The specs an owner actually needs.
Independent, numbers-first answers about the equipment in your home: what the spec actually means, what size you need, and which part fits.
Short answer
21,000 grainsA four-person household on 10 gpg water, regenerating every 7 days, needs 21,000 grains of softener capacity between regenerations. Nearest nominal size: 24,000 grains.
Diagnostic guides: a symptom or a lab result in, a treatment class out, with the EPA or NSF limit cited by number.
37 guides
Sizing calculators for home water treatment: grain capacity, flow rate, micron rating and more, with the formula shown.
13 calculators
Model number to replacement part: which cartridge, membrane or lamp fits, on what interval, and which cross-fit alternatives exist.
42 parts
System comparisons segmented by the problem being solved, chosen against a stated method from published specifications.
25 reviews
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Best 20 inch Big Blue filter housing: port size, 90 psi rating, material, wrench, bracket and NSF/ANSI 42, by published spec
Pentek 150233: 1 in NPT, 90 psi max, NSF/ANSI 42 listed housing. iSpring WGB32B if you want a full 15 GPM system with wrench and bracket in the box.
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Best gravity water filter: Berkey alternatives compared on stated NSF/ANSI standards, element life and tank size
LifeStraw Home states NSF/ANSI 42, 53, 401 and a 264 gal membrane; Waterdrop King Tank states 42, 372 and 6,000 gal carbon. No listing confirmed.
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Best water softener resin cleaner: iron-specific vs general cleaners, dose per maker label, and who should skip one
Iron fouling: Iron OUT powder, 1 cup per 0.5 gal, then 0.25 cup per 40 lb salt. No iron: Res Care at 4 oz per cubic foot. No fouling: skip it.
Diagnose
Black specks in water after a new carbon filter: carbon fines, manganese or rubber, and how much to flush
Black specks after a filter change are carbon fines: GE says flush 2 to 3 gallons. Specks that outlast the flush point to manganese at 0.05 mg/L or rubber.
Match
Bluevua RO100ROPOT Replacement Filters: Which Filter Fits the RO100ROPOT and RO100ROPOT-UV, and How Often to Change Each One
Bluevua RO100ROPOT and RO100ROPOT-UV take the same PP, CTO, RO and PCF filters: 12 months for PP, CTO and PCF, 12-24 months for the RO membrane.
Match
Brita Faucet Filter Replacement: Which Mount Takes the FR-200, What Basic, Complete, Elite and Elite Advanced Mean, and the Cross-Fit That Claims to Fit
Every Brita faucet mount (FF-100/SAFF-100, Elite or Elite Advanced) takes the FR-200: 100 gallons, 0.58 gpm, WQA-certified to NSF/ANSI 42, 53 and 401.
Why the numbers here check out
Most equipment questions have one correct answer and it is a number. How many grains of capacity for four people on 22 gpg water. What micron rating a sediment filter needs on a well. Which cartridge fits a housing you already own.
Those answers are usually published by whoever wants to sell you the hardware. OwnerSpec publishes them independently: the formula, the inputs, the working, and the source it came from, so you can check it rather than trust it.
Independent
No manufacturer, dealer or installer owns a number on this site. Affiliate links are disclosed on every page that carries one.
Cited to the source
Limits come from EPA, NSF/ANSI and extension services by number. Capacities and part numbers come from the manufacturer's own manual or data sheet.
Dated
Every page shows when its facts were last verified and last updated, so you know how old the number is before you use it.