Water Softener vs Water Filter: Which One Do You Need?

It trades hardness minerals for sodium

Hard water is high in dissolved calcium and magnesium, USGS explains. Those minerals leave soap scum, cloudy glasses and scale, and when hard water is heated, scale can form in water heaters and lower their efficiency.

A water softener, also called an ion exchange unit, removes those minerals. Penn State Extension describes the process: water flows through a resin coated with sodium ions, and the resin releases sodium while it holds on to the calcium and magnesium. When the resin is full of hardness minerals, it stops softening, and the unit runs a cycle called regeneration to recharge it. Penn State estimates each regeneration uses about 50 gallons of water.

• What it removesCalcium and magnesium (hardness)
• What it addsSodium; Penn State estimates 7.5 mg per quart for each grain per gallon of hardness removed
• Where it goesOn the main water line, so it treats the whole house
• Certification to look forNSF/ANSI 44, the standard for hardness reduction

It reduces the contaminants it is certified for

"Water filter" covers many products: pitchers, faucet filters, under-sink carbon filters, reverse osmosis systems and whole-house units. What they share is that each is built to reduce particular contaminants.

The Minnesota Department of Health says no single treatment unit can remove all contaminants in water, and advises choosing a unit certified by NSF, UL or the Water Quality Association for the contaminant you are concerned about. EPA's guidance on lead, for example, points to filters certified against NSF/ANSI Standards 53 and 42. For PFAS, EPA found that the activated carbon, ion exchange and reverse osmosis point-of-use systems it studied can greatly reduce PFAS levels.

Two products, two different problems

The quickest way to choose is to start with the problem you actually have.

Not in the way most people mean

A softener changes the minerals in your water, but it is not a general-purpose filter. Penn State Extension notes that water softeners will not necessarily remove any of the more serious drinking water contamination problems, and describes softening as removing hardness minerals that are a nuisance, not a threat to human health.

There are a few overlaps. Penn State says a softener can remove colorless (dissolved) iron, but iron that has been exposed to air or chlorine will clog the resin, so it suggests filtering before the softener in that case. It also notes that turbid water may need filtering, and water with bacteria may need disinfecting, before it reaches the softener.

Start with your water record or a test

On a public water system, your utility's yearly water quality report and PurityMap's city pages show what public records report, where data is available. On a private well, a lab test tells you both.

• If Your problem is scale, spots or soap scumThat is hardness. Check your city's hardness level, then look at a softener certified to NSF/ANSI 44.
• If Your water record or test shows a contaminant such as lead or PFASChoose a filter certified for that contaminant. A softener is not the tool for it.
• If You have hard water and a contaminantSome homes use both: a softener on the main line for scale, and a certified filter at the kitchen tap for drinking and cooking water.

A few practical points

Not every tap needs softened water. Penn State suggests softening showers, sinks and laundry hookups, and says toilets, outside spigots and basement sinks can be bypassed. If your home is on a septic system, Penn State also points out the extra water from regeneration adds load to the drainage field.

You may also see salt-free "conditioners". The Water Quality Association describes these anti-scale devices as aiming to reduce or prevent certain scale deposits, which is not the same as removing the hardness minerals.

Whatever you install, the Minnesota Department of Health advises testing the treated water to make sure the treatment is working, and keeping up with maintenance. It warns that units that aren't maintained lose effectiveness and, in some cases, can make water quality worse.

1. 01
   Find your hardness and contaminants

   Look up your city's water report, or test your well.

2. 02
   Match the product to the problem

   Softener for hardness; certified filter for a contaminant.

3. 03
   Check the certification

   NSF/ANSI 44 for softeners; the standard for your contaminant for filters.

4. 04
   Test and maintain

   Test the treated water after installing, and follow the maintenance schedule.

Look up your water first

Check how hard your water is and what your utility has reported before choosing either product.

• Hard water by cityReported hardness for US cities, with what it means at home.Find your city →
• Water softener buying guideHow softeners work, how to size one, and what to look for.Read the guide →
• Compare water treatment optionsSofteners, carbon, reverse osmosis and more, side by side.Compare options →
• Choosing a water filter by contaminantWhich certified filter fits lead, PFAS, arsenic or nitrate.Read the guide →