Measure a pool filter cartridge

A wet cartridge is hard to compare by eye. Start with the least ambiguous evidence—the printed part number—then measure only if the number is missing or unreadable.

Before removing or handling the cartridge, follow the filter maker’s directions for shutting the system down and releasing pressure.

Decision 1: can you read a part number?

Check the top and bottom end caps. A maker may also mark the core or another molded surface. Wipe away loose debris and turn the cartridge under good light. Repeated wetting, cleaning and handling may leave a used cartridge’s mark faint, partly obscured or worn.

If you find a number:

1. Copy every letter, number, dash and suffix.
2. Enter it in the part-number lookup.
3. Compare the result’s dimensions and end caps with the cartridge in your hands. A cross-reference is evidence to check, not permission to ignore a visible mismatch.

If you cannot read a number, continue with measurements.

Decision 2: measure diameter, length and end caps

Use a rigid ruler or tape marked in small fractions of an inch. Record the measurement you see rather than rounding to the nearest whole or half inch.

Outside diameter

Measure the outside diameter across the widest part of the cartridge. Measure through the center, from outside edge to outside edge.

The Unicel replacement method calls for outside diameter in inches, ±1/32 inch.

Do not wrap the tape around the cartridge and record its circumference. The lookup needs the straight distance across the center.

Cartridge length

Measure from the outer face of one end cap to the outer face of the other. Include the end caps. Exclude handles and other extensions.

The Unicel method calls for length in inches, ±1/16 inch, and recommends measuring through the center, when possible, instead of along the outside edge. That distinction matters when an end cap is domed, stepped or fitted with a handle.

Top and bottom end caps

Record the configuration at each end separately. Note whether it is open, closed, handled, threaded, slotted, gasketed or fitted with an adapter. The end-cap guide shows the terms used in this site’s data.

If either end is open, measure its inside diameter. The Unicel method calls for an open-end inside diameter in inches, ±1/32 inch.

Also record the OEM name and filter model, if you can find them. Those details can separate cartridges that look alike.

Fraction-to-decimal quick table

The size finder accepts fractions, but a decimal tape or caliper may be easier to read. These are exact sixteenth-inch conversions:

For a mixed number, keep the whole inches and convert only the fraction. For example, the dataset value 4-15/16" is 4.9375 inches.

Decision 3: count the pleats and check the core

Choose a starting pleat, mark it lightly if needed, and count once around the cartridge. The Unicel method allows a pleat count of ±2.

Look down the center and record whether a center core is present. Pleat count and core presence are confirmation details; they do not replace diameter, length and end-cap measurements.

Decision 4: make the match

Enter the outside diameter and cartridge length in the size finder. Use end-cap configuration, open-end inside diameter, pleat count, center core, OEM name and model to narrow the result.

Before ordering, compare all of these:

• Printed part number or cross-reference, when available
• Outside diameter
• Length including end caps but excluding handles and extensions
• Top and bottom end-cap configuration
• Inside diameter of every open end
• Pleat count
• Center core presence
• OEM name and filter model

If a result carries a conflict flag, open its source values. Do not resolve a catalog disagreement by choosing the value closest to your measurement; confirm the housing model, OEM part number and physical end connections first.