Guide

How to measure a pool filter cartridge

Three measurements identify a pool filter cartridge: overall length, outer diameter, and the diameter of the openings at each end. Get those three right and you can match any cartridge without a part number.

When the part number on your cartridge has worn off, or you never had it, measuring is the reliable way to find the correct replacement. Cartridges are defined by a small set of physical dimensions, and matching them settles the identification. This guide covers exactly what to measure, how to measure it, and the mistakes that send people the wrong size.

The three measurements that matter

You need a tape measure or a ruler. A caliper is nice for the openings but not required. Measure these three, in inches, to the nearest sixteenth:

Measurement	What it is
Length	The full height of the cartridge, end cap to end cap
Outer diameter	The width across the outside of the pleats, through the center
Opening diameter	The hole in the center of each end cap, top and bottom

How to measure each one

Length. Lay the cartridge flat on a level surface. Measure from the outer face of one end cap to the outer face of the other. Include the end caps, not just the pleated fabric. The end caps are part of the length and leaving them out is a common source of a wrong reading.

Outer diameter. Measure straight across the widest point of the pleats, passing through the center of the cartridge, not across a chord off to one side. If you don't have a ruler long enough, wrap a cloth tape around the cartridge to get the circumference and divide by 3.14, but measuring straight across is simpler and less error-prone.

Opening diameter. Look at the round hole in the center of each end cap. Measure its diameter. Check both ends, because some cartridges have the same opening at top and bottom and some do not. The opening is how the cartridge seats onto the filter's manifold, so it has to match even when the length and diameter already do.

The mistakes that lead to a wrong size

Confusing outer diameter with opening diameter. This is the single most common error. Both are round, both are diameters, and it is easy to record one when you meant the other. The outer diameter is the full width of the cartridge across the pleats. The opening is the small hole in the end cap. On a typical Clean & Clear Plus cartridge, for example, the outer diameter is 7 inches and the opening is 3 inches, very different numbers. Mixing them up guarantees a wrong order.

Measuring only the pleated section for length. The end caps add height. A cartridge that is 20 1/16 inches overall might measure a full inch shorter if you only measure the fabric. Always measure cap to cap.

Measuring a worn or collapsed cartridge. An old cartridge whose pleats have spread or whose caps have deformed can read slightly off. Measure to the nearest sixteenth and, if the number falls between two standard sizes, compare against known cartridge dimensions rather than trusting a distorted part.

Rounding too aggressively. Neighboring cartridge sizes can be close. Small differences in length or diameter distinguish one size from the next, so measure precisely rather than rounding to the nearest inch.

Matching your measurements to a cartridge

Once you have the three numbers, you can match them to a known cartridge. As a reference, here are the dimensions of the Pentair Clean & Clear Plus family, one of the most common cartridge lines, so you can see how much the sizes differ:

Filter	Length	Outer diameter	Openings
Clean & Clear Plus 240	14 1/8 in	7 in	3 in
Clean & Clear Plus 320	20 1/16 in	7 in	3 in
Clean & Clear Plus 360	20 1/16 in	7 in	3 in
Clean & Clear Plus 420	26 1/16 in	7 in	3 in
Clean & Clear Plus 520	32 1/16 in	7 in	3 in

Note that the 320 and 360 share the same dimensions. They cannot be told apart by measuring alone, because they differ in media density (80 versus 90 square feet), not physical size. For those two, read the part number on the end cap or identify the filter housing. Every other size in this family is distinguished by its length.

If your measurements match one of these, the Clean & Clear Plus family page lists the cross-reference part numbers for each. If they don't match, you have a different cartridge line, and the same three measurements will identify it against that line's specs.