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 vessel size.
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
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 volume: 55 litres
The calculator also gives hardness removed per cycle 3 kg as CaCO₃; salt per regeneration 8.8 kg NaCl; salt per month 264 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.
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