What Size Water Softener Do I Need? Grains, People and Hardness

The short answer

Size from your water, not the box. Grains a day = people × 75 gallons × hardness in grains per gallon, adding about 4 gpg for each ppm of iron. Multiply by about 7 days between regenerations. Grain ratings are full-salt figures; at an efficient salt setting a tank gives about three quarters of its rating, so pick a unit rated about a third higher than your weekly load. Our sizing calculator does this and lists the units that keep up.

Weekly load by household and hardness

Grains between weekly regenerations (75 gal per person per day)
People	10 gpg	15 gpg	20 gpg	25 gpg
2	10,500	15,750	21,000	26,250
3	15,750	23,625	31,500	39,375
4	21,000	31,500	42,000	52,500
5	26,250	39,375	52,500	65,625

From weekly load to grain rating

Divide the weekly load by 0.75 for the rating to look for. Four people on 15 gpg water need about 31,500 grains a week, so a unit rated about 42,000 grains or more. The Aquasure Harmony 48,000 and AFWFilters Fleck 5600SXT 48,000 sit there; a family of five on 20 gpg water needs about 70,000, the territory of the Aquasure Harmony WS-EM-64 and up.

Penn State Extension's worked example uses the same arithmetic: 75 gallons a person, 10 gpg, four people, 3,000 grains a day, so a 20,000-grain unit regenerates about every six to seven days.

Don't forget flow

Capacity covers how much water; service flow covers how fast. A unit whose stated service flow is below your peak use drops pressure when showers and appliances run together. More bathrooms usually means a higher peak; compare each unit's stated flow with yours.

Iron changes the size

Each ppm of ferrous iron takes up resin capacity like about 4 gpg of hardness, so 15 gpg water with 2 ppm of iron sizes like 23 gpg. Most softeners handle up to about 2 ppm; above about 5 ppm, or with red water, put an iron filter ahead of the softener.