Water softener size calculator

By Ajay Singh. Updated November 10, 2026.

Enter how many people live in your home and how hard your water is. The calculator works out how many grains of hardness you use a day and suggests the softener capacity to match, so it regenerates about once a week. A family of four on 10 gpg water usually needs a 32,000-grain softener.

How the calculator works

A softener's resin can hold a set amount of hardness before it needs recharging with salt, which is called regenerating. That amount is its capacity, measured in grains. Sizing a softener is a matter of matching that capacity to the hardness your household runs through it.

1. Daily water use. People × gallons per person per day. Penn State Extension's worked example uses 75 gallons per person; the EPA's WaterSense program gives 82 gallons a day at home as the US average, based on USGS estimates for 2015. If you know your actual use from a water bill, divide it by the number of people and days and use that.
2. Hardness. If your test gives mg/L or ppm, it's converted at 17.1 mg/L to 1 grain per gallon, the factor the Connecticut Department of Public Health uses.
3. Iron allowance. Dissolved iron also uses up resin capacity. The calculator adds 5 gpg for every 1 ppm of clear-water iron, which is the figure in A. O. Smith's softener manual. We couldn't find an extension source that gives a number, and dealers use anything from 3 to 5, so treat this as a manufacturer's rule of thumb.
4. Grains a day. Daily gallons × hardness (plus the iron allowance). This is Penn State's method.
5. Grains per cycle. Grains a day × days between regenerations. Seven days is a common target, not a rule; Penn State's own example works out at every 6 to 7 days.
6. Rated size. A softener's headline grain rating is what it delivers at a high salt setting. At a lower, more efficient setting it delivers less. Solenis, a water treatment chemicals company, gives about 30,000 grains per cubic foot of resin at 15 lb of salt; Water Conditioning & Purification magazine gives about 24,000 at 8 lb. That's roughly three quarters, so the calculator divides your weekly grains by 0.75 and picks the next common size up.

A worked example

Four people, 75 gallons each a day, water at 10 gpg, no iron:

• 4 × 75 = 300 gallons a day
• 300 × 10 = 3,000 grains a day
• 3,000 × 7 = 21,000 grains a week
• 21,000 ÷ 0.75 = 28,000, so the next size up is 32,000 grains

At a low salt setting that softener gives about 24,000 grains per regeneration, enough for 8 days. Add 2 ppm of iron and the same household needs about 64,000 grains, which shows why a well test matters before you buy.

Things that change the answer

• The grain rating isn't standard across brands. The resin volume behind a "48,000-grain" label varies. Whirlpool's WHES48 has 1.33 cubic feet of resin, for example, and Morton's M45C has 1.26. Check the capacity the manufacturer lists at each salt setting, not just the headline number.
• Efficiency-rated models. Softeners certified as efficiency rated under NSF/ANSI 44 remove at least 3,350 grains of hardness per pound of salt and use no more than 5 gallons of water per 1,000 grains, at their stated settings.
• Demand-initiated regeneration. Most modern softeners measure water use and regenerate only when needed. The Connecticut Department of Public Health recommends regenerating by volume of water used rather than on a timer.
• Too much iron. Extension sources put the usual softener limit at 2 to 5 mg/L of dissolved iron. Above that, an iron filter goes first. See iron in well water.
• Set the hardness right. The Minnesota Department of Health notes that a softener set for harder water than you have costs more to run and wastes water.

Next, see what your softener will cost to run, or compare models in water softeners compared.