How to Measure Your Hot Tub Filter Cartridge

Measuring your spa filter cartridge may seem complicated, but it’s actually quite simple! Our Spa Headquarters Filter Measuring Guide shows you the proper way to easily find and confirm correct spa cartridge dimensions. Once all the elements are measured, finding the appropriate replacement filter cartridge for your spa is easy. Take a look at our step-by-step measuring guide here:


Width (Diameter)

First, measure the width (diameter) of your old filter cartridge across the outside edges of the cap. The cap will be measured at its widest point.

Please Note: Due to expansion and shrinkage, measurements may vary. For example, if you measure a 5-inch diameter and no filters seem to match, the filter you need might actually be 4-15/16 inches instead.


Next, measure the length of the cap from one outside end to the other. Remember, like mentioned above,

measurements may vary due to expansion and shrinkage – so specs maybe off 1/8 inch or more. If your filter has handle or other fitting on top, do not include in measurement.

Opening Diameter

If your filter has an opening on one end (or both), measure the actual diameter of the hole (excluding the taper).

Slotted-hole filter: Measure the shortest way excluding the slot.

Closed top filter: If your filter is closed topped and has pinholes, do not include pinholes in measurement.

Filter Threads

Thread Type

After taking measurements, you will need to identify the filter thread type. There are two main types of threads on the bottom of spa filters, a course thread (SAE) and a fine thread (MPT). The course thread only has a few, larger spirals while the fine thread has many more thinner and closer together spirals.

From there, you will have to determine if the thread is a male thread that comes up from the bottom of the filter, or a female thread , that goes inside the filter. Most hot tub filters that are threaded have the male type connectors.

Thread Size (Male)

Once thread type is determined, you need to figure out the thread size. Use our tables below to determine your match. Below you will see the outside diameters (O.D.) measured at the widest point.

Fine Thread Measurements:

Nominal Thread SizeMeasured O.D.
1-1/2” 1-7/8”
2” 2-1/4”

Course Thread Measurements:

Nominal Thread SizeMeasured O.D.
1-1/4” 1-11/16”
1-1/2” 1-13/16”
2” 2-5/16”

Thread Size (Female - Course)

Female sizes are stated as nominal pipe thread sizes, which is not the actual thread inside diameter (I.D.) measurement. A 1-1/2” nominal female course thread fitting measures 1-7/8” inside diameter.

Please note: Measure at the widest I.D. – not including threads. Course threads are also known as SAE.

Removable Thread Fittings

You may need to unscrew the male pipe fitting from your spa’s filter – if it did not come out with the old filter. Many people don’t realize it’s even inside the filter!

Male Slip Fittings

If your filter cartridge comes with a smooth and non-threaded slip fitting on one end, the size is the overall outside diameter (O.D.) measured at the fittings widest point.

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