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  How to Measure Hot Tub Spa & Pool Filter Cartridges

Get out your tape measure or ruler. We'll show you the proper way to confirm your correct spa or pool filter cartridge by dimensions, so that you can find the appropriate replacement from our Clarathon Premium Ultra Filters cross-reference guide.

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Measuring Diameter

Always start by measuring the width (diameter) of your old cartridge across the outside edges of the end cap at its widest point.

Note: Measurements may vary by about 1/8" due to expansion, shrinkage, etc. For example, if you measure as a 5" diameter, and can't find a match, it may actually be a 4-15/16" filter, etc.

Measuring Length
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" or more due to expansion, shrinkage, or manufacturer's variances.

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

Measuring Opening

Opening Diameter
Your filter may have an opening in one end (with a handle top or closed top), or it may have openings in both ends. Measure the actual hole diameter, excluding taper, if any. If your filter has a slotted-hole, measure the shortest way, not including the slot.

Closed-top filters often have tiny pin-holes to allow trapped air to escape when installed.  Ignore pin-holes for measurement purposes.

Note: See below instructions for measuring cartridges with threaded or slip fittings.

Thread Types

Filter Thread Types
For filters with male threaded fittings, you will first need to determine the type of thread. There are two types of threads which are easily identified by referring to the left photo:
  • Fine threads, also called MPT or NPT
  • Coarse threads, also called SAE

See notes on Removable Fittings below.

Measuring Threads

Male Thread Sizes
After identifying thread type, you will need to determine the thread size. Screw fitting sizes are stated as nominal pipe thread sizes (1-1/2", 2", etc.) NOT the actual thread diameter measurement. If in doubt, use our tables to determine your match. Shown below are the overall outside diameters (O.D.) measured at the widest point.

Fine Thread Measurements:

Nominal Thread Size Measured O.D.
1-1/2" 1-7/8"
2" 2-1/4"

Coarse Thread Measurements:

Nominal Thread Size Measured O.D.
1-1/4" 1-11/16"
1-1/2" 1-13/16"
2" 2-5/16"

Female Threads

Female (Internal) Coarse Threads
Female screw fitting sizes are stated as nominal pipe thread sizes (such as 1-1/2") which is NOT the actual thread inside diameter measurement. A 1-1/2" nominal female coarse threaded fitting actually measures approx. 1-7/8" inside diameter (I.D.).

  • Measure at the widest inside diameter, not including threads.
  • Coarse threads are also known as SAE.

Pleatco Adapter Removable Threaded Fittings
If replacing a Pleatco brand threaded cartridge, be aware that older types had a removable male pipe thread fitting which slip-fit into a 1-15/16"D cartridge opening. You may need to reach down 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 inside). Clarathon's screw fittings are permanently attached.

Male Slip Male Slip Fittings
Some filter cartridges come with a smooth (non-threaded) male slip fitting on one end. The size dimension is the overall outside diameter (O.D.) measured at the fitting's widest point.


Read Tape Measure

Reading a Tape Measure
Tape measures have fractional lines & inch marks across the face. The scale between inch marks is divided into 1/16" increments. There are four different sizes of lines between the inch marks. The longest line represents 1/2", the second longest 1/4", then 1/8" and finally 1/16".

The value of any line on the scale can be described in sixteenths of an inch, but it's customary to give measurements with the lowest common denominator -- so 6/16" is stated as 3/8".

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