The short answer
The fastest match is the part number on your old cartridge or filter lid. Unicel, Pleatco and Filbur cross-reference each other and the original Hayward, Pentair and Jandy numbers, so one cartridge often lists several codes. If the label has worn off, measure length, outside diameter and the top and bottom holes.
Cross-referencing
For example, the Pleatco PA120 and the Cryspool 08055 both list Hayward's CX1200RE; the Unicel C-4950 is a common spa size. Every cartridge page shows the numbers its listing says it replaces and other cartridges that share them.
Measuring
- Length end to end, excluding handles.
- Outside diameter of the pleats.
- Top and bottom hole diameters (open or closed, threaded or not).
- Filter area in square feet, if printed.
When to replace
Hose cartridges off when the pressure gauge rises 8 to 10 psi above clean. Replace pool cartridges every one to three seasons and spa cartridges about once a year with regular use.
Products mentioned
Questions readers ask
Are Unicel and Pleatco cartridges interchangeable?
Where they list the same original part number, yes; they're made to the same dimensions.
Does a bigger filter area fit better?
Only among cartridges with the same dimensions; area changes cleaning intervals, not fit.