A softener is a bed of resin beads doing an ion exchange, and it is easily ruined by things it was never meant to handle. A prefilter is there to keep sediment and oxidized iron out of that bed. Whether you need one depends entirely on what your water carries, not on what the installer would like to sell you.
What actually damages softener resin
Suspended sediment packs into the bed and channels the flow, so water finds a path around the resin rather than through it. Oxidized iron is worse: Penn State Extension notes that softeners can remove dissolved iron under the right conditions, with pH above 6.7, hardness between 3 and 20 grains per gallon and dissolved iron under 5 mg/L, but that oxidized iron fouls resin. Chlorine attacks the resin structure over years on a municipal supply. Each of those has a different prefilter answer.
Choosing the prefilter for your water
For grit and silt, a sediment cartridge or a spin-down separator ahead of the softener is enough, chosen coarse enough not to blind in weeks. On a well producing sand, use a separator first and a cartridge behind it. For chlorine on a city supply, a carbon prefilter extends resin life, though it needs its own replacement schedule. For iron above what the softener can take, the prefilter is not a cartridge at all, it is a proper oxidizing iron filter installed ahead of the softener.
Sizing so you do not lose pressure
A softener needs a decent service flow to work and a strong backwash flow to regenerate, so a prefilter that restricts the line hurts both. Size the housing on the household demand rather than on the pipe diameter, use the coarsest micron rating that fixes the problem, and fit shutoff valves either side so a change takes minutes. Keep the prefilter serviceable: a cartridge that never gets changed becomes the very restriction you installed it to prevent.
When you can skip it
Plenty of homes on clean municipal water run a softener for years with no prefilter at all and nothing goes wrong. If your water is clear, has no measurable iron and the aerators stay clean, a prefilter is a cartridge cost and a pressure drop bought for nothing. Test before adding one, and let the result decide. Softeners are sold in stepped capacities with published flow ratings, and our water softener installation guide covers where each part goes in the line.
Questions people ask about prefilter for water softener
Does every water softener need a prefilter?
No. Only water carrying sediment, sand, oxidized iron or a chlorine residual you want kept off the resin genuinely justifies one.
What micron prefilter is right before a softener?
Use the coarsest rating that stops the sediment you actually have. Too fine a cartridge blinds quickly and drops the flow the softener needs.
Should the prefilter go before or after the softener?
Before it. The point is to keep sediment and oxidized iron out of the resin bed, which only works if the filter sees the water first.
Can a prefilter fix iron staining?
Only if the iron is already oxidized into particles. Dissolved iron passes straight through and needs an oxidizing iron filter instead.