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.
Service Flow — A Note on “Rules of Thumb”
If you enter peak flow, we compare it to a very rough rule of thumb service-flow estimate (≈ 7 gpm per 1.0 cu ft of resin). Actual service flow depends on tank size, valve, plumbing, pressure drop, and manufacturer ratings—always confirm with product specs.
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 system’s service flow rating, a twin-alternating system can keep softened water online 24/7 and improve convenience.
Related Home System Calculators
Frequently Asked Questions
What size water softener do I need?
Use a water test for hardness, iron, and manganese. This calculator adjusts hardness (Hadj = H + 4×Fe + 2×Mn), estimates daily grains from people × water use, and chooses the smallest resin size that meets your target regeneration interval after reserve capacity.
How often should a softener regenerate?
A common target is about 7 days. Shorter intervals can increase salt and water use, while very long intervals may reduce efficiency depending on the system.
How much salt will I use per year?
Annual salt is estimated from salt per regeneration (resin volume × salt dose) and regenerations per year (365 ÷ expected days). Divide by 40 to estimate 40-lb bags/year.
Do I need an iron filter?
If iron is high (often above ~2 mg/L or ppm), a dedicated iron filter or specialty media is commonly recommended. Confirm with manufacturer guidance and a water professional.
Important Estimate Disclaimer
These calculations are general estimates only and should not be used as final system design specifications. Actual sizing may vary based on demand requirements, code compliance, and manufacturer guidelines.
See our Methodology and Data Sources for calculation details.
Mechanical, plumbing, and electrical systems should be verified by qualified professionals prior to installation.