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.
Important Estimate Disclaimer
The results provided by this calculator are intended as general estimates only and should not be considered precise engineering measurements. Actual material requirements, costs, and quantities can vary significantly based on site conditions, local building codes, soil type, reinforcement requirements, climate exposure, and the intended use of your project.
These calculators are designed to help you plan and budget your project more effectively, but they do not replace professional advice from a qualified contractor, engineer, or building inspector. Always verify your calculations and assumptions with trained professionals before placing material orders or commencing construction.
For more information on the assumptions, formulas, and data used in these tools, see our Data Sources and Methodology pages.
Material estimates, cost figures, and volume calculations are approximate and may not reflect your actual requirements. Always double-check dimensions and project-specific needs, and consider adding a contingency allowance for waste, over-excavation, and unforeseen conditions.
If you are unsure about any aspect of your project — including structural requirements, reinforcement needs, drainage, or load-bearing capacity — consult with a licensed professional before proceeding.