How to Measure a Pool Filter Cartridge (But First: the Number on the Cap Beats Your Tape Measure)
Measure three numbers if you must: outside diameter × total height (end-cap to end-cap, including handles) × top and bottom opening diameters. But measurement is the fallback — the OEM part number printed on the end-cap is the primary key, because two cartridges can share every dimension and still differ in filtration square footage, which is a different filter. Note the sq-ft rating along with the numbers: same body, different cloth area, different performance.
Every pool-cartridge measuring guide on the internet puts the tape measure first, because measuring makes a better step-by-step. The field reality is reversed: the cap number resolves in thirty seconds what measurements resolve ambiguously. Unicel prints its numbers on the top cap (e.g., C-9419 on a Pentair Clean & Clear 200 cartridge); Pleatco’s are on the cap or wrapper; Filbur runs FC- series numbers. When the number exists, use it — the tape measure is for cartridges whose caps have gone smooth with age.
Quick Answer: The Measurement Set
| Measurement | How to take it | Typical trap |
|---|---|---|
| Outside diameter | Across the widest point of the cartridge body, outside edge to outside edge | Measuring the core (inner tube) instead of the outer pleat diameter |
| Total height | End-cap to end-cap, including handles/slings if present | Excluding a molded handle — which can add 2″+ to the true height |
| Top opening diameter | Inside the top cap hole | Confusing top and bottom when they differ (very common) |
| Bottom opening diameter | Inside the bottom cap hole | Same — measure both, label which is which |
| Square footage | Printed on cap or spec (e.g., “200 sq ft”) | Ordering same-size/lower-sqft and throttling the filter |
ClearFit manufactures an OEM-equivalent C 9419 replacement cartridge with the same filter-media square footage.
Order of Operations: Number, Brand, Tape
Step 1 — Find the part number. Inspect both end-caps for molded-in or printed numbers: Unicel’s C-xxxx, Pleatco’s PA/PAP/PAE/PXST-style codes, Filbur’s FC-xxxx, Hayward OEM CX-xxxxRE, Pentair OEM R17xxxx. Often more than one appears (aftermarket cartridges frequently print the numbers they replace). Any one of them resolves the cartridge — enter it in the cross-reference tool and you’re done measuring.
Step 2 — Identify the filter brand & model. No surviving number? Read the filter tank itself: the label on the tank body names the system (Pentair Clean & Clear 200, Hayward Star-Clear C900, SwimClear, XStream…). The system maps to its cartridge list — e.g., a Star-Clear C900 takes the cartridge cross-referenced as Unicel C-8409 / Pleatco PA90 / Filbur FC-1292 / Hayward CX900RE (90 sq ft, 8-15/16″ × 17-3/8″, 174 pleats). The system→cartridge path is the cross-reference guide.
Step 3 — Tape-measure fallback. Only when both the cap and the tank label are gone. Then the measurement set above, plus the sq-ft question below.
Why this order beats measure-first: pool cartridges cluster dimensionally. Dozens of cartridges are 7″ × 19-5/8″; the cap number is what separates them. Measuring first turns a lookup into a guessing game — the “looks right, doesn’t fit” return-shipping story that pool stores hear every Saturday.
The Square-Footage Trap (Same Size, Different Filter)
The measurement that most guides omit: filtration square footage. Two cartridges can share diameter and height while carrying meaningfully different media areas — 100 vs 125 sq ft at the same 8-15/16″ × 23-5/16″ body happens in real catalogs (e.g., the Hayward Star-Clear Plus family cross-references C-1200/PA120/FC-1293 at 125 sq ft). What sq ft changes:
- Flow capacity: more cloth = more water filtered per pass at the same pump pressure. An undersized-sqft cartridge raises pressure and shortens cleaning intervals even though it “fits.”
- System rating match: your filter tank is rated for a specific sq ft (it’s often in the tank’s model name — Clean & Clear 200 = 200 sq ft, four-pack). Dropping sq ft below rating silently downgrades the filter.
- The rule: when two replacements match your dimensions, take the one matching your tank’s rated square footage — never the cheaper smaller one.
