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Water softener sizing calculator

Four inputs, one answer, and the math shown so you can check it against any plumber's quote. No data leaves your browser.

Every person uses about 75 gallons per day.

Each value is read straight from the utility's latest water quality report — sources and check dates on the hardness index.

Pick your city above to prefill this, or enter the value from your utility's annual report.

Each ppm of iron adds roughly 4 gpg of load on the resin.

Formula & constants documented in our methodology.

Systems that fit this result

Enter your household and hardness values above to see matching systems.

How this calculation works

How the four inputs become a capacity numberPeople4 × 75 galHardness14 gpgRegen every7 daysDaily demand300 gal/dayInterval load29,400 grains+ 25% margin37kgrainscapacityHow the four inputs become a capacity numberPeople4 × 75 galHardness14 gpgRegen every7 daysDaily demand300 gal/dayInterval load29,400 grains+ 25% margin37kgrains capacity

Daily softening demand = household members × 75 gallons × compensated hardness (your gpg plus an adjustment for dissolved iron). Multiply by days between regenerations, then add a 25% safety margin so the resin never exhausts early. A properly sized system regenerates about once a week; much more often means undersized, much less often means you're paying for idle capacity.

The full formula, constants, and their sources are documented in our methodology. If a plumber's quote disagrees with this math, ask them which assumption differs.

Not sure whether to soften at all? Read salt vs. salt-free, or see the full best water softener ranking.

Is your water hard?

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