Total Dissolved Solids
The combined concentration of all inorganic and organic substances dissolved in water — minerals, salts, metals, and trace organics — measured in parts per million (ppm).
TDS is one of the most common single-number measures of water quality. It's reported in parts per million (ppm) — equivalent to milligrams per liter (mg/L) for dilute solutions — and represents everything dissolved in the water, not just contaminants. That includes beneficial minerals like calcium and magnesium alongside whatever else has leached in along the way.
How TDS is measured
Most consumer TDS meters infer the value from electrical conductivity: dissolved ions carry current, so more dissolved material means a higher reading. Lab measurements use gravimetric analysis (evaporate a known volume, weigh the residue) for higher accuracy.
What TDS does (and doesn't) tell you
A high TDS reading is a flag, not a diagnosis. It tells you something is dissolved in the water — but not what. Two samples at 300 ppm can have wildly different health implications: one may be naturally mineral-rich spring water, the other may carry lead, arsenic, or nitrate. Use TDS as a screening signal; pair it with targeted testing (see related contaminants) when the reading is unusual or when health risk is the question.
Typical ranges
- < 50 ppm — very low; common after reverse osmosis filtration.
- 50–300 ppm — typical municipal tap water in most of the US.
- 300–600 ppm — hard or mineral-rich; often noticeable in taste.
- > 1000 ppm — EPA secondary standard threshold (aesthetic, not health-based).