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Quick answer from a licensed plumber: Measure two things: length and diameter. On Pentair’s spec sheets a “10-inch” cartridge is really 9.75 to 9.88 inches long and a “20-inch” is 20 inches. Diameter splits them into the slim class (about 2.5 inches, for Standard and Slim Line housings) and the 4.5-inch Big Blue class. Length has to match the housing’s sump and diameter has to match the housing family.
Standard 10″ x 2.5″ housing? A 5-micron sediment cartridge like the Pentek P5 is the common refill. See the Pentek P5 on Amazon
Filter Cartridge Sizes at a Glance
Two numbers decide whether a replacement cartridge fits: its length (the “10-inch” or “20-inch”) and its diameter (the slim “2.5-inch” class or the 4.5-inch Big Blue class). Here is how Pentair’s own spec sheets size the four common combinations:
| What it is called | Actual size on Pentair’s sheets | Example Pentek cartridges | Housing it goes in |
|---|---|---|---|
| 10″ standard (“10 x 2.5”) | 2.38″ x 9.88″ (P5) · 2.63″ x 9.75″ (R30) · 2.88″ x 9.75″ (RFC-93) | P1, P5, P25, R30, R50, RFC-93 | 10″ Standard (3/4″ ports) or 10″ Slim Line (1/4″–1/2″ ports) |
| 20″ standard (“20 x 2.5”) | 2.38″ x 20″ (P5-20) · 2.63″ x 20″ (R30-20) · 2.88″ x 20″ (RFC-20) | P5-20, R30-20, RFC-20 | 20″ Standard or 20″ Slim Line |
| 10″ Big Blue (“10 x 4.5”) | 4.50″ x 9.75″ (R30-BB, RFC-BB) · 4.50″ x 9.88″ (DGD-5005) | R30-BB, R50-BB, DGD-5005, RFC-BB | 10″ Big Blue |
| 20″ Big Blue (“20 x 4.5”) | 4.50″ x 20″ | R30-20BB, DGD-5005-20, RFC20-BB | 20″ Big Blue |
Two things jump out of that table. A “10-inch” cartridge is really 9.75 to 9.88 inches long on Pentair’s sheets, and the “2.5-inch” class is a family name, not one exact size: Pentair says its Standard housings “will accommodate a wide range of 2-1/2″ to 2-7/8″ diameter cartridges,” while its P5 measures 2.38 inches across. Big Blue housings, per Pentair, accept “up to 4-1/2″ diameter cartridges.”
Step 1: Measure the Old Cartridge
- Length, end cap to end cap. Just under 10 inches is a 10-inch cartridge; about 20 inches is a 20-inch cartridge. Pentair also makes 4.88-inch P and R Series cartridges for short 5-inch housings, so a cartridge around 5 inches long is its own size.
- Outside diameter. Under 3 inches puts it in the standard/slim class. About 4.5 inches is Big Blue. There is nothing in between on these sheets, so this measurement settles it.
- The printed model number. Pentek cartridges carry model numbers like P5, R30-20 or DGD-5005-20. On Pentair’s naming, “-20” means 20-inch and “BB” means Big Blue.
Step 2: Or Measure the Housing
If the old cartridge is gone or crushed, the housing tells you the same thing. These are the overall housing dimensions Pentair prints (height x width):
| Housing | Size on Pentair’s sheet | What it takes |
|---|---|---|
| Slim Line #10 (3/8″ or 1/2″) | 11.75″–12.13″ tall x 4.38″–4.63″ wide | 10″ standard-class cartridge |
| Standard #10 (3/4″) | 12.25″–12.63″ tall x 5.13″–5.25″ wide | 10″ standard-class cartridge |
| Standard #20 (3/4″) | 22.38″ tall x 5.13″ wide | 20″ standard-class cartridge |
| Big Blue #10 (3/4″, 1″ or 1-1/2″) | 12.75″–13.45″ tall x 7.38″–7.45″ wide | 10″ x 4.5″ cartridge |
| Big Blue #20 (3/4″, 1″ or 1-1/2″) | 23.04″–23.74″ tall x 7.38″–7.45″ wide | 20″ x 4.5″ cartridge |
A housing about 7.4 inches across is a Big Blue. One about 5 inches across is a Standard or Slim Line, and its height tells you 10 or 20.
What a 20-Inch Cartridge Buys You
Same diameter, twice the length, so more media and a lower pressure drop at the same flow. Pentair’s own numbers show it:
- Pressure drop. Pentair’s P Series sheet says 20-inch cartridge pressure drop “can be estimated by dividing the psi above by 2” compared with the 10-inch figure at the same flow.
- Media area. Pentair says the standard 9.75-inch R Series cartridge has “more than four square feet” of polyester fabric and the Big Blue version “more than 16 square feet.” That jump comes with the wider Big Blue diameter, so it needs a Big Blue housing.
- Carbon capacity. For chlorine taste and odor, Pentair rates the RFC-93 (2.88″ x 9.75″) at more than 5,000 gallons at 1 gpm, the RFC-20 at more than 10,000 gallons at 2 gpm, the RFC-BB at more than 50,000 gallons at 2 gpm and the RFC20-BB at more than 100,000 gallons at 4 gpm, based on its internal testing.
That is the practical case for going bigger on a whole-house line: fewer changes and less pressure loss. It only works if the housing is already that size. Changing the housing means cutting the line, and that is a different job from swapping a cartridge.
