A TDS meter does not measure total dissolved solids directly. It measures how well water conducts electricity and converts that into an estimated number, because dissolved salts and minerals carry current. The scale therefore tells you how much is dissolved in the water, not what it is, and that distinction is the whole point of understanding the reading.
How to read the scale honestly
A reading near zero means almost nothing dissolved, which is what you would expect from distilled or reverse osmosis water. A middling reading is normal municipal or well water carrying ordinary mineral. A high reading means a lot is dissolved and is worth investigating. What no reading tells you is whether any of it is harmful, because harmless calcium and hazardous sodium or nitrate all push the number in the same direction and the meter cannot tell them apart.
Why a low number is not a certificate
Some of the contaminants households worry about most barely move the meter. Bacteria carry no charge worth speaking of, and many organic compounds are similarly invisible to a conductivity probe. So water can read reassuringly low and still be unfit, while water that reads high may be perfectly good water from a mineral-rich aquifer. Treat the meter as a change detector, useful for checking that a reverse osmosis membrane is still working, and not as a safety test.
Where hardness sits on the scale
Much of a typical household reading is simply hardness. The USGS defines hardness as dissolved calcium and magnesium, and bands it 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 that. Those minerals push a TDS reading up while causing nothing worse than scale and spotted glassware. A hardness test tells you far more about that complaint than any conductivity number, and our best TDS meter guide explains what the instruments do measure well.
Questions people ask about tds scale
What is a good TDS reading for drinking water?
There is no single good number. Perfectly safe supplies vary widely by geology, and the reading says nothing about which dissolved substances are present.
Can a TDS meter detect lead or bacteria?
No. Lead appears at concentrations far too small to shift the reading, and bacteria do not conduct meaningfully. Both need proper laboratory testing.
What is a TDS meter actually good for?
Checking a reverse osmosis membrane. Compare the feed water reading to the product water reading over time; a rising product reading means the membrane is tiring.