Do you need one at all

Minnesota Department of Health puts the line at greater than 7 grains per gallon or 120 mg/L.

That is a comfort, scale and appliance-life threshold. It is not a health threshold — hard water is not a health hazard, and anyone selling softening on health grounds is selling something else.

For context, USGS classifies hardness as soft to 60 mg/L, moderately hard to 120, hard to 180, and very hard above that. The softening threshold sits exactly at the boundary between moderately hard and hard.

Checking that threshold yourself

The two units get used interchangeably and rarely explained. Here is where the conversion comes from, using NIST’s published factors:

1 grain      = 64.79891 mg     (NIST SP 811, grain to milligram)
1 US gallon  = 3.785412 L      (NIST SP 811, gallon to litre)

1 grain per gallon = 64.79891 mg / 3.785412 L = 17.118 mg/L

So 120 mg/L divided by 17.118 is 7.01 grains per gallon — which reproduces Minnesota Health’s 7 gpg figure exactly. The two numbers in that guideline are the same number in different units, and now you can prove it rather than assume it.

What softening actually does

It is an exchange, not a removal. Minnesota Health: “Resin beads inside the softener trap the calcium and magnesium and exchange them for sodium or potassium.”

Something comes out and something goes in. That has two consequences most product pages skip:

  1. Softened water carries added sodium. If anyone in the household is on a sodium-restricted diet, Minnesota Health’s suggestion is to “regenerate your softener with potassium chloride instead of sodium chloride (salt).”
  2. Minnesota Health recommends you keep an un-softened tap for cooking and drinking. That advice appears in a state health department FAQ and almost never in a softener listing, for obvious reasons.

Sizing

Capacity is measured in grains of hardness removed between regenerations.

capacity = hardness (gpg) x 75 gallons per person per day x people x days per cycle
Household Hardness Grains/day 7-day cycle Buy
2 people 10 gpg 1,500 10,500 16,000 grain
4 people 15 gpg 4,500 31,500 32,000 grain
4 people 25 gpg 7,500 52,500 64,000 grain
6 people 20 gpg 9,000 63,000 64,000 grain

Longer cycles need more capacity but use less salt per grain removed. Undersizing is the more expensive mistake: a unit regenerating daily wears out its resin and its valve far sooner than the rating suggests.

Before you size anything: two disqualifiers

Penn State Extension is specific about when a softener is the wrong purchase. Softeners are recommended only where pH is above 6.7, hardness is between 3 and 20 gpg, and dissolved iron is below 5 mg/L.

And the failure that ends softeners early: “Oxidized forms of iron and manganese will foul the softener resin.” If your iron is already oxidised — particles visible the moment you draw a glass — it must be removed upstream. The iron page has the test that tells you which form you have.

Where iron is going through the resin, add roughly 5 grains of capacity per gallon for every 1 ppm of dissolved iron. At 2 ppm that is the equivalent of another 10 gpg of hardness, which is usually the argument for removing it first instead.

Salt-free alternatives

A salt-free “softener” is a different product with a different objective, and there is an objective test for the distinction — NSF/ANSI 44 is scoped to salt-regenerated cation exchange, so a saltless device falls outside it structurally. → Salt-free vs salt-based, and the certification test

Where it sits in the system

Fourth. After pH correction, after iron and sulfur removal, before UV. The full sequence and the reasoning is worked through in the sizing method.