Water Softener Size Calculator
A water softener size calculator finds grains per day from people, hardness, and iron. 4 people at 15 gpg with 1 ppm iron need 39,900 grains over 7 days.
Water softener size calculator: grains per day and tank capacity
A water softener size calculator multiplies the hardness your softener must remove by the water you use each day. Hardness is counted in grains per gallon (gpg). Add 4 gpg for every 1 ppm of iron. A household of 4 with 15 gpg hardness and 1 ppm iron removes 5,700 grains a day, so a 7-day cycle needs 39,900 grains of capacity, or a 40,000 grain unit.
The formula in plain words
Picture a bucket the softener fills with hard minerals. Each day it takes in a bit more. The tank has to be big enough to hold a week of it before it empties the bucket and starts again.
- Daily water use = people × gallons per person (75 by default).
- Hardness in gpg = ppm ÷ 17.1, if you start from ppm.
- Compensated hardness = hardness + (iron in ppm × 4).
- Grains per day = compensated hardness × daily water use.
- Capacity needed = grains per day × days between regenerations.
- Round up to the next standard size.
Worked examples
These match the calculator exactly.
Example 1: 4 people, 15 gpg, 1 ppm iron, 7 days.
- Daily water: 4 × 75 = 300 gal
- Compensated hardness: 15 + 1 × 4 = 19 gpg
- Grains per day: 19 × 300 = 5,700
- Capacity: 5,700 × 7 = 39,900 grains
- Next standard size: 40,000 grains
Example 2: 3 people, 171 ppm hardness, no iron, 5 days.
- Hardness: 171 ÷ 17.1 = 10 gpg
- Daily water: 3 × 75 = 225 gal
- Grains per day: 10 × 225 = 2,250
- Capacity: 2,250 × 5 = 11,250 grains
- Next standard size: 24,000 grains
Standard sizes
| Mineral tank | Full salt regeneration | Efficient regeneration |
|---|---|---|
| 8 in × 44 in | 24,000 | 18,000 |
| 9 in × 48 in | 32,000 | 24,000 |
| 10 in × 44 in | 40,000 | 30,000 |
| 10 in × 54 in | 48,000 | 36,000 |
| 12 in × 48 in | 64,000 | 48,000 |
| 13 in × 54 in | 80,000 | 60,000 |
| 14 in × 65 in | 110,000 | 84,000 |
The calculator rounds up against the full salt column. If you plan to run the unit in efficient mode, compare your result with the right-hand column instead.
How to measure
- Get a water test for hardness and iron. Many municipal reports list hardness. A well needs its own test.
- Read the units on the report. Grains per gallon and ppm are not the same number.
- Count the people who live in the home most of the year.
- Check your flow rate with the GPM calculator if the unit also has to handle peak flow.
Common mistakes
- Using ppm as if it were gpg. A reading of 171 is 10 gpg, not 171.
- Ignoring iron. A small amount raises the compensated hardness a lot.
- Sizing for a tiny regeneration interval. Very frequent regeneration wastes salt and water.
- Assuming a softener fixes high iron. Above about 2 ppm, add a filter.
- Buying exactly the number you calculated with no rounding up.
Sources
- Nelsen Corporation, Softener Sizing: 75 gallons per person, 4 gpg per ppm of iron, ppm ÷ 17.1, 3 to 5 days between regenerations, and the tank capacity table (nelsencorp.com/tech/softener-sizing).
- SoftPro Water Systems sizing guides: dealer practice of 5 to 7 days and a 5 gpg per ppm iron factor (softprowatersystems.com).
Dealers do not all use the same iron factor. The calculator uses Nelsen’s 4. If your supplier uses 5, add one more grain for each ppm of iron by hand.
Related calculators
Check flow with the GPM calculator. Homes on a septic system should look at the septic tank size calculator, because a softener sends regeneration water to the drain. The pipe volume calculator shows how much water sits in a plumbing run.
Frequently asked questions
How many gallons of water does one person use per day?
Softener makers commonly size at 75 gallons per person per day, and that is the calculator default. Some dealers use 75 to 80, and EPA indoor-use estimates run higher. Raise the number if your household uses a lot of water.
How do I convert ppm hardness to grains per gallon?
Divide parts per million (or mg/L) by 17.1. A water test that shows 171 ppm is 10 grains per gallon. The calculator does this when you pick the ppm option.
How does iron change the size?
Dissolved iron uses up the resin like hardness does. Nelsen adds 4 grains per gallon for each 1 ppm of iron. Some dealers use 5. Most softeners handle up to about 2 ppm of iron, so higher levels may need a separate iron filter.
How many days between regenerations should I plan for?
Nelsen suggests 3 to 5 days as the typical range, while some dealers plan for 5 to 7. The calculator uses 7 by default. A shorter interval gives a smaller tank that regenerates more often. A longer interval gives a larger tank that uses salt more efficiently.
Which standard size should I buy?
Round up to the next common size: 24,000, 32,000, 40,000, 48,000, 64,000, 80,000, or 110,000 grains. These are full salt regeneration capacities from Nelsen. Efficient regeneration cuts the capacity by about a quarter, so use the lower rating when you set the unit for efficiency.
Estimates only. Verify quantities with your supplier before purchasing.