Hard water is water carrying dissolved calcium and magnesium. It is not a health concern, and it is what forms scale, spots glassware and consumes soap.
The units
Grains per gallon (gpg) — the US convention, and the unit softeners are rated in.
Parts per million (ppm) or milligrams per litre, which are the same thing.
gpg = ppm ÷ 17.1
A laboratory report in ppm has to be converted before it means anything to a softener specification.
The scale
| gpg | ppm | Description |
|---|---|---|
| 0–1 | 0–17 | Soft |
| 1–3.5 | 17–60 | Slightly hard |
| 3.5–7 | 60–120 | Moderately hard |
| 7–10.5 | 120–180 | Hard |
| Over 10.5 | Over 180 | Very hard |
Softening is generally considered worthwhile above about 7 gpg, and the case strengthens sharply above 10.
How to test
A titration test kit — drops added to a sample until the colour changes, with the drop count giving the hardness. Inexpensive, quick and accurate enough for sizing.
Test strips, which are cheaper and coarser. Adequate for a rough answer, not for sizing a unit precisely.
A laboratory test, which gives hardness alongside iron, manganese, pH, total dissolved solids and anything else requested. This is the one to use on a private well.
The utility's annual report, on municipal water, which publishes average hardness — though it varies through the year and by source.
Test the right water
Before any treatment. A sample from a tap downstream of an existing softener measures the softener, not the water.
From an untreated outside tap, or from the inlet side of the equipment.
On a well, test the well. Water quality changes with the season, with the water table and with pump activity, and a single test is a snapshot.
What else to test on a well
Hardness is the least important item on a private well.
Coliform bacteria and E. coli — annually, and after any work on the well or any flooding.
Nitrate, particularly in agricultural areas and critically important where there is an infant in the house.
Iron and manganese, which affect both treatment and staining.
pH, which affects corrosion and how other treatment behaves.
Arsenic, radon, uranium, fluoride, depending on the local geology — the state or county health department publishes what is a concern in the area.
Lead and copper, from the plumbing rather than the source.
A softener addresses none of these. It removes hardness, and it is not a purifier.
Interpreting a report
A report gives numbers against limits. The two that matter most:
Anything above a health-based limit is a treatment requirement, not a preference.
Hardness, iron and pH determine what treatment equipment is appropriate and in what order — iron removal before softening, pH correction where needed.
Retesting
A private well, annually, at minimum for bacteria and nitrate, and after any change in taste, smell or appearance.
A full panel every few years, or after any nearby land use change, flooding, or work on the well.
After installing treatment, to confirm it is doing what it was bought to do — which is a step almost nobody takes, and it is the only way to know.
Where to send a sample
State-certified laboratories perform drinking water analysis, and the state or county health department publishes the list.
Many offer a well-owner package covering the common parameters at a modest price, with sample bottles posted out and specific instructions on how to collect — which matter, particularly for the bacterial sample, where a contaminated collection produces a false positive and an unnecessary alarm.
Free tests offered by companies that sell treatment equipment measure hardness and iron accurately enough and are not a substitute for a certified panel.
Softened water and sodium
Ion exchange replaces calcium and magnesium with sodium, in proportion to the hardness removed.
For most households the amount is small relative to dietary sodium. Where it is a concern — a specific medical restriction — the usual answers are a potassium chloride regenerant, or leaving the cold kitchen tap unsoftened, which is a plumbing decision made at installation.