The manufacturers’ engineering frame, worth knowing even if you never use it: cartridge sq ft is designed against pump flow (GPM) — roughly 3–5 sq ft of cloth per GPM of pump output in typical cartridge-filter design practice. A 100 sq ft cartridge is happy around a 25–30 GPM system. This is why sq ft, not body size, is the real filtration spec. inference from industry design convention
Three Traps in the Tape-Measure Fallback
1. Handles and slings count. A cartridge with a molded top handle measures end-cap-to-handle-top for total height — the handle occupies the space a plain-cap cartridge’s clearance would. Order the handled height when the original had a handle (Jacuzzi spa cartridges like the 5CH-402 / PJW40SC-F2M / FC-2811 set are classic handled-and-threaded examples: 5-3/16″ × 10-3/4″, bar-handle top, male-thread bottom).
2. Top and bottom openings are often different. Measure both and record which is which — one common pattern: open gasket seat on top, plain open on bottom. An upside-down mental model orders the wrong cartridge.
3. Skimmers vs filter cartridges. Skimmer baskets and filter cartridges get conflated in search results and in kitchen-table measurements. If what you’re holding is a basket with a handle and no pleats, that’s a skimmer replacement — different aisle.
From Measurement to Order
Enter the cap number — or the tank brand and model, or the three measurements — into the cross-reference tool for exact-fit matches across Unicel, Pleatco, and Filbur plus the OEM references. The three-number-systems explainer is pool filter cross-reference (with real worked examples); the brand-comparison angle (which of the three to buy) is Unicel vs Pleatco vs Filbur. Cartridge life and the spring-replacement timing: how often to replace — and every cartridge you log lands on one dashboard in the Home Filter Calendar, pool season and all.
FAQ
What size pool filter cartridge do I need?
The one matching your cartridge’s end-cap part number first (Unicel C-xxxx, Pleatco PA/PAP-series, Filbur FC-xxxx, or OEM CX/R numbers); the filter tank’s brand and model second; and only as a fallback, the measurements: outside diameter × total height including handles × top and bottom opening diameters, plus the square-footage rating.
How do you measure a cartridge if the numbers are worn off?
Outside diameter at the widest point, total height end-cap to end-cap (handles included), and both opening diameters — labeled top vs bottom. Then also note the tank’s brand/model from its label, because dimensionally identical cartridges differ in square footage, and the tank model is what disambiguates them.
Does square footage matter if the size matches?
Yes — two cartridges with identical dimensions can differ 20%+ in media area. Lower sq ft raises pressure, shortens cleaning intervals, and downgrades the tank’s rated capacity. When dimensions tie, match the tank’s rated square footage (often in the model name, e.g., Clean & Clear 200 = 200 sq ft).
Are pool cartridge openings always the same top and bottom?
No — differing top/bottom openings are common (gasket-seat on one end, plain open on the other; threads or handles on spa cartridges). Measure both and mark which is which before ordering.
Can I upgrade to a higher square footage cartridge?
Within the same tank family, higher-sqft cartridges sometimes exist as upgrades (e.g., Star-Clear to Star-Clear Plus configurations), but the tank must be designed for it — more cloth in the same shell needs the right internal clearance. Verify by tank model cross-reference, not by body dimensions alone.
Sources & References
- Cartridge manufacturer catalogs (Unicel, Pleatco, Filbur, Hayward OEM) — measurement specs, sq-ft ratings, part-number placements. Official documentation (explicit).
- Cross-reference dataset — the equivalences cited (C-8409 set; C-1200/PA120/FC-1293 125-sqft example; 5CH-402 spa set). Cross-reference database data (explicit).
- Cartridge filter design practice — sq-ft-per-GPM sizing conventions. inference (industry design convention).
- r/pools measurement threads — worn-cap fallbacks, handle-height confusion, skimmer conflation. Community discussion (paraphrased).
Related Guides
- Pool Filter Cross-Reference — the numbers, decoded.
- Unicel vs Pleatco vs Filbur — picking among equivalents.
- How Often to Replace Pool Filter Cartridge — timing the repurchase.
- How to Clean a Pool Filter Cartridge — maintaining what you measured.
Shop this guide: Pool & spa filter cartridges