How to Measure a Hot Tub Filter Cartridge

The part number is the first thing to go. It’s printed on the top end cap, in ink that spends its whole life sitting in hot, chlorinated water, so by the time you actually need to reorder, it’s a smear. That’s the exact spot I was in the first time my own tub needed a filter, and it’s a big part of why this site exists.

Here’s the good news: you don’t need the part number. A filter cartridge is a simple object, and three measurements will tell you what you have. Grab a tape measure. Calipers are nicer if you happen to own a pair, but a cloth or metal tape does the job.

The three things that decide which cartridge fits

Every cartridge on this site is pinned down by three properties: outer diameter, length, and end-cap style. Get those right and you’ve gone from a few hundred possibilities down to one, or close to it.

Outer diameter

Measure straight across the widest part of the pleated body, edge to edge, through the middle. Not the cap, not the center hole. The body. Across the cartridges I’ve catalogued, diameters run from about 3⅞ inches on the little inner filters up to just past 10 inches on the big Pentair pool cartridges. Most hot tub cartridges land somewhere between 5 and 8½ inches.

If your tape lands between two marks, that’s normal. Cartridges get sized in oddly specific fractions, like 4-15/16 or 5-3/16 inches. Write down the closest reading you can and don’t agonize over the last sixteenth. When you match it against a size, anything within about an eighth of an inch is the same cartridge.

Length

Measure the pleated body from the top edge to the bottom edge. This is the one people get wrong: measure the filter media itself, not a handle or a molded knob sticking off the top. A cartridge with a handle can look a couple of inches longer than it actually filters. Lengths in the database run from under 6 inches to nearly 33, so length is usually the measurement that tells two same-diameter cartridges apart. Sta-Rite’s 50 and 70 square foot filters, for instance, use an identical can and differ only in how much media is packed inside, while Hayward’s Star-Clear line keeps one diameter and just gets taller as the model number climbs.

End caps, the part everyone forgets

This is the measurement that isn’t really a measurement, and it’s the one that catches people out. Look at both ends and note how they’re built, because a cartridge that’s the right diameter and length still won’t seal if the ends are wrong. There are only a few styles:

• Open ends. A plain hole top and bottom, usually an inch or two across, and the cartridge drops straight into the filter well. This is by far the most common. Of the 49 cartridges I track, 37 have open tops.
• A handle. A molded loop or bar on top that you grab to pull the cartridge out. Six of them have one.
• A closed top. No hole and no handle, just a solid cap. Another six.
• A threaded base. Some cartridges screw into the housing on a coarse thread, often 1½, 2, or 3 inches across. A slip-fit cartridge won’t stay put in a threaded housing, and a threaded one won’t seat in a slip-fit well, so this detail is not optional.

If you can, measure the diameter of the openings too. A 2-inch top hole versus a 2⅛-inch top hole is the difference between a cartridge that seats and one that rattles.

Turning three numbers into a part

Once you have a diameter, a length, and the cap style, you’ve got two ways to go.

If any part of the old number is still legible, the fastest path is the cross-reference. A Pleatco, Unicel, Filbur, or OEM number all point at the same physical cartridge, so one is enough to find the rest. I wrote a whole separate guide on how those cross-reference numbers work.

If the number is truly gone, use the dimensions. Browse by size and you’ll see every cartridge in the index at that diameter and length. Match the cap style against the list, confirm it against the tubs each one fits, and you have your part.

A few habits that save a reorder

Measure the cartridge, not the housing. The plastic canister the cartridge sits in is always wider and taller than the cartridge itself. Pull the old element out and measure that.

Watch the model year. A handful of tub makers quietly changed cartridge sizes partway through a model’s life. Jacuzzi’s J-400 is the one I run into most: the earlier tubs and the 2011-and-later ones use cartridges that look nearly identical and aren’t. If your tub sits right around a transition year, measure instead of assuming.

Keep the old cartridge until the new one is in your hand. It’s the best reference you have, and holding the two side by side takes about ten seconds to settle any doubt.

None of this needs special tools or any real expertise, which is the whole point. I’m a software engineer who got annoyed at filter part numbers, not a pool tech. If you can work a tape measure, you can figure out what your tub takes.

Next: Pool and spa filter cross-reference numbers, explained walks through why one cartridge ends up with five different part numbers.