How to find your spa filter size
By MySpaFilter Editorial Team · Last reviewed
First look for a part number printed on the end cap or label and search it here. If there is none, measure the diameter across the widest point, the overall length end to end, and note the top and bottom fittings (open hole size, thread, handle or closed cap), then match them on the size lookup. These are the same fields Unicel and Pleatco catalogs list for every cartridge[5][2].
Step 1: look for a number
OEM, Unicel (e.g. C-xxxx) and Pleatco (e.g. Pxxx / PRBxx) numbers can all be looked up from the homepage search. Our cross-reference comes from the Unicel 2026 Replacement Guide and the Pleatco 2023 catalog[5][2].
Step 2: measure
| Measurement | How | Why it matters |
|---|---|---|
| Diameter | Across the widest point of the pleats / end cap | Must fit the filter well or housing |
| Length | End to end, including any handle or threaded nipple | Must fit the housing height |
| Top fitting | Open hole (measure inside), handle, or closed | Affects how it is held in place |
| Bottom fitting | Open hole size, or male/female thread size | Must seal on the standpipe or the water bypasses the media |
Catalog tables list diameter, length, top and bottom fittings and media area (sq ft) for each cartridge[5][2]. See an example with a to-scale diagram: Caldera C-4335.
Square footage is not the deciding factor
Media area (sq ft) helps distinguish two cartridges with the same dimensions, but match size and fittings first. Our measuring guide covers this in more detail.
FAQ
Where is the part number on a hot tub filter?
Usually printed on an end cap or the label/band on the pleats. It may be an OEM spa-maker number or an aftermarket Unicel or Pleatco number.
What if two cartridges have the same diameter and length?
Compare the top and bottom fittings and the square footage listed in the catalog.
Do I measure the handle?
Measure the full overall length end to end; then compare with catalog length values and your old cartridge.
Sources
This guide summarizes published manufacturer guidance; always follow your own spa’s owner’s manual. See our disclaimer.