How To Measure a Filter
The 5 Steps to Identifying Spa Filters by Size
 Step 1 - Outside Diameter
Measure the outside diameter of the filter cartridge in inches.
(Your measurements may be within 1/32" of the manufacturer's specifications) |
 Step 2 - Overall Length
Measure the overall length of the filter cartridge in inches. If your filter has a fitting on the bottom do not include that as a part of the length. The filter pictured above is 13", note that the fitting adds an extra inch which should not be included. |
 Step 3 - Top of Filter
Examine the top of the filter cartridge and note whether it is open or closed. If there is an opening measure the diameter of the hole in inches. 
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 Step 4 - Bottom of Filter
Examine the bottom of the filter cartridge and note whether it is open or closed. If there is an opening measure the diameter of the hole in inches. 
If your spa has Threads, Identify the Threads. Standard vs. S.A.E
Threads
 | As you can see in this graphic, when looking at a filter's threads from the side the difference is very obvious. Standard, sometimes called "MPT" or "Male Pipe" threads are smaller and have a higher number of threads than the S.A.E. style. The S.A.E. threads are large and are square in nature almost resembling a child's toy. |
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Step 5 - Finding Your Filter
Click on the number that corresponds with the outside diameter of your filter cartridge. This will display a table with all of the filters in that size range. They are listed in order by Outside Diameter, Overall Length and hole size. |
