A water softener removes hardness (calcium and magnesium) by ion exchange; a water filter removes other things, such as chlorine taste, sediment or specific contaminants, depending on its type. Most filters don’t soften water, and a softener doesn’t filter out contaminants. NSF certifies them under different standards: NSF/ANSI 44 for softeners, and 42, 53 or 58 for filters and reverse osmosis.
On this page
Many homes end up with both. Here’s what each one does, which certification to look for, and how to combine them.
What each one removes
| System | NSF/ANSI standard | What it’s certified for | Hardness |
|---|---|---|---|
| Water softener (ion exchange) | 44 | Swapping calcium and magnesium for sodium or potassium | Removes it |
| Carbon filter (taste and odour) | 42 | Aesthetic impurities such as chlorine, taste and odour | No |
| Carbon filter (health claims) | 53 | Contaminants with health effects, as set by the EPA and Health Canada | No |
| Reverse osmosis | 58 | Contaminants removed by a pressurised semi-permeable membrane | Reduces it, but scales up on hard water |
| Emerging contaminants filter | 401 | Up to 15 emerging contaminants, such as some pharmaceuticals | No |
Why a filter doesn’t fix hard water
Hardness is dissolved minerals, not particles, so sediment and carbon filters let it straight through. Reverse osmosis does reduce dissolved minerals, but hard water works against the membrane: the Vermont Department of Health notes that hardness scales up reverse osmosis membranes (and the bulbs of UV systems), making them less effective. Vermont cites industry guidance to keep hardness at or below about 120 to 170 mg/L (7 to 10 gpg) for membrane systems, which may mean softening first.
Why a softener doesn’t make water “clean”
A softener’s resin targets calcium and magnesium. It isn’t designed or certified to remove chlorine taste, lead, nitrate or bacteria. The City of Phoenix’s water FAQ makes a related point from the other side: its tap water meets drinking water standards, so filters are typically not necessary for safety unless a physician advises one or you choose one for personal preference. If your concern is taste or a specific contaminant, a filter certified for it is the right tool; a softener is not.
Which do you need?
| Problem | Fix |
|---|---|
| Scale on kettles, taps and heaters; poor lather | Softener |
| Chlorine taste or smell | Carbon filter (NSF/ANSI 42) |
| A specific health contaminant from a test or report | A filter or RO unit certified for that contaminant (53 or 58) |
| Cloudy water or sediment from a well | Sediment filter |
| Hard water and an RO system you want to protect | Softener before the RO unit |
| Drinking water without the sodium a softener adds | Unsoftened kitchen tap, or RO at the kitchen sink |
Start from your water report or a test. Our guides on reading a water quality report and testing hardness show where to find the numbers, and the do I need a softener checklist helps with the hardness side.
Using both: the usual order
- Sediment filter first, especially on well water, to protect everything downstream.
- Iron filter if a test found high iron, since manuals such as Pentair’s say softeners handle iron only up to about 3 ppm.
- Water softener for the whole house.
- Carbon or RO filter at the kitchen sink for drinking water.
Running costs to compare
A softener’s main cost is salt: about 349 lb a year for a family of four on 10 gpg water with an efficiency-rated unit, roughly $76 at October 2026 prices (see how much salt a softener uses). Filters cost replacement cartridges or membranes at the interval the manufacturer sets, and RO units also send part of the water to drain. Compare like with like over several years.
Frequently asked questions
Do I need a water filter if I have a softener?
Only if you also want to change taste or remove specific contaminants. A softener doesn’t do either.
Will a pitcher filter remove hardness?
Carbon pitcher filters are usually certified for taste, chlorine or specific contaminants rather than for softening. Check the product’s NSF listing for what it actually reduces.
Is reverse osmosis the same as a softener?
No. RO reduces many dissolved substances, including hardness, but its membrane scales on hard water. On hard supplies it usually works best behind a softener.
Should the softener go before or after a filter?
Sediment and iron filters go before the softener; a drinking-water filter or RO unit usually goes after it, at the kitchen sink.
