A water cartridge filter is the replaceable element inside a housing, and almost everything about choosing one comes down to three numbers on the label: the physical size, the micron rating, and the flow rate the cartridge can sustain. Get those right and the housing itself barely matters.
The standard sizes, and why they exist
Household cartridges are made to a handful of industry sizes, which is what lets you buy a replacement from a different brand than the housing. The slim ten inch cartridge is the under-sink standard. The same length in a fatter diameter, commonly called big blue, holds far more media and is what goes in a whole-house housing. Twenty inch versions of both exist for higher flow or longer intervals. Measure the old cartridge and note both length and diameter before ordering, because a slim element rattling around inside a wide housing will simply let water past it around the outside.
Micron rating and the pressure trade
The micron rating is the size of particle the cartridge is designed to stop. Tighter is not better. A very fine cartridge on silty water blinds within days, the pressure at the shower collapses and the household concludes the filter is faulty. The usual approach on a well is staging: a coarse cartridge first to take the sand and grit, then a finer one behind it to catch what got through, so each does the job it is suited to and the fine one lasts. Note also that nominal ratings describe most particles of that size while absolute ratings are a guarantee, and the two are not comparable.
Sediment, carbon, and the ones that do both
Sediment cartridges are mechanical: spun polypropylene, pleated polyester or string wound, and they trap particles. Pleated cartridges have far more surface area and some can be rinsed and reused a few times, which suits seasonal silt. Carbon cartridges are chemical: they adsorb chlorine, taste and odor compounds and many organics, and a solid carbon block also filters particles as a side effect. Combination cartridges put both in one element, which is convenient in a single housing but means you throw away good carbon when the sediment side plugs. Two housings is usually the better economics on dirty water.
Changing one without making a mess
Shut the inlet valve, open a downstream tap to drop the pressure, then press the red relief button on top of the housing if it has one. Have a bucket and a towel ready, because a big blue housing holds a surprising amount of water. Use the supplied wrench rather than pliers. Inspect the O-ring, wipe the groove clean and lubricate it with silicone grease, never petroleum jelly, and reseat it properly: a pinched or dry O-ring is the reason most housings weep after a change. Write the date on the new cartridge with a marker, and keep pressure drop, not the calendar, as the real signal.
Questions people ask about water cartridge filter
How do I know what size cartridge I need?
Measure the old one for length and diameter and match it. The common household formats are the slim ten inch under-sink element and the wider big blue used in whole-house housings.
Is a lower micron rating always better?
No. A finer cartridge plugs faster and drops pressure sooner. On silty water a coarse cartridge ahead of a finer one gives better water and much longer life than one very fine element.
How often should I change a cartridge?
Change on pressure drop rather than the calendar. Fit a gauge before and after the housing if you can, and note that a clogged carbon block can release taste back into the water.
Can I rinse and reuse a cartridge?
Pleated sediment cartridges can be rinsed a few times, which suits seasonal silt. Spun, string wound and carbon cartridges cannot be usefully cleaned and should be replaced.
Why does my housing leak after a change?
Almost always the O-ring. It needs to be clean, undamaged, seated in its groove and lightly greased with silicone, and the housing needs to be tightened with the proper wrench.