Water Softener Sizing
Compensated hardness, daily grain load, days between regenerations.
Example
You enter
- Occupants 4
- Water use per person (gal/day) 75
- Total hardness (grains/gal) 20
- Dissolved iron (ppm, optional) 2
- Usable resin capacity (grains) 32000
- Salt per regeneration (lb) 15
You get
- Compensated hardness 28
- Daily grain load 8400 grains/day
- Days between regenerations 3
- Annual salt 1825
Checked against published worked examples from: NSF/ANSI 44 / Water Quality Association.
Details, formula, and sources
ScopeCompensated hardness, daily grain load, days between regenerations, and salt per regen and per year for a water softener.
Formula or tablecomp_hardness = hardness_gpg + iron_ppm x 4; daily_gal = people x use_per_cap; grain_load = daily_gal x comp_hardness; days_between = floor(capacity / grain_load); annual_salt = salt_per_regen x (365 / days_between). 1 gpg = 17.1 ppm.
Source and editionNSF/ANSI 44 and the Water Quality Association (WQA) hardness/iron-compensation practice by name; first-principles ion-exchange grain budget.
Free accessNSF/ANSI 44 is a published consensus standard (NSF International). The hardness conversion (1 gpg = 17.1 ppm) and the WQA iron compensation are public.
What governsEstimate. AHJ and licensed professional govern.
API field namespeople, use_per_cap, hardness_gpg, iron_ppm, capacity, salt_per_regen, comp_hardness, grain_load, days_between, annual_salt
- Iron compensation ~4 gpg of hardness added per ppm of dissolved ironWater Quality Association practice
- Default water use 75 gal per person per daystandard residential design figure