Total dissolved solids
The total mass of dissolved substances in water. TDS is the most misunderstood number in consumer water testing, because the cheap meters that report it do not measure dissolved solids at all.
Key facts
- Unit
- mg/l
- Metric
- Total dissolved solids
What it measures
Everything dissolved: minerals, salts, and small amounts of organic matter. It says nothing about what those substances are, and therefore nothing about safety.
How it is measured
Properly, by evaporating a filtered sample and weighing the residue. In practice, almost always inferred from electrical conductivity using a conversion factor between roughly 0.5 and 0.7 — a factor that depends on the ionic composition the meter cannot know.
How to read a value
| Value (mg/l) | What it indicates |
|---|---|
| Below 50 | Very low mineral content. Typical of distilled, deionised or reverse-osmosis water, and of some soft upland supplies. |
| 50 – 250 | Low to moderate. Typical of soft public supply. |
| 250 – 500 | Moderate. Typical of harder groundwater supplies. |
| 500 – 1,000 | High. Increasingly noticeable taste; the WHO acceptability guidance sits around 1,000. |
| Above 1,000 | Very high. Palatability declines; may indicate saline influence. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Geology of the catchment or aquifer
- Seawater intrusion in coastal aquifers
- Road salt and agricultural runoff
- Evaporative concentration in arid regions
Why it matters
- A low TDS reading is not evidence of purity. Reverse-osmosis water contaminated with a soluble organic compound would still read near zero
- A high TDS reading is not evidence of contamination. Mineral water prized for its composition often reads several hundred
- TDS is an aesthetic and operational parameter in regulation, not a health standard
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain