How to use this calculator
- Enter daily soft water use and the peak flow.
- Enter feed hardness as CaCO₃.
- Choose how often you want to regenerate and the salt dose.
- Read resin volume, salt use and the recommended vessel from the list of standard sizes.
How do you size a water softener?
Load per cycle (g) = Q (m³/day) × (cycle hours ÷ 24) × hardness (mg/L)
Resin (L) = load ÷ operating capacity (g CaCO₃/L)
Salt (kg) = resin (L) × salt dose (g/L) ÷ 1,000
Vessel area ≥ peak flow ÷ service velocity
Max resin (L) ≈ area × (height − D/2) ÷ 1.5 × 1,000, leaving 50% freeboard
Pick the smallest standard vessel that holds the resin with a bed of at least 0.75 m
Worked example
With soft water needed 10 m³/day, peak flow rate 2 m³/h, feed total hardness 300 mg/L as CaCO₃, time between regenerations 24 hours, salt dose and resin capacity: Standard: 160 g salt/L resin, 57 g/L capacity, maximum service velocity 25 m/h, resin safety margin 10 %:
Resin volume: 55 litres
The calculator also gives hardness removed per cycle 3 kg as CaCO₃; salt per regeneration 9.6 kg NaCl; salt per month 288 kg.
Assumptions and limits
- Resin capacities are typical manufacturer values for counter or co-current regeneration at room temperature.
- Assumes soft water target near zero hardness.
- Single vessel; use twin vessels for continuous supply.
- Vessel list covers FRP sizes from 8" × 44" to 63" × 67" and MS vessels from 500 to 3,500 mm diameter with the height made to suit.
Frequently asked questions
How much salt does a water softener use?
At a standard dose of 160 g salt per litre of resin, a 25 litre resin softener uses 4 kg per regeneration. Lower salt doses save salt but lower capacity per cycle.
What hardness needs a water softener?
Water above about 120 to 180 mg/L as CaCO₃ causes scale in geysers, boilers and pipes. BIS IS 10500 sets 200 mg/L as the acceptable limit for drinking water.
What is the difference between a softener and RO?
A softener swaps calcium and magnesium for sodium but leaves total dissolved solids almost unchanged. RO removes most dissolved salts, including hardness, but wastes reject water.
References
- DuPont, Water Solutions Ion Exchange Resins Product Data and Design Guides
- BIS IS 10500:2012, Drinking Water Specification
- Water Quality Association, softener sizing practice