I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.
Length Is Not the Only Thing That Has to Match
- End style. Standard residential cartridges are open at both ends; Pentair’s Slim Line sheet says those housings are “compatible with DOE (double open end) filters.” Pentair’s commercial catalog also lists fin, flat and 222/226 end caps, which are for different housings. If the old cartridge has a fin or a plug on one end, match that too.
- Media for your water. The size can be right and the media wrong: Pentair’s American Plumber sheet says its pleated cellulose W20CLA is “designed for municipal/chlorinated water (not recommended for well water applications).” Micron rating is covered in our sediment filter micron guide.
- Flow for your house. The same cartridge is rated for different flows in different housings. Pentair’s American Plumber chart rates its W5P at 5 gpm in a 3/8″ Slim Line housing and 10 gpm in a 3/4″ Standard housing, because “flow rates are calculated for cartridges according to the housing in which they are being used.”
What to Buy, by Size
All Pentair Pentek, all shipped and sold by Amazon when we checked, sized by the table above:
- 10″ standard, sediment: Pentek P5 (10″ x 2.5″, 5 micron). Its listing says it is a drop-in for any standard 10″ x 2.5″ housing.
- 10″ standard, coarse or reusable: Pentek R30 (10″ x 2.5″, 30 micron pleated).
- 20″ standard, carbon: Pentek RFC-20 (2.88″ x 20″ radial flow carbon).
- 10″ Big Blue: Pentek DGD-5005 (4.5″ x 10″, 5 micron) for sediment, or Pentek RFC-BB (4.5″ x 10″ carbon) for chlorine taste and odor.
- 20″ Big Blue: Pentek DGD-5005-20 (4.5″ x 20″, 5 micron) or Pentek R30-20BB (4.5″ x 20″, 30 micron pleated).
Parts You’ll Also Need
Worth having before you open the housing:
- A spare sump O-ring for a Standard housing: Pentek 151120 #241 O-ring, which Pentair lists for its Standard housings. A flattened or nicked O-ring can make a housing weep after a cartridge change.
- Food-grade silicone grease: Captain O-Ring NSF food-grade silicone grease. Pentair’s Slim Line housing sheet says to lubricate the O-ring with food-grade silicone before the sump goes back on, and that is good practice on any housing.
When to Call a Licensed Plumber
- You want to change housing size or add a housing. That means cutting into the water line, and a whole-house filter should have shutoff or bypass valves around it.
- The housing cap is cracked or leaks at the pipe threads even with a new O-ring.
- The shutoff valves on either side of the filter will not close.
FAQ
Is a 10-inch water filter really 10 inches?
No. Pentair lists its 10-inch cartridges at 9.75 inches (R30, RFC-93) or 9.88 inches (P5, DGD-5005) long. Its 20-inch cartridges are listed at 20 inches.
What is the difference between a 2.5-inch and a 4.5-inch filter?
Diameter. The 2.5-inch class fits Standard and Slim Line housings; Pentair says its Standard housings take 2-1/2 to 2-7/8 inch cartridges. The 4.5-inch class is Big Blue, and Pentair says Big Blue housings accept cartridges up to 4-1/2 inches across.
Can I use a 10-inch cartridge in a 20-inch housing?
Not on its own. A cartridge has to seat at both ends of the sump, and one that is too short leaves a gap where water can go around it. Pentair does list cartridge couplers as housing accessories (155003 for Standard cartridges, SH144229 for 4-1/2-inch cartridges), but the simple fix is to buy the length the housing was built for.
Is a 20-inch filter better than a 10-inch?
It lasts longer and restricts less at the same flow. Pentair says 20-inch P Series pressure drop can be estimated by dividing the 10-inch figure by 2, and it rates its 20-inch RFC carbon cartridges for more gallons than the 10-inch versions. It only fits a 20-inch housing, though.
How do I tell a Big Blue housing from a standard one?
Measure the sump. Pentair lists Big Blue housings at about 7.4 inches wide and Standard housings at about 5.1 to 5.3 inches. The height then tells you 10 or 20 inch.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia journeyman plumber. Every part number, size and fit on this page comes from the manufacturer pages and documents listed below, which we opened and read; product details come from those makers and from the Amazon listings, not from hands-on testing. Each linked product was shipped or sold by Amazon, new and in stock, when we checked on September 24, 2026. Nobody pays to be included here. Where a link goes to Amazon we may earn a commission at no extra cost to you. More on who writes this and how we pick.
Sources
- Pentair: Pentek P Series cartridge spec sheet (310060)
- Pentair: Pentek R Series cartridge spec sheet (310066)
- Pentair: Pentek DGD Series cartridge spec sheet (310061)
- Pentair: Pentek RFC Series cartridge spec sheet (310075)
- Pentair: Pentek Traditional Standard housing spec sheet (310051)
- Pentair: Pentek Traditional Slim Line housing spec sheet (310050)
- Pentair: Pentek Big Blue Heavy Duty Series housing spec sheet (310053)
- Pentair: Pentek Big Blue Heavy Duty Series
- Pentair American Plumber: sediment cartridges sheet (300026)
- Pentair Engineered Filtration: filter cartridge catalog
Bottom Line
Measure the length and diameter of the old cartridge, or measure the housing, and buy the same size: 10 or 20 inch, standard or Big Blue. Then pick the media for the job. A longer or wider cartridge lasts longer, but only if the housing was built for it.