Water Softener Calculator

How this calculator works

A softener swaps the calcium and magnesium that make water hard for sodium, held on beads of ion-exchange resin. Each litre of resin can only hold so much hardness. When it is full, the softener regenerates: it flushes the resin with brine (salt water) to strip the hardness off, then rinses and goes back into service.

Everything in this calculator follows from three numbers: how much resin, how much salt per regeneration, and how hard the water is.

Capacity per regeneration

Resin capacity depends on the salt dose. More salt recovers more capacity, with diminishing returns:

Salt dose	Salt per litre of resin	Capacity (g CaCO₃ per litre)
Economy	80 g	about 42 g
Standard	120 g	about 52 g
High capacity	160 g	about 59 g

These are typical figures for a standard strong-acid cation resin. Your resin’s datasheet has the exact curve.

We hold back a 15% reserve, so hardness never breaks through before the softener regenerates:

Capacity per cycle (g) = resin (L) × capacity (g/L) × (1 − reserve)

OBR: output between regeneration

OBR (L) = capacity per cycle (g) × 1,000 ÷ hardness removed (mg/L)

Hardness is entered as mg/L (ppm) as CaCO₃ — the unit Indian water reports use. If your report is in grains per gallon, multiply by 17.1. For German degrees, multiply by 17.8; for French degrees, by 10.

Worked example

A home of five on borewell water at 400 ppm, with a 25 L softener on a standard dose:

Step	Value
Capacity per cycle	25 × 52 × 0.85 = 1,105 g
OBR	1,105 × 1,000 ÷ 400 = 2,762 L
Softened water per day	5 people × (135 − 45 L flushing) = 450 L
Regenerates	2,762 ÷ 450 = every 6.1 days
Salt per regeneration	25 × 120 g = 3 kg
Water per regeneration	about 6 bed volumes = 150 L

How often it regenerates — and how to set the valve

Days between regenerations = OBR ÷ softened litres per day

• Meter (volume) valves regenerate when the set volume has passed through. Set them a little under the OBR. They suit homes and buildings where daily use varies.
• Time-clock valves regenerate every N days, usually at 2 AM when nobody is drawing water. Set N to the whole number of days the OBR lasts, rounded down.
• Never let resin sit more than 7 days. Even if the capacity would last longer, a weekly regeneration keeps the resin bed from compacting and growing bacteria.
• More than one regeneration a day means the softener is undersized, and the building gets hard water while it regenerates. Use more resin, or a twin-vessel softener that alternates so one is always in service.

Flow: the second limit

Hardness has to stay in contact with the resin long enough to be exchanged. At peak flow, keep the water passing through at no more than about 20 bed volumes an hour:

Minimum resin (L) = peak flow (L/h) ÷ 20

A home drawing 1,500 L/h at peak needs at least 75 L of resin for flow, even if 25 L would cover the hardness. The calculator checks both limits and recommends the larger.

Right-sizing a new softener

Resin needed (L) = daily load (g) × days between regenerations ÷ (capacity × (1 − reserve))

where the daily load is softened litres × hardness (mg/L) ÷ 1,000. The calculator rounds up to the next standard vessel: 844, 1044, 1054, 1252, 1354, 1465 and larger.

What softening doesn’t fix

A softener removes hardness: the scale on taps, tiles, geysers and heat pump heat exchangers. It does not lower total dissolved solids, remove iron above a few mg/L, or disinfect. If your water report shows high TDS, iron or bacteria, the treatment train needs other stages — filtration, RO or UV — ahead of or instead of a softener.