Total dissolved solids measures everything dissolved in the water, in parts per million.
The range
SCA target — around 150 ppm, acceptable 75 to 250.
Under 50 ppm — too little to extract properly. Coffee tastes thin and hollow.
Over 300 ppm — harsh, chalky extraction and rapid scaling.
Why the number alone is not enough
TDS does not say what the solids are.
Water at 150 ppm of magnesium sulphate brews beautifully. Water at 150 ppm of sodium bicarbonate brews badly — same number, opposite result.
What matters is the split between:
Hardness (calcium and magnesium) — 50 to 175 ppm as CaCO₃, which does the extracting.
Alkalinity (bicarbonate) — around 40 ppm, which buffers acidity and should be low.
Sodium — low, since it contributes nothing.
Measuring it
A TDS meter costs very little and reads total dissolved solids by conductivity.
It will not tell you the composition. For that you need hardness and alkalinity test strips, which are equally cheap.
Use both.
Where it is genuinely useful
Checking a filter is working — the TDS should drop measurably after filtration.
Checking RO output — should be under 10 ppm before you remineralise.
Comparing bottled waters quickly.
The honest summary
Get TDS into range, then worry about alkalinity. Alkalinity is where most tap water goes wrong.
Last reviewed 8 August 2026