Water Softener Sizing — How This Calculator Works
Sizing a softener is about matching usable resin capacity to your household’s daily grain load while hitting a reasonable regeneration interval (often 7 days). We adjust hardness for iron and manganese (rules of thumb: +4 gpg per 1 ppm iron, +2 gpg per 1 ppm manganese), then estimate daily grains from people × use per person. The calculator scans common resin sizes (0.75–2.5 cu ft) and picks the smallest size that meets your target days after subtracting a reserve.
Choosing a Salt Dose
Salt dose trades off capacity and efficiency. Lower doses (6–8 lb/ft³) give the best grains per pound but regenerate slightly more often; higher doses (10–15 lb/ft³) push maximum capacity but use more salt.
- 6 lb/ft³ → ~22k grains/ft³ (≈3,600 grains/lb)
- 8 lb/ft³ → ~26k grains/ft³ (≈3,250 grains/lb)
- 10 lb/ft³ → ~29k grains/ft³ (≈2,900 grains/lb)
- 15 lb/ft³ → ~31k grains/ft³ (≈2,100 grains/lb)
8% vs 10% Resin
On chlorinated city water, 10% crosslink resin often lasts longer. Use 8% on most private wells unless chlorine is added. For high iron, consider pre‑filtration or specialty resin.
When to Consider a Twin (Duplex) Softener
If your expected regeneration interval falls under ~3 days, or your peak flow demand is high compared to the resin’s service flow rating, a twin‑alternating system keeps softened water online 24/7 and improves efficiency.
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Disclaimer: Planning estimates only. Follow manufacturer instructions and local plumbing codes, and consult a certified water treatment professional for final selection.