How to measure your hot tub filter

If the part number on the end cap has worn off, four measurements will usually identify the cartridge. Use a tape measure with 1/16" marks, and measure a clean, dry filter if you can.

  1. Outside diameter. Measure straight across the widest part of the end cap. Most spa cartridges are close to 4-5/8", 5" (often listed as 4-15/16"), 6", 7" or 8-1/2". Unicel's guide asks for accuracy to about ±1/32".
  2. Length. Measure from end cap to end cap, through the center if you can. Don't count handles, threaded fittings or other extensions. Catalogs list length to about ±1/16".
  3. Top and bottom ends. Note what each end looks like:
    • Open hole: measure the hole's inside diameter (common sizes: 2-1/8", 2-1/16", 1-15/16", 3").
    • Handle (solid top with a handle), closed (solid, no hole) or castle (notched top, common on Sundance).
    • Threaded: note the size and type, e.g. 1-1/2" MPT (male pipe thread), 1-1/2" or 2" SAE, or 2" ACME on some Bullfrog filters. Threads of the same size but a different type are not interchangeable.
  4. Your tub brand and model, and if possible the number of pleats (±2) and whether there is a center core.

Then search by size

Use the size search on the home page, or scan the full chart, which is sorted by diameter and length. If two cartridges share a size, the end fittings decide which one fits.

Why measurements differ between brands

Pleatco, Unicel and Filbur each publish their own measurements, and they can differ by 1/16"–5/8" for the same interchangeable cartridge, usually because one includes part of a fitting in the length. Filter area (sq ft) also varies by brand because of pleat count and media. That's why every filter page here shows all three makers' numbers side by side.

Method based on the measuring instructions in the Unicel 2026 Replacement Guide (p.3) and the Pleatco catalog (p.30).