How to find the right replacement hot tub filter

The fastest way to find a replacement hot tub filter is the part number printed or molded on one of its end caps. Search that number, or cross-reference it between Pleatco, Unicel, and Filbur. If the number is gone, measure three things: the length between the end caps, the outside diameter, and the size and type of each end (open hole, closed, or threaded). A filter that matches those will fit.

Method 1: Use the part number

1. Turn off power at the breaker and remove the filter.
2. Rinse the end caps and look for a number molded into the plastic or printed in ink. Check both ends.
3. Note the prefix. It tells you the maker.

Brand	What the numbers look like	Example
Pleatco	Start with P, then letters and digits	PRB25-IN
Unicel	Often C- followed by four digits, or digits then CH	C-4326
Filbur	FC- followed by four digits	FC-2375
Tub maker (OEM)	Varies by brand	Printed in the owner's manual

Cross-reference examples

The same filter is sold under all three aftermarket numbers. A few of the most common:

Pleatco	Unicel	Filbur	Description
PRB25-IN	C-4326	FC-2375	25 sq ft, about 5 inch diameter, open both ends
PRB50-IN	C-4950	FC-2390	50 sq ft, about 5 inch diameter, open both ends
PWW50-P3	6CH-940	FC-0359	50 sq ft, about 6 inch diameter, handle top, threaded bottom
PWK30	C-6430	FC-3915	30 sq ft, about 6 inch diameter, threaded bottom

Always confirm the dimensions on the listing against your old filter before ordering. Tub makers sometimes change filters mid-year, and a previous owner may have fitted something that was close but not right.

Method 2: Measure it

Use a tape measure or ruler. Measure to the nearest 1/16 inch.

1. Length. Measure end cap to end cap, the flat surfaces only. Do not include handles, threads, or anything that sticks out past the cap.
2. Outside diameter. Measure across the end cap at its widest. Measure the cap, not the pleats, which bulge with age.
3. Top end. Note what it is: open hole, closed, handle, or threaded. If it is a hole, measure the inside diameter.
4. Bottom end. Same again. If it is threaded, measure the thread's outside diameter and note whether the threads are coarse or fine.
5. Pleat count and square footage, if you want to match capacity. Count the folds around the outside.

End cap types

Type	What it looks like	What to record
Open	Plain round hole in the center	Inside diameter of the hole
Closed	Solid cap, no hole	Just note "closed"
Handle	Molded grip, cone, or loop on a closed cap	Handle style
Male threaded (MPT)	Threaded stub sticking out	Thread outside diameter. 1.5 inch and 2 inch are most common. Coarse or fine.
Female threaded	Threads inside the hole	Thread inside diameter
Slotted or castle	Notched ring that locks with a twist	Number and width of slots

Threads are where most wrong orders happen. A 1.5 inch coarse thread and a 1.5 inch fine thread look alike on a screen and will not interchange. Compare your old filter's thread side by side with the listing photo, and count threads per inch if in doubt.

Method 3: Use the tub's make, model, and year

The owner's manual lists the filter part number. Many filter listings also name the tub brands and model years they fit. This is a good cross-check, but measure anyway, because tubs get modified and model names get reused.

Write it down once

Record these and keep them with your tub notes:

• Part number and brand
• Length
• Diameter
• Top end type and size
• Bottom end type and size
• How many filters the tub uses

With that you can find a match in hot tub filters in a minute, every time.

What square footage means

The number in many part codes, such as the 25 in PRB25 or the 50 in PRB50, is the square feet of filter fabric. More fabric means more dirt-holding capacity. If the physical dimensions match, a little more square footage is fine. Much less means more frequent cleaning and more risk of flow errors.

Does the brand matter?

The three major aftermarket brands all make solid cartridges. Differences are in fabric weight, core strength, and end cap material. Very cheap unbranded filters often have fewer pleats and thin fabric that collapses, which leads to low flow and codes such as FLO. Compare pleat count and square footage, not just price.

Special filter types

• Disposable microfiber or depth filters: replaced every few months, not chemically cleaned.
• Ceramic filters: long-life cartridges used by some brands, with their own cleaning method.
• Two-part or nested filters: an inner and outer cartridge. Measure and order each.
• Circulation pump filters: some tubs have a separate small filter on the circ pump line.

If yours is one of these, go by the tub maker's part number.

When you install the new one

1. Turn off power at the breaker.
2. Rinse the new filter to remove loose fibers.
3. Seat or thread it fully so no water can bypass it.
4. Restore power and check for good flow.
5. Note the install date.

Keep the old one as a spare if it is still in fair shape. Rotating two filters extends the life of both. Cleaning routine is in how to clean a hot tub filter, and the full calendar is in the maintenance schedule.