Multiply your hardness in grains per gallon by your household's daily water use to get grains removed per day, then size the softener to regenerate about every seven days. A family of four using about 300 gallons a day with 15 gpg hardness removes 4,500 grains a day, or about 31,500 a week, so a 32,000 to 40,000 grain unit fits. Add about 4 to 5 gpg for each ppm of iron.
Softener capacity is rated in grains of hardness removed between regenerations. Getting the size right keeps salt use low and the resin healthy.
Step 1: Find your hardness
Use a lab test, a hardness test strip, or your water utility’s report. Hardness is often reported in ppm or mg/L.
| Hardness | ppm (mg/L) | Grains per gallon |
|---|---|---|
| Soft | 0 to 60 | 0 to 3.5 |
| Moderately hard | 61 to 120 | 3.5 to 7 |
| Hard | 121 to 180 | 7 to 10.5 |
| Very hard | Over 180 | Over 10.5 |
To convert, divide ppm by 17.1.
Step 2: Estimate daily use
A common sizing figure is about 75 gallons per person per day. Your water bill gives a better number if you have one.
Step 3: Do the math
Daily grains = hardness (gpg) × gallons per day. Weekly grains = daily grains × 7.
| Household | Gallons per day | 10 gpg | 15 gpg | 20 gpg |
|---|---|---|---|---|
| 2 people | 150 | 10,500 | 15,750 | 21,000 |
| 4 people | 300 | 21,000 | 31,500 | 42,000 |
| 6 people | 450 | 31,500 | 47,250 | 63,000 |
Weekly grains, rounded. Choose the next size up.
Step 4: Allow for iron and salt settings
Iron also loads the resin. A common rule is to add 4 to 5 gpg for every ppm of iron. Above roughly 3 ppm iron, a dedicated iron filter is usually the better tool; see our whole-house filter guide.
Rated capacity assumes the maximum salt dose. Many owners run a lower salt setting for efficiency, which gives a lower working capacity, so check the manual’s capacity table at the salt setting you plan to use.
Salt-based or salt-free
Salt-based softeners remove calcium and magnesium by ion exchange, certified under NSF/ANSI 44. Salt-free conditioners using template-assisted crystallization change how minerals behave so they form less scale, but the water is still hard. If you want softer-feeling water and less soap use, you need ion exchange.
Planning reverse osmosis too? Softening ahead of it extends membrane life. See RO vs carbon, or browse whole-house and softeners.
Common questions
How do I convert ppm hardness to grains per gallon?
Divide ppm (mg/L as calcium carbonate) by 17.1. For example, 257 ppm is about 15 gpg.
What size softener does a family of four need?
At moderate hardness (10 to 15 gpg), usually a 32,000 to 40,000 grain unit. Very hard water or high use needs more.
Is a bigger softener always better?
No. An oversized unit regenerates rarely, and resin can foul if it sits too long. Aim for regeneration roughly weekly.
Do salt-free conditioners soften water?
No. They reduce scale formation but do not remove hardness minerals, so soap still behaves as it does in hard water.