Hardness is the number people most often have an opinion about and least often actually know. It arrives in units nobody uses in conversation, it has no health limit to anchor it, and the only people offering to interpret it are usually selling a softener.
Here is the number, plainly.
What hardness is
Dissolved calcium and magnesium, picked up as groundwater moves through limestone, chalk and dolomite. That is all of it. It is not a contaminant, it is not a sign of pollution, and across much of the country it is simply what the local rock is made of.
Because it is minerals rather than a hazard, there is no EPA limit, primary or secondary. Nothing is being violated at any concentration.
The two units, and the conversion
This is where reports lose people.
- mg/L as calcium carbonate (CaCO₃) — what the lab usually prints
- Grains per gallon (gpg) — what softener equipment is specified in
gpg = mg/L ÷ 17.1
That factor is exact rather than a rounding: one grain in one US gallon is 17.1 mg/L. So 342 mg/L is 20 gpg, and 120 mg/L is 7 gpg.
If a supplier quotes in one unit and your report is in the other, convert before comparing. Confusing them by a factor of seventeen is a remarkably easy way to buy the wrong machine.
The classification scale
| Classification | mg/L as CaCO₃ | gpg |
|---|---|---|
| Soft | 0 – 60 | 0 – 3.5 |
| Moderately hard | 61 – 120 | 3.5 – 7 |
| Hard | 121 – 180 | 7 – 10.5 |
| Very hard | over 180 | over 10.5 |
Seven grains per gallon is the line that matters in practice. It is where "hard" begins on the scale, it is roughly where households start noticing, and it is also the maximum feed-water hardness for UV disinfection — above it, scale films the quartz sleeve and blocks the light while the lamp keeps glowing.
What hardness actually costs you
Not your health. Your equipment and your patience.
- Scale in anything that heats water. Heater elements, pipe interiors, dishwashers, kettles. Scale insulates, so the heater works harder for the same result and fails sooner. The effect accelerates with temperature, which is why the water heater goes first.
- Soap that will not lather, and the grey film on the bath.
- Spotting on glassware and fixtures.
- Stiff laundry and colours that dull.
- Restricted flow in old pipework over decades.
When softening is worth it
Below about 7 gpg (120 mg/L): usually leave it. Few households see real problems, and softening is not free — salt, regeneration water, equipment, plus added sodium.
7 to 10 gpg: judgement. Depends how much the soap scum bothers you and whether you have had heater trouble.
Above 10 gpg: the appliance case is usually clear-cut, and the payback in heater life alone is real.
Two cautions either way:
Softened water is slightly more corrosive. If you already have low pH or blue-green staining, fix the acidity first — softening alone can nudge lead and copper leaching upward.
Softening adds sodium in proportion to the hardness removed. On very hard water that becomes a real number, which is why potassium chloride exists for sodium-restricted households, at several times the cost.
Sizing, if you go ahead
Do not accept a size without the arithmetic. Four multiplications and one correction that most online calculators leave out — iron, because softener resin removes iron along with hardness and pays capacity for the privilege.
Our softener sizing guide has worked examples. The short version: hardness in gpg, plus 5 gpg for every mg/L of iron, times people, times 75 gallons a day, times 7 days between regenerations, rounded up to the next stocked size.
Any supplier who sizes a softener without asking for your iron number is guessing.
Measure it, do not estimate it
Hardness is cheap and sits in almost every comprehensive panel. Test strips give a usable ballpark for nothing.
What you want on record is the number in both units, your iron and manganese beside it, and your pH — because those four together determine not just the size of a softener but whether a softener is the right machine at all.
Frequently Asked Questions
What is a good hardness level for well water? There is no health-based standard — hardness is not harmful to drink. The USGS classification puts soft at 0 to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180, and very hard above 180. Households generally start noticing scale and soap problems above about 7 grains per gallon, which is 120 mg/L.
How do I convert mg/L hardness to grains per gallon? Divide by 17.1. A hardness of 342 mg/L as calcium carbonate is 20 grains per gallon. The conversion is exact by definition: one grain per US gallon is 17.1 mg/L.
Is hard water bad for you? No. Hardness is dissolved calcium and magnesium, both dietary minerals, and there is no evidence that drinking hard water is harmful. The case for softening is about plumbing, appliances and laundry, not health.
Does hard water damage appliances? It shortens the life of anything that heats water. Scale forms on heater elements and inside pipes, cutting efficiency and eventually causing failure. Water heaters, dishwashers, washing machines and kettles all suffer, and the effect accelerates with temperature.
Should I soften water that is only moderately hard? Often not. Below about 7 grains per gallon most households see few problems, and softening costs money to run, adds sodium, and makes water slightly more corrosive. Between 7 and 10 it is a judgement call. Above 10 the appliance case is usually clear.