Why do two spa filters the same size have different square-foot ratings?

Last reviewed November 4, 2026

The square-foot number is the total area of filter media folded into the cartridge, not its outside size. Two cartridges with the same diameter, length and end caps can hold different amounts of media because one has more pleats: in Unicel’s 2026 table, 106, 158 and 212 pleats in the same 4-15/16" × 13-5/16" body are rated 25, 37 and 50 sq ft^[5]. So size alone cannot tell you the rating; the part number, or the pleat count, can.

How the number is calculated

Unicel publishes its method for computing a cartridge’s area: count the pleats and measure the pleat height (the “amplitude”), measure the usable length of the media (excluding end caps and handles), then multiply the pleats by 2, by the pleat height, and by the usable length, and divide by 144. Its worked example is a cartridge with 75 pleats of 0.625" height and 9-1/4" of usable length: (75 × 2 × 0.625 × 9.25) ÷ 144 = 6.02 sq ft^[5]. So area depends on three things: how many pleats, how tall each pleat is, and how long the media is. The outside diameter and overall length you can measure with a tape do not by themselves fix any of the three, which is how two cartridges that look identical can carry different amounts of media.

The same-size pairs in Unicel’s spec table

Unicel’s 2026 Replacement Guide lists each cartridge’s outside diameter (OD), length, end-cap openings, pleat count, media weight and rated area^[5]. Where several cartridges share the same OD, length and openings, the pleat count is what separates them. These are the groups in the table (Unicel’s own figures, quoted as printed from the 4000, 5000 and 6000 Series pages, printed pp. 5, 6 and 8; the area method is on p. 74):

Within each group, more pleats means a larger rated area. (The table does not list pleat height, so we cannot show the full calculation for these rows.) In the first group, Unicel prints all four cartridges in one block of its table, and its guide says that cartridges listed between horizontal lines are interchangeable^[5]. Two rows also show that media weight is a separate spec: C-4326 and C-4625 both have 106 pleats and 25 sq ft, but one uses 3 oz media and the other 4 oz^[5]. Matching the square footage alone does not mean you are matching the cartridge.

What Unicel says about the “upgrade” cartridges

Unicel’s maintenance page tells dealers to check its guide for an “upgraded surface area” cartridge when selling a replacement; as its example it names C-4637 as an upgrade for C-4625, with 12 more square feet and “all other dimensions the same”^[4]. The guide marks the same relationship in its notes: C-4637 and C-4950 are listed as upgrades to the 25 sq ft C-4326 and C-4625^[5]. Two Marquis replacements are described the same way: C-5423 (“the original was 25 sq. ft. (115 pleats). This replacement has been upgraded to 35 sq. ft.”) and C-5434 (original 40 sq ft, 109 pleats; upgraded to 60 sq ft)^[5].

Unicel’s stated reason is flow per square foot: “cartridge filters can work at any flow rate. In fact, the lower the flow per square foot, the more efficient the system works”^[4]. In a spa, where the pump moves a fixed amount of water, a cartridge with more media spreads that flow over more area. That is arithmetic from Unicel’s statement, not a test result we ran.

What we could not find: a spa manufacturer statement that says a higher-area cartridge of identical outside dimensions is approved, or not approved, for a particular spa. Unicel’s “upgrade” notes apply to cartridges whose other dimensions are the same, so check the end-cap openings and length as well as the area, and ask your spa dealer if your spa is under warranty.

A real example where the sources disagree: Hot Spring Limelight (2018 and newer)

Unicel’s Hot Spring cross-reference ties both C-6350 (50 sq ft, 142 pleats) and C-6355 (75 sq ft, 229 pleats) to the Limelight cartridge, OEM 1642301-1 / 78161, and notes that the original is 6-3/8" OD while its replacements are 6"^[5]. A Hot Spring dealer’s product page for 78161 gives 50 sq ft, 14" long, about 6-1/4" OD with open 2" ends^[24]. Pleatco’s 2023 catalog lists the same OEM number as PWK35B at 6-3/8" × 14" and 35 sq ft^[2]. That is three different ratings (75, 50 and 35 sq ft) for what the sources treat as one factory part.

We cannot tell from these documents which number is “right”, and we are not going to pick one. Dealer listings are not a reliable tie-breaker either: the same dealer’s pages list 260 pleats for both the Limelight 78161 (50 sq ft) and the Caldera 73531 (75 sq ft)^[24][25], so we use pleat counts only from Unicel’s table. For a Limelight, order by the OEM number (78161 or 1642301-1) and compare the cartridge’s OD, length and end caps, rather than by the square-foot figure.

How to tell which one you have

1. Read the part number on the end cap or label. It beats every other method.
2. If it has worn off, measure the OD, the length and the size of the top and bottom openings, and note whether the top has a handle or thread. Our how to find your spa filter size guide and the size lookup do this step.
3. Count the pleats if two candidates share the same body: mark one fold with tape and count every fold around the circumference. Unicel’s own lookup instructions include counting the pleats (±2) and noting whether there is a center core^[5], so a count within two of a listed number is a match.
4. Use square footage last. Look at it only after the dimensions and end caps agree, and expect different catalogs to round it differently.

Once you know the cartridge, OEM vs Unicel vs Pleatco explains how the brands’ numbers line up.

Beachcomber prints this split on a current hot tub filter. Its 25 sq ft microfilter (SKU 3005845) and 50 sq ft microfilter (SKU 3005815) are both 4-15/16" × 13-5/16" with a 2-1/8" open core, and Beachcomber counts 107 and 212 pleats^[63][64]. Unicel lists those stock numbers as C-4326 (106 pleats) and C-4950 (212 pleats)^[5]. The one-pleat difference, and the shorter pre-1995 set, are on the Beachcomber filter by year page.

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Questions

Sources

2. Pleatco 2023 Catalog (Pentair)
4. Unicel: Maintenance & FAQ
5. Unicel 2026 Replacement Guide
24. Leisure Time Inc.: Hot Spring Limelight 78161 filter cartridge (specifications)
25. Leisure Time Inc.: Caldera 73531 filter cartridge (specifications and compatibility)
63. Beachcomber Hot Tubs: Microfilter 25 sq ft, SKU 3005845 (107 pleats, 4-15/16" × 13-5/16")
64. Beachcomber Hot Tubs: Microfilter 50 sq ft, SKU 3005815 (212 pleats, same body as the 25)

This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.