The bookkeeping
A softener is sized by matching three quantities:
1. The hardness load — how many grams of hardness (as CaCO₃) arrive per day:
Load (g/day) = daily water use (m³/day) × hardness (mg/L, as CaCO₃)
A guesthouse using 4 m³/day of 300 mg/L borehole water carries 1 200 g of hardness a day. (Convert the analysis to mg/L as CaCO₃ first — the hardness converter does this.)
2. The resin capacity — how much hardness a litre of resin holds between regenerations. This is not a fixed property: it depends on the salt dose used to regenerate (see below). Practical working figures run around 45 – 65 g CaCO₃ per litre of resin at typical doses — use the resin datasheet's curve for real work.
3. The regeneration interval — how often the system may sensibly regenerate. Every 2 – 4 days is a comfortable design point: daily regeneration wastes salt and water on a bed barely exhausted at the margin, while stretching past 4 – 6 days risks hardness bleed near exhaustion and long stagnant intervals.
Then: resin volume = load per interval ÷ capacity. The guesthouse at a 3-day cycle needs 3 600 g ÷ ~55 g/L ≈ 65 L of resin, taking 55 g/L as the mid-dose working capacity from above — read your own resin's curve at the salt dose the valve is actually set to before committing to a vessel. That is not a 10" × 54" vessel: its ~65 L gross volume holds only about 37 L of resin once the 75% freeboard the resin needs for backwash expansion is allowed. Nor is it a 13" × 54" — ~108 L gross works out at about 62 L of resin, still short of the 65 L the arithmetic asked for, and a bed you have already sized short has no margin left for a fouling day. The next size up, a 14" × 65" (~152 L gross, ~87 L of resin), carries it with room to spare. Check any vessel in the vessel media fill calculator rather than assuming a nominal size fits.
The salt-dose economics
Salt dose is set on the valve, in grams of salt per litre of resin, and it buys capacity at falling efficiency: a low dose (~80 g/L) restores perhaps two-thirds of the capacity a high dose (~200+ g/L) would, but uses far less than two-thirds of the salt. High doses chase the last capacity expensively; low doses regenerate cheaply but more often. For most duty a middling dose (100 – 160 g/L) is the sensible economic setting — and it is worth checking, because valves ship at factory defaults nobody revisits, and salt is the softener's entire running cost.
The two checks that catch bad sizing
- Service flow check. Resin needs contact time: keep the duty flow within roughly 15 – 30 bed volumes per hour (our 65 L bed: about 1 – 2 m³/h continuous). A softener sized purely on daily volume can still be undersized for the peak flow — hardness slips through a bed run too fast, even a freshly regenerated one. Size the resin volume for whichever is larger: the capacity arithmetic or the peak-flow requirement.
- Valve match check. The valve's flow rating must cover the peak, its brine system must be configured for the actual resin volume, and the drain must accept the backwash flow. An oversized bed behind an undersized valve is still an undersized softener.
Duplex or simplex?
If hard water during a regeneration cycle is acceptable (homes, most commercial), a simplex unit regenerating at 02:00 is fine. Where softened water must be continuous — boiler feed, process, hospitality laundry — specify duplex (twin alternating vessels) rather than stretching one large vessel, and gain a second benefit: volumetric switching between vessels tracks actual consumption exactly.
Rule of thumb: litres of resin ≈ (daily use in m³ × hardness in mg/L × days between regenerations) ÷ 55 — where 55 g/L is a mid-range working capacity for a salt dose around 100–160 g/L, not a property of the resin. Take the real figure off your resin's own capacity-against-salt curve; at a low dose it can be a third less, and sizing on 55 when the site regenerates at 80 g/L undersizes the bed. Then verify the peak flow stays under ~30 bed volumes per hour and the valve matches the bed. If the two sizings disagree, the peak flow wins.