Silo
Water treatment is sold on fear and bought on guesswork, largely because the buyer rarely has a test result in hand. Everything here starts from the opposite end: get a real number for what is in your water — free on a public supply, cheap on a well — and the equipment question becomes arithmetic.
Enter your hardness, iron and household and it returns the grain capacity as a range — because the two published conventions disagree — plus your peak flow in GPM and the exact certifications to ask a seller for in writing.
Open the calculatorOn a public supply your utility must deliver a Consumer Confidence Report by 1 July every year, covering the previous calendar year: the source of the water, the monitoring results for detected contaminants and any violations. Your hardness figure is often in it, and it costs nothing.
On a private well nobody is testing but you. EPA states that private wells are not regulated federally under the Safe Drinking Water Act and that the owner is responsible for the water being safe, and it recommends testing annually for total coliform bacteria, nitrates, total dissolved solids and pH.
What we will not do is guess your hardness from your ZIP code. The lookup tools that offer this are mostly reading the USGS national hardness map, and USGS says on that same page that the map rests on 1975 sampling and should not be used to decide for an individual household. A map is a fine way to know roughly what to expect and a bad way to size a tank.