Water
Total Dissolved Solids
TDS
The combined concentration of dissolved substances in water, usually inferred from electrical conductivity. A bulk figure that says nothing about which substances are present or whether any of them matter.
In more detail
Total dissolved solids is the combined mass of everything dissolved in water — minerals, salts, some organics. It is rarely measured directly. A handheld meter reads electrical conductivity, which rises with dissolved ionic content, and applies a conversion factor to report an estimated figure in mg/L.
Two limits follow. Conductivity responds to charged species, so uncharged dissolved compounds contribute little or nothing to the reading. And the conversion factor assumes a composition, so the same conductivity can correspond to different actual TDS depending on what is dissolved.
More importantly, TDS is a total. Dissolved calcium from limestone and dissolved lead from a service line both raise it, and nothing in the number distinguishes them. The EPA lists TDS under secondary drinking water standards, with a 500 mg/L guideline based on taste and nuisance effects rather than health. Water at 400 mg/L may be entirely benign mineral content, and water at 50 mg/L can still carry a contaminant that matters at parts per billion.
Why it matters
A TDS meter is the most common piece of water instrumentation in a house and the least informative, because the number it produces is usually used to make a decision it cannot support. It is genuinely useful for one thing: watching a trend on a known supply, such as confirming an RO membrane is still rejecting. It is not a water quality test, and a house that relies on one has not tested its water.
The common misunderstanding
That a low reading means the water is safe and a high one means it is not. The figure is a bulk estimate of dissolved ionic content with no compound-level information in it. It cannot detect lead, PFAS, most pesticides, or bacteria — and it reads high on mineral-rich water that is perfectly fine to drink.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.