How to identify and replace a hot tub filter

1. Look for the part number first

Most cartridges have a part number printed or molded on the end cap. It might be an OEM number from the spa maker (for example a Sundance 6540-xxx number), a Unicel number (C-xxxx, 4CH-xx), or a Pleatco number (Pxxxx). Search for any of them here: each part page lists every equivalent number we found in the source catalogs.

2. Measure if there's no number

• Diameter: measure across the widest point of the pleats, not the end cap's lip.
• Length: measure the overall length, including any handle or threaded fitting.
• Top and bottom: note whether each end is open, closed, has a handle, or is threaded, and measure any opening.

Then use the size lookup. Two cartridges can share a diameter and length but have different fittings, so always compare the top and bottom too.

3. Understand square footage

The sq ft rating is the total pleated media area. Within the same size and fittings, more area can mean longer intervals between cleanings. A cartridge that is physically different won't seal in your housing, whatever its rating.

4. When to clean and replace

Rinse cartridges regularly and soak them in a filter cleaner periodically, following your spa maker's schedule. Replace a cartridge when the pleats are torn, flattened, or permanently discolored, when the end caps crack, or when flow stays weak after cleaning.

About our data

We list 50 cartridges, 30 of them fully cited. The rest are clearly labeled unverified. Sources are the Unicel 2026 Replacement Guide and the Pleatco 2023 catalog, with page numbers on every cartridge page.