A water hardness meter is meant to tell you how much dissolved calcium and magnesium your supply carries, since that figure decides whether a softener is worth buying and how it must be set. The problem is that the cheapest devices sold under that name do not measure hardness at all, and the number they display can be badly misleading.
What hardness actually is
The USGS defines hardness as dissolved calcium and magnesium, and classes water as 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 mg/L. Those bands are the reference everything else is judged against, and they are why an exact figure matters: the difference between the top of moderately hard and the middle of hard changes both whether a softener pays and what capacity it needs. Softener settings are usually entered in grains per gallon, so expect to convert.
Why a TDS pen is not a hardness meter
The inexpensive pens sold in this category measure electrical conductivity and convert it to total dissolved solids. Conductivity responds to every dissolved ion, so sodium, chloride, nitrate, bicarbonate and sulfate all contribute to the reading alongside calcium and magnesium. Two supplies with identical readings can have completely different hardness. Worse, softened water often reads about the same as it did before treatment, because sodium replaced the calcium ion for ion, which leads people to conclude a working softener has failed.
The methods that do measure hardness
Titration is the honest domestic method: a reagent is added drop by drop until the sample changes color, and the drop count gives the hardness directly. It is cheap, quick and specific to calcium and magnesium. Test strips using the same chemistry are less precise but perfectly adequate for deciding a band and for checking that a softener is still working. Dedicated electronic hardness meters exist and are used commercially. A laboratory analysis is worth it once, before buying equipment, because it gives hardness alongside iron, pH and everything else the design depends on.
Testing so the result means something
Sample from a cold tap, with the aerator off, after running until the water is cold and steady, so you measure the supply rather than what sat in the pipes. If a softener is fitted, test both before and after it, because the pair of readings is what proves the unit is working. Retest after any change to the supply and periodically on a well, where hardness moves with the season. Our water hardness testing guide covers where to send a sample when the number is going to justify an equipment purchase.
Questions people ask about water hardness meter
Does a TDS pen measure hardness?
No. It measures conductivity from all dissolved ions, so sodium, chloride and sulfate count towards the reading alongside calcium and magnesium.
Why does softened water still read high on a TDS pen?
Because sodium replaced the calcium and magnesium ion for ion. Conductivity barely changes, which is why a working softener appears to have failed.
What is the cheapest reliable method?
A titration kit. Reagent is added drop by drop until the color changes, and the drop count gives calcium and magnesium hardness directly.
Where should I take the sample?
A cold tap with the aerator removed, after running until the water is cold and steady. Test before and after any softener to prove it is working.