What size water softener do I need?

Multiply your water's hardness in grains per gallon by the number of people in your home, the gallons each person uses per day and the days you want between regenerations. That is the number of grains the softener must remove each cycle. You need a model that removes it at the salt setting you will actually use, because the size on the box is reached only at the highest salt dose.

The formula

Grains per regeneration = hardness (gpg) × people × gallons per person per day × days between regenerations.

Our softener size calculator does the math. What to enter:

• Hardness. Water quality reports give hardness in mg/L (the same as ppm) of calcium carbonate, or in grains per gallon (gpg). One gpg equals 17.118 mg/L, so divide mg/L by 17.1 (Maryland Geological Survey). A report showing 257 mg/L means about 15 gpg. See how to read your water quality report.
• Gallons per person per day. EPA puts average home use at 82 gallons per person per day (EPA WaterSense), and the calculator starts there. Roughly 70 percent of household use is indoors (EPA WaterSense), so if your outdoor taps skip the softener, about 57 gallons is closer. Your water bill gives your real number: gallons billed, divided by days, divided by people.
• Days between regenerations. Your choice. The calculator starts at 7.
• Iron. If a lab found clear-water (ferrous) iron, the Morton, Whirlpool, Rheem and WaterBoss manuals all say to add 5 gpg to the hardness for each 1 ppm of iron (Morton manual). On a well, read softener sizing for well water first.

Example: 15 gpg × 4 people × 82 gallons = 4,920 grains a day, or 34,440 grains for 7 days.

The salt setting changes the capacity

A softener's grain rating depends on how much salt it uses per regeneration. Each manual lists the rated capacity at three salt doses:

At the lowest dose these softeners remove about 5,000 grains per pound of salt. At the highest they remove about 2,400 to 2,650, so the top rating takes roughly twice the salt for each grain. Aquasana's SimplySoft WH-SF40, sold as a 40,000-grain softener, lists a single rating: 13,269 grains at 3 lb of salt (Aquasana data sheet).

These are laboratory figures. The manuals say the efficiency a softener reaches once installed is typically lower than its rating, because of hardness, water use and other contaminants (Whirlpool WHES40 manual).

What it means for the example

The household above needs 34,440 grains a week.

• Only the highest salt settings of the WHES40, Rheem, WaterBoss and Morton cover a full week.
• At its lowest dose the WHES40 removes 17,300 grains, so it would regenerate about every 3.5 days. All five are demand softeners that regenerate based on the water used (Morton manual), so a smaller capacity means more frequent regenerations.
• The home removes about 1.8 million grains a year. On paper, the WHES40 needs about 350 lb of salt a year for that at its 3.4 lb setting and about 700 lb at its 15.5 lb setting.

The low setting saves salt but uses more regeneration water per grain. The WHES40 uses 3.0 gallons per 1,000 grains at the minimum dose. At the maximum dose a whole regeneration uses 37.1 gallons, or about 0.9 gallons per 1,000 grains (Whirlpool WHES40 manual).

Two rules from the manuals. An efficiency-rated softener under NSF/ANSI 44 has a rated salt efficiency of at least 3,350 grains per pound of salt (Morton manual, Whirlpool WHES30 manual). In California, the Morton, Whirlpool and Rheem manuals say the salt-efficiency setting must be on, which keeps the softener at 4,000 grains per pound or more (Morton manual, Whirlpool WHES30 manual, Rheem manual).

Before you buy

• Check the maximum hardness in the manual: 90 gpg for the WaterBoss 900, 95 for the WHES30, 110 for the M34, 125 for the WHES40 and 140 for the RHW42 (manuals above).
• If you will use potassium chloride instead of salt, WaterBoss says to raise the hardness setting by 12% (WaterBoss manual).
• Check the certification. IAPMO lists the WH-SF40 under NSF/ANSI 44 for hardness: Certified (IAPMO listing, checked 16 Oct 2026) (IAPMO certificate). The Morton, Whirlpool, Rheem and WaterBoss manuals say NSF International certified those softeners to NSF/ANSI 44: Brand says certified, not found in IAPMO's listing; check NSF in NSF's listings.