How the calculator works
- Hardness load per regeneration = hardness (mg/L as CaCO₃) × water between regenerations (m³), in grams.
- Resin capacity for standard strong acid cation resin is about 46 g CaCO₃ per litre at 96 g salt per litre, 57 g at 160 g salt and 69 g at 240 g salt (the industry 20,000 / 25,000 / 30,000 grains per ft³ figures).
- Resin volume = load ÷ capacity, plus a 10% margin.
- Vessel: the smallest FRP vessel whose resin fills no more than about 60% of its volume, leaving freeboard for backwash. See the FRP vessel size chart.
- Salt per regeneration = resin litres × salt level.
Above about 30 m³ a day, two softeners in duplex (one in service, one regenerating) give soft water round the clock. Remove iron above about 0.3 ppm before the softener or it will foul the resin.
Related: water softener systems, FRP softener vessels, multiport valve positions.
FAQs
How much resin does a softener need?
Resin litres ≈ (hardness in ppm × litres between regenerations ÷ 1000) ÷ resin capacity in g/L. At 350 ppm and 20,000 litres a day, that is about 135 litres of resin for a daily regeneration at the standard salt level.
How much salt does a water softener use?
Between 96 and 240 grams of salt per litre of resin per regeneration. Standard industrial practice is about 160 g/L.
Does a softener reduce TDS?
No. It swaps calcium and magnesium for sodium, so TDS stays about the same. To reduce TDS you need RO or a DM plant.