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Water Softener Sizing Calculator

Grains per day, recommended capacity, and how often it regenerates. Every assumption is shown and editable.

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Grains per day

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Recommended capacity

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Regenerates about every

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at a salt-efficient setting

Hardness check

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How the math works

  1. Compensated hardness = hardness (gpg) + iron (ppm) × 4. Rule-of-thumb iron adders range from about 3 to 5 gpg per ppm; we use 4. High iron (several ppm) usually needs a dedicated iron filter, so ask a water-treatment professional.
  2. Grains per day = people × gallons per person × compensated hardness.
  3. Capacity needed = grains per day × 7 days ÷ 0.67. We assume you run the softener at a salt-efficient setting that uses about 67% of its maximum rated capacity, and target about 7 days between regenerations. Both are our assumptions.
  4. We round up to a common size (24k, 32k, 40k, 48k, 64k, 80k, 96k, 110k grains). Brands label sizes differently, so compare against each maker's own rating.

Hardness classes follow the USGS (checked 2026-10-19): 0 to 60 mg/L soft, 61 to 120 moderately hard, 121 to 180 hard, over 180 very hard. The 17.1 ppm per gpg factor is a unit conversion (1 grain = 64.8 mg; 1 US gallon = 3.785 L).

Also check flow rate: a softener must handle your peak gallons per minute, which makers list next to capacity. Read the step-by-step in How to Size a Water Softener.

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