Water Softener Size Calculator (Grains per Day and Capacity)
How big a water softener you need: grains of hardness removed per day from your household size and water hardness, the grain capacity for your regeneration interval, and how often a given softener will regenerate.
Formula
Grains per day = people × gallons per person per day × hardness (gpg). Capacity needed = grains per day × days between regenerations. Days between regenerations = rated capacity ÷ grains per day. gpg = mg/L ÷ 17.1.
Worked example
Inputs: People in the home = 4; Softened water per person per day = 75 gal; Water hardness = 10; Hardness unit = grains per gallon (gpg); Target days between regenerations = 7 days; Softener capacity from its spec sheet (optional) = 0 grains.
| Hardness gpg | 10 |
|---|---|
| Hardness class | Hard |
| Water softened gal per day | 300 |
| Grains removed per day | 3,000 |
| Capacity needed grains | 21,000 |
| Days between regenerations at rated capacity | — |
| Regeneration water gal per month | 214 |
Method from Penn State Extension: people × gallons per person per day × hardness (gpg) = grains per day; capacity ÷ grains per day = days between regenerations. 1 gpg ≈ 17.1 mg/L. Rated grain capacity depends on the salt setting, so use the figure for the salt dose you will run. Iron and manganese use extra capacity; ask the softener maker how to adjust. About 50 gallons of water per regeneration.
FAQ
What size water softener do I need for a family of 4?
At 75 gallons per person and 10 gpg: 4 × 75 × 10 = 3,000 grains per day, so a 7-day interval needs about 21,000 grains of capacity. At 15 gpg it is 4,500 grains per day and 31,500 per week.
What about 2 people?
2 × 75 × 10 gpg = 1,500 grains per day, or 10,500 grains for a weekly regeneration.
How hard is hard water?
Penn State Extension: soft is under 1 gpg (17 mg/L), slightly hard 1–3.5, moderately hard 3.5–7, hard 7–10.5 and very hard over 10.5 gpg (180 mg/L). Below about 7 gpg, scale and soap film are usually minor.
How do I convert mg/L (ppm) to grains per gallon?
Divide by about 17.1. A lab result of 171 mg/L as calcium carbonate is 10 gpg.
How often should a softener regenerate?
Divide its capacity by your grains per day. Penn State's example: a 20,000-grain unit at 3,000 grains per day regenerates every 6 to 7 days. Each regeneration uses about 50 gallons of water, which matters if you are on a septic system.
Sources
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Last reviewed 2026-10-30.