This free water softener size calculator works out the grain capacity you need from your household size and water test. Oversized softeners waste money and undersized ones regenerate constantly. Enter the number of people, your water hardness and any iron, and this calculator works out the grains you remove each day, the capacity you need between regenerations, the best standard softener size, and how much salt you will use.
How to use the Water Softener Size Calculator
- Enter the number of people in the home and water use per person per day.
- Enter your water hardness and the dissolved iron in mg/L (ppm).
- Set how many days you want between regenerations.
- Press Size my softener to see grains per day, the capacity you need and yearly salt use.
Choose the next standard size at or above the water softener size calculator result rather than the one just below it. Look for softeners certified by the Water Quality Association or a similar body.
How water softener sizing works
Softeners are sold by their maximum grain capacity, the grains removed between regenerations at the highest salt dose (about 15 lb of salt per cubic foot of resin). At that dose they use salt inefficiently. At an efficient 6–8 lb/ft³ dose, the same softener gives about 65–75% of its rated capacity. That is why the recommended size here is one step larger than the bare minimum.
Water hardness scale
| Classification | gpg | mg/L (ppm) |
|---|---|---|
| Soft | < 1 | < 17 |
| Slightly hard | 1 – 3.5 | 17 – 60 |
| Moderately hard | 3.5 – 7 | 60 – 120 |
| Hard | 7 – 10.5 | 120 – 180 |
| Very hard | > 10.5 | > 180 |
Don’t know your hardness? City water users can find it in the annual Consumer Confidence Report. Well owners should get a lab test, which is also the time to check iron, manganese and pH (see our well water test interpreter).
Frequently asked questions
What size water softener do I need for a family of 4?
With 10 gpg hardness and no iron, a family of four uses about 3,000 grains per day, or 21,000 grains per week. A 32,000-grain softener is the usual choice. With 20 gpg or some iron, step up to 40,000–48,000 grains.
How do I convert ppm hardness to grains per gallon?
Divide mg/L (ppm) by 17.12. For example, 250 ppm ÷ 17.12 = 14.6 gpg.
Why add iron to the hardness?
Softener resin also removes dissolved iron, but iron uses more capacity than calcium and magnesium. The common industry rule is to add about 4 gpg for each 1 ppm of iron (some manufacturers use 3–5).
Is a bigger softener always better?
Not always. A heavily oversized unit regenerates too rarely, which can let the resin bed channel and bacteria grow. Aim for regeneration every 7–10 days at an efficient salt setting.
What is compensated hardness?
Compensated hardness adds an allowance for iron, which a softener also removes. Rules of thumb add about 3 to 5 grains per gallon for each 1 mg/L of iron, so 15 gpg hardness with 1 mg/L of iron is sized as roughly 18 to 20 gpg.
What does water softener grain capacity mean?
Grain capacity is how many grains of hardness a softener removes before it has to regenerate. A 32,000-grain unit treating 19 gpg water at 280 gallons a day (5,320 grains a day) lasts about 6 days between regenerations at its rated salt setting.
Related tools
Estimates only. Actual capacity and salt use depend on the softener model, valve settings and resin. Check the manufacturer’s specifications. Built by a water-testing laboratory professional.