Watkins 31250 Filter Cartridge
Watkins OEM number • 35 sq ft • 6″ OD × 9″ long
31250 is Watkins's number for this cartridge, not a part you have to buy from Watkins. The filter itself is a 35 square foot cartridge measuring 6 inches across and 9 inches long, and Pleatco, Unicel, and Filbur all build it. If you read 31250 off an old cartridge or found it in your manual, any of the numbers below drops into the same housing.
Fitment notes
1-15/16 inch top and bottom openings. Watkins and Unicel rate this cartridge at 33 sq ft while Pleatco rates the same part at 35 sq ft. Unicel has discontinued the C-6433, so the Pleatco and Filbur numbers are the ones still in production. Not the same cartridge as the PWK35B, which is 14 inches long and fits Limelight models. Replaces Watkins OEM 31250.
What to buy instead
Replacements for the Watkins 31250
Each button searches Amazon for one of the numbers this cartridge is sold under. The OEM version is usually the most expensive of the set and the aftermarket versions are the same physical filter, so it is worth checking all of them before you order.
Every number for this filter
| Brand | Part Number | Type |
|---|---|---|
| Watkins | 31250 | The number you searched |
| Pleatco | PWK35 | Aftermarket equivalent |
| Unicel | C-6433 | Aftermarket equivalent |
| Filbur | FC-3920 | Aftermarket equivalent |
Full specs, the cartridge diagram, and the complete compatibility list live on the PWK35 cartridge page. You can also see every 6″ × 9″ cartridge if you are checking a measurement rather than a part number.
Frequently Asked Questions
What filter replaces the Watkins 31250?
The PWK35 replaces it directly, and so does every cross-reference for it: C-6433, FC-3920. All of them are the same 35 sq ft cartridge at 6" outer diameter and 9" long. Buy whichever is cheapest at the time with solid recent reviews.
Is the Watkins 31250 the same as the PWK35?
Yes, physically the same filter. Watkins 31250 is an OEM number, meaning it is the code Watkins put on the cartridge that shipped with the equipment. Watkins does not manufacture cartridges. Pleatco, Unicel, and Filbur do, and they sell the identical part under their own numbers at a lower price. Match the dimensions against your old cartridge before ordering and you are covered.
Do I have to buy the Watkins version?
No. Nothing about the housing requires an OEM-branded cartridge, and the warranty on your equipment is not voided by using an aftermarket filter that meets the same spec. The reason to buy OEM is convenience, not performance. The reason not to is price, which is often a meaningful gap for an identical part. What matters is that outer diameter, length, and end-cap style all match.
What size is the Watkins 31250?
6 inches in outer diameter, 9 inches long, with 35 square feet of filter media. Both measurements need to land within about an eighth of an inch, and the end caps have to match so the cartridge seats correctly when you close the lid. Measure your old one rather than trusting a number that has worn off the label.
Last verified August 2026