Understanding water hardness

Hardness is the total dissolved calcium and magnesium in water. Both are picked up as groundwater moves through limestone, chalk and dolomite, which is why hardness is a matter of local geology rather than anything wrong with a well.

It is not a contaminant and there is no EPA limit for it, primary or secondary. Hard water is not a health problem — if anything the minerals are mildly beneficial, and some studies associate hard water areas with marginally better cardiovascular outcomes.

What hardness does is chemical. Calcium and magnesium react with soap to form the insoluble scum that leaves rings on baths and dulls laundry, and they precipitate out of hot water as scale on heating elements and inside pipes. Everything people dislike about hard water follows from those two reactions.

Two units are in common use and they produce very different-looking numbers for the same water, which is the single most common source of confusion.

Grains per gallon (gpg) is the American convention and what equipment is sized in. Milligrams per litre (mg/L) is the scientific one, and is identical to parts per million (ppm) for drinking water. Both express hardness as calcium carbonate equivalent.

The conversion is a single number: 1 grain per gallon = 17.1 mg/L. So divide mg/L by 17.1 to get grains, or multiply grains by 17.1 to get mg/L. Water at 200 mg/L is 11.7 gpg — hard, but nothing unusual.

• mg/L to grains: divide by 17.1
• Grains to mg/L: multiply by 17.1
• mg/L and ppm are the same thing here
• Some labs report calcium and magnesium separately; total hardness is what equipment is sized on

The standard classification, used by the USGS and the water treatment industry:

• 0–1 gpg (0–17 mg/L) — soft. Nothing to do.
• 1–3.5 gpg (17–60 mg/L) — slightly hard. Rarely noticed.
• 3.5–7 gpg (60–120 mg/L) — moderately hard. Some spotting, mild scale over years.
• 7–10.5 gpg (120–180 mg/L) — hard. Visible scale, soap struggles, softening starts to pay.
• Above 10.5 gpg (180+ mg/L) — very hard. Scale accumulates quickly and appliance life is measurably shorter.

Around 7 grains per gallon is where the economics start to work, and it is worth being honest that below this it is a comfort decision rather than a plumbing one.

Above 7 gpg, scale builds on water heater elements and inside pipes at a rate that measurably shortens equipment life and costs a little efficiency. Add the soap, shampoo and detergent you stop over-using and the payback lands in the range of a few years.

Between 3 and 7 gpg, a softener will make a noticeable difference to how things feel and look, but it will not save you money. Below 3 gpg it is difficult to justify at all, and anyone selling you one at that level should be asked what problem it is solving.

Iron changes this calculation. Even modest hardness with iron often justifies a softener, because it deals with both — up to about 3 mg/L of iron, above which the iron needs its own filter.

Size a softener for your hardness→

Hardness is one of the analytes where a home test is genuinely adequate. Strips and titration drop kits are accurate enough to size equipment with, and being able to retest freely is useful — particularly after fitting a neutraliser, which raises hardness as it works.

Test at a cold tap upstream of any treatment, and if you already have a softener, test both the inlet and the outlet. A softener outlet should read close to zero. If it does not, the unit is not regenerating properly, is out of salt, or is fouled — most often with iron.

If you are on a public supply for comparison, the utility publishes hardness in its annual water quality report. On a well, nobody publishes anything, so testing is the only way to know.

What else to test for→