Minerals
Tea books quote a scatter of different mineral targets for brewing water, and they do not agree with each other because they are not measuring the same thing. Some are counting total dissolved solids, the sum of everything ionic in the water; others mean calcium and magnesium hardness specifically; and at least one figure in wide circulation traces to no verifiable source at all. A cheap TDS pen gives you a single number for all of it, which tells you less than it looks like it does.
What it is
A TDS pen does not weigh anything dissolved in your water. It measures electrical conductivity and multiplies by an assumed conversion factor to estimate total dissolved solids in mg/L; the SCA's own brewing standard specifies a '4-4-2 conversion' for exactly this reason. Different dissolved ions conduct electricity differently per unit of mass, so an identical conductivity reading maps to a different true TDS depending on what is actually dissolved. And because TDS is a single summed figure, it cannot tell 130 ppm of calcium and bicarbonate apart from 130 ppm of sodium and chloride, two waters that behave nothing alike in a pot of tea.
What it does to your tea
Franks et al. (2019) measured individual ions rather than a summed TDS figure, deliberately, and found harder tap water suppressed EGCG extraction in green tea by roughly half compared with low-mineral water; black tea was barely affected either way. That is a mineral effect measured directly, not inferred from a TDS reading. The two ions doing most of the work, calcium and bicarbonate, are covered on their own pages (hardness and alkalinity); TDS as a single number cannot isolate either one, which is the practical reason a hardness or alkalinity reading tells you more about tea than a TDS reading does.
How to find out what yours is
A TDS pen is cheap and instant, and fine for noticing when your water supply has changed, but read the number as a rough index rather than a precise one, for the reasons above. For tea specifically, a hardness test strip and your utility's public water quality report, which list calcium, magnesium and alkalinity separately, tell you more than a single TDS figure. Bottled water usually prints the individual mineral breakdown on the back label; that is worth reading over any TDS claim on the front.
What to aim for
The SCA's Water for Brewing Specialty Coffee standard (2009) is the only widely quoted figure that traces to a real primary document: calcium hardness at 68 mg/L as CaCO₃ (17 to 85 mg/L range), alkalinity at 40 mg/L, pH near 7.0. A figure of 50 to 150 ppm TDS, often attributed to the Tea Association of the USA, does not appear anywhere in that association's own published brewing material, and its actual source cannot be traced. It is still worth knowing, since you will see it everywhere, but treat it as a widely repeated number rather than a standard anyone actually set.
What you can do about it
If a bottled or filtered water's label prints an actual mineral breakdown, calcium, magnesium, bicarbonate and sodium in mg/L, trust that over any TDS figure, front-label or pen-measured, since it tells you what is dissolved rather than just how much. The same levers used for hardness and alkalinity work here too: reverse osmosis or distillation strips almost everything out, and blending that back with tap water at different ratios lets you dial the total up again. Some brewers remineralise stripped water with a measured mineral product rather than guessing at a target from a TDS reading alone.
What people get wrong
It is tempting to treat a TDS pen reading as the one number that decides whether water suits tea. It measures how much is dissolved, not what: 130 ppm of calcium and bicarbonate, generally good for tea, reads about the same as 130 ppm of sodium and chloride, which behaves nothing like it. Two waters can carry an identical TDS number and produce a completely different cup. A hardness reading and an alkalinity reading, taken separately, get you closer to predicting what happens in the pot than TDS ever will.
Sources
- TDS pens measure conductivity and apply an empirical conversion factor ('4-4-2 conversion') Specialty Coffee Association, Water for Brewing Specialty Coffee standard, v.21NOV2009A
- harder tap water suppressed EGCG extraction in green tea by roughly half; black tea barely affected Franks, M., Lawrence, P., Abbaspourrad, A. & Dando, R. (2019). Nutrients 11(1), 80
- SCA calcium hardness 68 mg/L (17 to 85 range), alkalinity 40 mg/L, pH near 7.0 Specialty Coffee Association, Water for Brewing Specialty Coffee standard, v.21NOV2009A
- 50 to 150 ppm TDS figure attributed to the Tea Association of the USA does not appear in their own material Widely attributed to the Tea Association of the USA; the claim could not be traced to any document the association has published