening of the cartridge. 

If there is a thread measure the inside edges of the thread.
The threads of hot tub filters are normally 50mm (2″)., 75mm (3″), 100mm (4″). Threads are normally 38mm (1 1/2″) or 50mm (2″).

D. Thickness of the thread (if it has a thread)

hot tub filter threadsThe thread is usually one of two thicknesses. 

Fine thread has approx. 6-8 twists. Whereas coarse thread will have approx. 3 thicker twists in the thread. You need to ensure both the diameter of the opening (above) and the thread thickness is correct for the hot tub filter to fit correctly.

Does a hot tub filter need to fit perfectly?

Do you have to use the exact same hot tub filter for your replacement?


There are three main filter cartridge manufacturers. Darlly, Pleatco and Unicel. You can usually find the equivalent model from another manufacturer to fit your hot tub.

You can usually use another similar sized hot tub filter to fit. Filter manufacturer sizes may vary slightly. So always allow for a variance of +/- 5mm.

If it has a threaded bottom, this needs to be the same size. And the thread thickness need to be the same in order to screw in. But otherwise, the filter can be slightly narrower, thicker, longer or shorter. 

As long as it fits within the allocated space. And as long as it does not come above the water line as this will cause the hot tub to draw in air.

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In summary

When hot tub filters get old, dirty or damaged you will need to replace them to ensure your hot tub is clean. To do this you will likely need to measure your current hot tub filter. 

This guide helps you measure your hot tub filter and you should try and stick to the same filter for maximum filter efficiency. But ultimately you can alter the size slightly as long as it fits.

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Debbie Ekins
Debbie Ekins
Eagle Leisure - Sales & Marketing Manager Mission = to arm you with the knowledge you need to make the best buying decisions. Fuelled by coffee (and naps). Explorer of Scotland and the world.
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