Hot Tub Filter Finder: Cross-Reference by Size

If you are holding a wet cartridge and trying to work out what to reorder, this page is the shortest route. Take four measurements, match them against the table, and buy whichever of the three cross-referenced part numbers is cheapest and in stock. They are equivalents, not upgrades.

The four measurements, and where to take them

1. Outer diameter. Across the widest point of the end cap, not across the pleats, which compress. Measure the plastic.
2. Length of the media body. Cap face to cap face, excluding any handle and excluding a threaded stub. This is the measurement people get wrong, and including a handle sends you looking for a cartridge one size up that does not exist.
3. The bottom fitting. An open hole, or a thread? If it is a hole, measure its internal diameter. If it is a thread, note that it is threaded and roughly how wide.
4. The top. Open hole, molded handle, or closed with a pull loop? These are three different families and they do not interchange even at identical dimensions.

Then check the cartridge itself for printing. Most carry the part number molded or printed on an end cap, and if yours is legible you can skip the measuring entirely and go straight to the cross-reference.

Common spa cartridges, cross-referenced

Every figure in this table was read from the published listing linked in the sources below. Where we could not verify a dimension against a published listing, the row is not in the table — we would rather the table were short and right than long and plausible.

The first two rows are worth reading together. They are dimensionally identical — same diameter, same length, same openings — and differ only in pleat count and therefore filtration area. They physically interchange, but they are not functionally equivalent, and the published guidance is to fit whichever version your spa originally shipped with.

The fittings matter as much as the size

This is the part that catches people. A cartridge can match on diameter and length and still be the wrong part, because yours threads into the skimmer and the replacement drops in with an open hole. Or because yours has a closed top with a pull loop and the replacement has an open end.

A cartridge that is nearly right is worse than one that obviously does not fit, because it will sit in the housing looking correct while water bypasses the media entirely. You get weak filtration, cloudy water and no visible clue that anything is wrong.

If nothing on this table matches

There are hundreds of spa cartridges and five rows here. If yours is not one of them, the reliable routes are, in order:

1. Read the number off the cartridge. Most carry one. Search it directly and the cross-references come with it.
2. Check the spa manual. Manufacturers list the OEM part number, which cross-references to all three majors.
3. Use a manufacturer’s own finder. Pleatco, Unicel and Filbur each publish cross-reference tools. They only return their own parts, but a part number from any one of them is enough to find the other two.
4. Photograph the end caps and measure carefully. A specialist retailer can usually identify a cartridge from a photo plus three numbers.

Where we would send you elsewhere

If your tub takes a proprietary format that only its manufacturer makes — the ceramic sets fitted to some Hot Spring and Caldera models, for example — the aftermarket does not have an equivalent, and buying from the manufacturer or a spa dealer is the right answer even though it is not the answer that earns us anything.

Once you have your number

Choose between the three majors on price and availability, using the cartridge roundup, and buy two rather than one so you can run a rotation. Then read how to clean a hot tub filter before the first one gets dirty, because how you clean it decides whether it lasts six months or a year. If you are here because the old cartridge has stopped working rather than because you want a spare, when to replace a filter is the page that says whether it is genuinely finished.