What water hardness means: GH, KH, TDS and ppm

Hardness is a measure of how much calcium and magnesium the water carries. USGS says hardness is caused by compounds of calcium and magnesium, and by a variety of other metals¹. A water report may show it under several names and units, which is where the confusion starts.

ppm and mg/L

Both are ways to say how much of something is dissolved. One ppm, or part per million, is usually defined as one milligram per liter of calcium carbonate when talking about hardness². So a report that says "100 mg/L as CaCO3" and one that says "100 ppm" are generally giving you the same figure².

Careful, though. Hardness isn't only given in ppm. Reports and manuals also use degrees of general hardness, German degrees, grains per gallon, English degrees and French degrees². One grain per gallon works out to 17.118 ppm².

GH and KH

GH stands for general hardness. It's one of the units used for the calcium and magnesium in water². Ask what the number refers to before you compare it with another.

KH stands for carbonate hardness. It measures the hardness caused by carbonate and bicarbonate in the water³. One degree KH equals about 17.848 ppm of calcium carbonate³.

KH is easy to confuse with alkalinity, a related number. The two don't always match when both are written in ppm³.

TDS

TDS means total dissolved solids. It counts all the inorganic and organic substances dissolved in the water, not only calcium and magnesium⁴. It's often given in ppm⁴.

That makes TDS a different question from hardness. TDS adds up everything that's dissolved. Hardness counts mainly calcium and magnesium⁴¹.

A TDS meter works from conductivity. Wikipedia says conductivity gives an approximate value for TDS, and it puts the accuracy at around 10 percent when compared with lab results⁴.

The USGS classes

USGS gives general guidelines for sorting water by hardness, in milligrams per liter as calcium carbonate¹.

These are general guidelines for water, not espresso rules. An espresso maker may set its own limits. Breville, for one, says water recommendations can vary by machine and local water hardness⁵.

What makers ask for

Some makers talk about hardness, some about TDS. Breville's support page suggests water that ideally measures between 50 and 150 ppm in dissolved solids, naming soft filtered water, spring water and well water as examples⁵. That's a TDS figure, not a hardness class.

Decent Espresso uses TDS too, and sets its softest-water threshold at about 30 ppm TDS⁶. The number only helps when you match it to the same kind of measurement.

Exceptions

Boiling doesn't fix everything. Wikipedia says permanent hardness is generally difficult to remove by boiling². Don't count on a kettle to change the number on your report.

Your own number comes from your supplier, as the finding your hardness page explains.