To find the correct filter replacement for your pool and spa, you'll need to follow a series of steps to ensure compatibility and proper fit. Here's a general guide based on the information provided by the websites you shared:

1. Identify the Current Filter:

Check your current filter for any labels, part numbers, or manufacturer information. This can often be found on the end cap or printed on the filter itself.A genuine quality hot tub filter will have a part number stamped into the end cap (see example). It may be a Darlly, Unicel, Pleatco or Filbur number. It can even be a manufacturer part number. Just find the number printed on your hot tub cartridge, and enter it into the box, and click the Find my Hot Tub Filter button.

2. Measure the Filter:

Use a tape measure to get the dimensions of your filter cartridge. You'll need to measure:

** Diameter (Width):

Across the widest part of the filter, typically the diameter of the cylindrical part.
Always start by measuring the width (diameter) of your old hot tub filter 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.

**Length:

From one end cap to the other, including the caps but not any protruding threads, handles, or attachments.
Now measure the length of your hot tub filter cartridge, from outside of end cap to outside of end cap (this is the hard-plastic end of the filter). 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.

**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 pinholes to allow trapped air to escape when installed. Ignore pinholes for measurement purposes.

**Thread Type and Size:

If the filter has threaded fittings, determine if they are fine or coarse threads and identify the size.


For hot tub 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 photo on the left:

• Fine threads, also called MPT or NPT
• Coarse threads, also called SAE

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.

 

Female screw fitting sizes

 

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.

 

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".