Water Hardness & Softener Dosage Calculator
Calculate your water hardness classification and determine the precise water softener salt dosage required to treat your water supply effectively.
Formulas Used
Unit Conversions to ppm (mg/L as CaCO₃):
- ppm × 1 = ppm
- gpg × 17.118 = ppm
- °dH × 17.848 = ppm
- °fH × 10.000 = ppm
Resin Capacity (grains):
Capacity = Resin Volume (L) × 45,000 mg CaCO₃/L ÷ 64.799 mg/grain
Daily Hardness Load (grains/day):
Load = (Source Hardness − Target Hardness) [gpg] × Daily Usage [US gal/day]
Days Between Regenerations:
Days = Resin Capacity [grains] ÷ Daily Load [grains/day]
Salt Dosage per Regeneration:
Salt (kg) = (Resin Capacity ÷ 1,000) × Salt Factor
Salt factors: NaCl Rock/Solar = 0.30 kg/1,000 grains; NaCl Evaporated = 0.28 kg/1,000 grains; KCl = 0.35 kg/1,000 grains
Brine Volume:
Brine (L) = Salt (kg) ÷ Saturated Brine Concentration (NaCl: 0.315 kg/L; KCl: 0.270 kg/L)
Assumptions & References
- Hardness classification follows WHO Guidelines for Drinking-water Quality and the Water Quality Association (WQA) scale.
- Ion exchange resin capacity assumed at 45,000 mg CaCO₃ per litre of strong-acid cation resin (industry standard for Type 1 sulfonated polystyrene resin).
- Salt efficiency values (0.28–0.35 kg/1,000 grains) are based on efficient regeneration protocols per WQA and NSF/ANSI 44.
- Saturated NaCl brine concentration ≈ 315 g/L; saturated KCl brine ≈ 270 g/L at 20 °C.
- Waste water estimate of 50 L per litre of resin is a midpoint industry estimate; actual values vary by system design (typically 40–60 L/L resin).
- 1 grain per US gallon (gpg) = 17.118 mg/L (ppm) as CaCO₃.
- Daily water usage is assumed constant; seasonal variation is not accounted for.
- Results are estimates. Consult a water treatment professional for system-specific sizing.
- References: WHO (2011) Guidelines for Drinking-water Quality, 4th ed.; WQA Technical Fact Sheet — Ion Exchange; NSF/ANSI Standard 44 (Residential Cation Exchange Water Softeners).