A water softener doesn’t filter water. It trades minerals.

That’s the clearest way to understand what’s happening. The device takes calcium and magnesium out of your water and puts sodium in. The mechanism that drives this exchange happens inside a tank of resin beads, and it’s been the standard approach to water softening for decades because it works reliably.

The Resin Beads

The mineral tank contains millions of tiny ion exchange resin beads. These are made from a styrene-divinylbenzene copolymer, a plastic matrix with negatively charged sites distributed throughout. Before use, each of these sites holds a sodium ion.

When hard water flows through the tank, calcium (Ca2+) and magnesium (Mg2+) ions contact those charged resin sites. Both calcium and magnesium carry a +2 charge. Sodium carries a +1 charge. The resin preferentially holds the stronger +2 ions. Calcium and magnesium bind to the resin. The sodium they displaced moves into the water flowing past.

Hard water in. Soft water out. Calcium and magnesium trapped on the resin.

The exchange continues until the resin sites are full. At that point the system needs to regenerate.

The Regeneration Cycle

Regeneration resets the resin. The softener pulls highly concentrated salt water from the brine tank and flushes it through the resin beads.

The brine is a very concentrated sodium solution, far more concentrated than the original hard water. That extreme concentration shifts the chemistry. Now there are so many sodium ions present that even though calcium binds more strongly to any individual site, the sheer number of sodium ions overwhelms the calcium. The calcium and magnesium get displaced from the resin and wash to drain. The sodium reloads onto the resin sites.

The resin is ready to soften again. The calcium and magnesium are gone down the drain.

The Brine Tank

The brine tank holds the salt. When the control valve triggers regeneration, it draws water into the brine tank to dissolve the salt into a concentrated solution, then routes that brine through the mineral tank.

You add salt to the brine tank as it depletes. Most households use 40-80 lb bags. Frequency depends on household size and water hardness. Typical range is every 4-8 weeks, but higher hardness levels or larger households go through salt faster.

Sodium chloride (regular water softener salt) is the most common and cheapest option. Potassium chloride is a direct substitute that adds potassium to the water instead of sodium. It costs 2-3x more but is the right choice for people managing sodium intake or for households that want to use the softened water for plants without salt concern.

Metered vs. Time-Clock Regeneration

Older softeners run on a fixed schedule, regenerating every 7 days regardless of how much water the household actually used. This wastes salt and water on low-use weeks.

Metered softeners track gallons processed and regenerate only when the resin capacity is approaching exhaustion. This is significantly more efficient. Most modern softeners are metered by default. If you’re replacing an old softener, this feature alone justifies the upgrade.

Sizing the Softener Right

A softener that’s too small will produce hard water between regeneration cycles. A softener that’s too large wastes salt from over-regeneration.

The calculation:

(household gallons per day) x (water hardness in gpg) = grains per day

A family of 4 using 80 gallons per day at 15 gpg processes 1,200 grains per day. A properly sized softener should have enough resin capacity to handle several days of that load before regeneration.

Most manufacturers spec their softeners at 20,000 to 80,000 grain capacity. Match the capacity to your household’s daily grain load, with a buffer for variability.

What a Softener Doesn’t Do

This matters as much as what it does do.

A water softener removes hardness minerals only. It doesn’t remove PFAS, lead, nitrates, arsenic, fluoride, bacteria, VOCs, or any chemical contaminant. If you have concerns about any of those, you need filtration in addition to the softener.

Many households pair an under-sink reverse osmosis system (for drinking water) with a whole-house softener (for appliances, laundry, and showers). The softener handles scale. The RO handles everything else.

What Gets Into Your Water

Sodium is added in proportion to how much calcium and magnesium were removed. At 15 gpg source water, softened water contains approximately 300 mg of sodium per liter. At 10 gpg, around 200 mg/L.

For context on whether that matters for your situation, the softened water safety page goes through the numbers in detail.

If you’re evaluating which softener to buy, see the water softener comparison. To understand how hardness levels in your area compare to national ranges, regional hardness levels by state has the breakdown. And for the salt-free alternative, water softeners vs. salt-free conditioners explains the key difference.