How Filbur, Unicel and Pleatco numbers compare on the same spa filter

Last reviewed October 27, 2026

They are four names for a match, not four shapes. The spa maker’s OEM number is the one molded for that brand. Unicel prints a C-number or a CH-number. Pleatco prints a P-number and a cross-reference to the Unicel number. Filbur prints an FC-number in its own catalog^[5][2][31]. When the size and the ends agree, the numbers are equivalents. When a catalog prints a different length or a different area, the page on this site says so instead of hiding the conflict.

Four columns, one cartridge

Retail sites often list a single part number and a photo. The catalogs list the same physical cartridge once for each company that sells it. Our cartridge pages put those numbers in one place so you can search whichever one is still readable. The columns are:

• OEM. The spa maker’s number. Hot Spring 31489, Caldera 74148, Sundance 6540-488, Softub 90-802, Bullfrog 10-2870. Some cartridges accumulated several OEM numbers over the years. C-6430’s row prints 31489, 71825 and 71826^[5].
• Unicel. Usually C- followed by four digits (C-6430, C-4950) or a size-series CH number (4CH-35, 6CH-940, 8CH-951). The 2026 Replacement Guide is both a cross-reference, by spa brand, and a spec table of diameter, length, ends, pleats and area^[5].
• Pleatco. A P-number such as PWK30-4, PRB35-IN, PCD50N or PSD125-2000. The 2023 catalog includes a Unicel-to-Pleatco cross-reference and its own spec tables^[2].
• Filbur. An FC-number such as FC-3915 or FC-2780, in the Filbur Catalog and Reference Guide, which also sorts parts by spa manufacturer^[31].

A worked example that agrees: Hot Spring OEM 31489 / 71825 / 71826, Unicel C-6430, Pleatco PWK30-4, Filbur FC-3915. All three catalogs are cited on the C-6430 page for a 6" × 10-1/2" cartridge open 1-15/16" at both ends, 30 sq ft. Another that agrees: Caldera OEM 74148, Unicel C-4335, Pleatco PRB35-IN, a 4-15/16" × 9-1/4" cartridge with 2-1/8" open ends. Searching any one of those strings from the homepage should land on the same family of pages. OEM vs Unicel vs Pleatco is the shorter version of this idea. This page adds Filbur and the rows where the books diverge.

Where the catalogs agree on size and disagree on a detail

Agreement on the body is not agreement on every cell.

• Softub 90-802. Unicel 6CH-25, Pleatco PTL20W-SV-P4 and Filbur FC-0305 all print 6" × 5-1/2" with a handle and a 1-1/2" MPT bottom. Unicel and Filbur rate it at 25 sq ft. Pleatco rates the same OEM number at 20 sq ft^[5][2][31]. The Softub page keeps the disagreement visible. Size, OEM number and fitting are the match. Area is not a single number.
• Intex S1. Unicel C-4317 and Pleatco PIN4PAIR agree on 4-1/4" × 2-15/16". They do not agree on the end opening (2-1/16" versus 1-5/8")^[5][2]. Details are in Intex and Bestway filters.
• Master Spas 6CH-352. Unicel prints 6" × 8". Filbur’s cross-reference to FC-0312 prints 6" × 7"^[5][31]. Measure. The Eco-Pur context is in Master Spas Eco-Pur filters.
• Hot Spring 78161 / 1642301-1. Unicel offers C-6350 at 50 sq ft and C-6355 at 75 sq ft, both 6" × 14". Pleatco lists PWK35B at 35 sq ft and 6-3/8" × 14". A dealer page lists 50 sq ft and a slightly different OD again^[5][2][24]. We do not collapse those into one rating. See Hot Spring sizes by collection.

Unicel’s area formula, pleat count times two, times pleat height, times usable length, divided by 144, is why two measurements of “the same” cartridge can still print different square footage when the pleat count differs^[5]. Why same-size filters differ walks through the groups in the spec table, including Unicel’s own upgrade notes such as C-4637 for a 25 sq ft cartridge of the same outside size.

When a column is blank on purpose

A blank is not a zero. It means that catalog did not give us a number we are willing to print.

• C-6350 has no Pleatco equivalent on the page, because PWK35B’s printed size and area do not match that Unicel row.
• Intex 29012E and Bestway Type VI have no Unicel, Pleatco or Filbur numbers. They are not in those books. The manufacturer pages identify them.
• Bullfrog PBF17 and PBF40 are Pleatco coreless parts with no Unicel number in the 2026 guide. Inventing a C-number would make the search box lie.
• Some rows are marked unverified and kept out of the search index when we have only one source or the sources conflict on the OEM number. The dataset still lists them, with the citation, so you can see the gap. The methodology is the rule we use.

What the spa maker says about “equivalent”

A cross-reference is the filter maker’s statement that its cartridge matches the OEM part’s size and fittings. It is not the spa maker’s statement that the aftermarket part is the one they want in the spa. Hot Spring’s maintenance page says to replace with genuine Watkins filters^[7]. Jacuzzi recommends filters designed for the specific model^[10]. Check the warranty before you switch. We are not Watkins, Jacuzzi, Unicel, Pleatco or Filbur, and the pages do not say an aftermarket cartridge is “genuine.” They say which numbers the catalogs tie together.

Filbur’s catalog is doing the same kind of work as the other two. On Marquis cartridges it lists the brand as Pacific Marquis, which is why a search for a Marquis OEM number can land on an FC-number you have not seen on the spa. The Marquis rows on this site cite that naming. If Filbur’s length or area differs from Unicel’s, the cartridge page carries the note, the way the Master Spas 6CH-352 page does.

How to use the numbers you can still read

1. Search the string on the cap, whatever prefix it has. C-, FC-, P-, PRB, PCD, PWK, 6540- and a plain OEM code all go through the same box.
2. Read all four columns on the page you land on. If one is blank, believe the blank.
3. Read the size line and the top and bottom. Equivalence means those match, not merely that two brands sell “a filter for this spa.”
4. If two pages share an OEM number and not an area or a plug or a bottom hole, open both. Sundance C-8326 and C-8327, and Jacuzzi C-4326 and C-4950, are the cases in look-alike Sundance and Jacuzzi filters.
5. Then use the retailer buttons on that cartridge page if you want to shop. A search for a Unicel number and a search for the OEM number should be the same physical cartridge only when our page says they are equivalents.

The whole table, including a source for every row, is the downloadable dataset. If a number on your cartridge is missing, the correction address is on the corrections policy.

Questions

Sources

2. Pleatco 2023 Catalog (Pentair)
4. Unicel: Maintenance & FAQ
5. Unicel 2026 Replacement Guide
7. Hot Spring Spas: Maintenance Tips
10. Jacuzzi: Hot Tub Filter Cleaning Made Easy
24. Leisure Time Inc.: Hot Spring Limelight 78161 filter cartridge (specifications)
31. Filbur Catalog & Reference Guide

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