Ion Exchange Softener Calculator
A softener runs out of capacity on a schedule you can predict. Enter flow, hardness, resin volume, and salt dose to get days between regenerations and the salt used per regeneration.
Softener Capacity
| Item | Value |
|---|
How the calculation works
Formulas
Hardness (gpg) = mg/L as CaCO3 ÷ 17.1
Daily hardness load (grains/day) = flow (gpd) × hardness (gpg)
Total capacity (grains) = resin (ft³) × capacity per ft³ (grains/ft³ at the selected salt dose)
Days between regenerations = total capacity ÷ daily load
Water treated per regeneration (gal) = total capacity ÷ hardness (gpg)
Salt per regeneration (lb NaCl) = salt dose (lb/ft³) × resin (ft³)
Salt efficiency (grains per lb salt) = capacity per ft³ ÷ salt dose
Hardness (gpg) = mg/L as CaCO3 ÷ 17.1
Daily hardness load (grains/day) = flow (gpd) × hardness (gpg)
Total capacity (grains) = resin (ft³) × capacity per ft³ (grains/ft³ at the selected salt dose)
Days between regenerations = total capacity ÷ daily load
Water treated per regeneration (gal) = total capacity ÷ hardness (gpg)
Salt per regeneration (lb NaCl) = salt dose (lb/ft³) × resin (ft³)
Salt efficiency (grains per lb salt) = capacity per ft³ ÷ salt dose
| Salt dose (lb NaCl / ft³) | Typical capacity (grains / ft³) | Salt efficiency (grains / lb) |
|---|---|---|
| 6 | 20,000 | 3,333 |
| 8 | 24,000 | 3,000 |
| 10 | 26,500 | 2,650 |
| 12 | 28,500 | 2,375 |
| 15 | 30,000 | 2,000 |
Notes & limitations
Capacities are typical values for standard 8% crosslinked gel strong-acid cation resin. Actual capacity varies by resin brand, age, and water chemistry — verify against the resin manufacturer’s specification sheet before sizing equipment.
- 17.1 mg/L as CaCO3 = 1 grain per gallon (gpg).
- Do not run the bed to theoretical exhaustion: hardness breakthrough and channeling occur near the end of the run. Program regeneration at roughly 90% of calculated capacity, or on a meter with a reserve.
- Iron and manganese foul cation resin. Pretreat if combined iron + manganese is elevated (many manufacturers recommend keeping iron below about 1 mg/L for softeners).
- Higher salt doses buy capacity but waste salt: compare the salt-efficiency column when salt cost or chloride discharge limits matter.
- This tool is for operational scheduling. Confirm regeneration settings against the softener manufacturer’s manual and lab hardness testing.
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