Hard water is a numbers problem. Here are the numbers.

Almost everything written about water softeners is about brands. The decision is actually about three figures, and once you have them the shortlist writes itself.

Grain capacity is how much hardness the unit removes between regenerations, and it is the one people get wrong. You do not want the biggest number you can afford. You want the capacity that lets the unit regenerate roughly once a week for your household and your water, because a softener that regenerates too often wears through salt and resin, and one sized far too large sits with water standing in it for days. The calculator above works this out from the two things you know, which are how many people live in the house and how hard the water is.

Salt per regeneration is what the unit costs to run, and it is the specification most often missing. Two units with the same headline capacity can differ substantially in the salt they burn to reach it, because the headline figure is measured at the maximum dose. Of the 18 units here, 16 are salt-based and only some of those publish the dose at all, which tells you something about the category on its own.

Iron tolerance only matters on well water, and then it matters more than anything else. Iron ruins softener resin that was not rated for it, and a unit with no published iron rating is not a unit that quietly handles a little iron. It is a unit whose maker never said. If you are on a well, start at the iron-rated comparison rather than the main chart.

Everything else, the warranty, the finish, the app, is a tiebreaker between units that already pass on those three. Sizing first is what stops you paying for capacity you cannot use, and it takes about twenty seconds.