How to Measure Spa & Hot Tub Filter Cartridges

Filter Replacement Guide  

Always start by measuring the width (diameter) of your old cartridge.  Keep in mind that measurements may vary by about 1/8" due to expansion, shrinkage, etc.  For example, if you measure as a 5" diameter, and cannot find the listing in the 5" series guide, it may actually be a 4 15/16" filter, etc.

Now measure the length, from outside of end cap to outside of end cap. Once again, remember that measurements may vary from specs by about 1/8" due to expansion or shrinkage.

Note: If your filter has a handle top and/or fitting on the other end,
do NOT include these in your length measurement.

Your filter may have a hole in one end (with a handle top or closed top), or it may have a hole in both ends.  Measure diameter(s).  This is the actual hole diameter, excluding taper, if any. (Closed-top filters often have small pin-holes to allow trapped air to escape when installed.  Ignore pin-holes for measurement purposes).  See notes on Removable Fittings below.

If your filter has a slotted-hole, measure the shortest way, not including the slot. The 2 most common slotted hole sizes are: 1 7/8" (1.9") and 2 3/8".

For filters with male threaded fittings, you will first need to determine the type of thread.There are only two types used:  Regular and SAE, so it's easy.

• Regular threads are fine, tightly spaced, with relatively sharp edges.
• The SAE type is a very coarse, widely spaced, flat-edge thread.

See notes on Removable Fittings below.

After identifying thread type, you will need to determine the thread size for filters with regular (fine) threads.  Regular threaded filters are listed in 2 nominal sizes: either 1 1/2" or 2" MPT (male pipe thread).  1 1/2" MPT actually measures 1 3/4" outside diameter (OD); 2" MPT measures 2 1/4" OD.

Since there are only a few SAE threaded filter types, it is only necessary to verify the SAE threaded filter's other dimensions and style to confirm that you have a perfect match.

If replacing a Pleatco brand threaded cartridge, be aware that these have a removable male pipe thread fitting which slip-fits into a 1.9" hole in the filter bottom.  You may need to unscrew the male pipe fitting from your spa's filter housing if it did not come out with the old filter (many spa owners are not even aware that the fitting is still in there). Most threaded Clarathon filters are molded as a single piece, so that the threaded fitting comes off with the filter.

Hot tub filters with smooth (non-threaded) male slip fittings come in only 2  nominal slip fitting sizes:

1-3/4" which is the actual outside diameter (OD), and 2" which measures approx. 2 1/4 to 2 3/8" OD, depending upon the filter.