Four free calculators for hard water: size a softener, convert hardness units, estimate salt use and look up your city’s hardness. Each one shows its formula and a worked example, and everything runs in your browser. Nothing you type is stored or sent.
Water softener size calculator
Grains per day, grains needed and the right size from 24,000 to 80,000 grains, with days between regenerations.
Water hardness converter
gpg, ppm, mg/L, Clark, German and French degrees and mmol/L, plus the USGS class.
Softener salt calculator
Pounds and bags of salt per month and year, the yearly cost and the water used to regenerate.
Water hardness by city
Official hardness from each city’s water utility, with a link to the report and the date we checked it.
Which tool to start with
Start with your hardness. Look up your city, or read it from your water quality report and convert it to gpg. Then size the softener, and finally estimate the salt it will use.
Where the default values come from
- Water use of 80 gallons per person per day: the low end of the USGS estimate of 80–100 gallons.
- Hardness classes (soft, moderately hard, hard, very hard): USGS.
- Salt efficiency of 3,350 grains per pound and 5 gallons per 1,000 grains: the NSF/ANSI 44 efficiency rating.
- Salt price: a 40 lb bag of softener salt at a major US retailer, checked on October 20, 2026.
You can replace every default with your own number.