Total Dissolved Solids—what it means and why it matters.
TDS (Total Dissolved Solids) measures everything dissolved in your water: minerals, salts, metals, and organic compounds. It's measured in parts per million (ppm) or milligrams per liter (mg/L)—they're equivalent.
TDS is not a measure of harmful contaminants specifically. It includes beneficial minerals (calcium, magnesium, potassium) alongside less desirable substances.
| TDS (ppm) | Classification | Notes |
|---|---|---|
| 0-50 | Very low | RO water, distilled water |
| 50-150 | Low | Typical of soft water areas |
| 150-300 | Moderate | Average tap water |
| 300-500 | High | Hard water, may taste mineral-y |
| 500+ | Very high | May affect taste significantly |
EPA recommends TDS below 500 ppm for drinking water, but this is a secondary standard (aesthetic, not health-based).
Not necessarily. High TDS often just means mineral-rich water. Many bottled mineral waters have TDS of 300-500+ and are marketed as healthy.
High TDS may be a problem if:
High TDS is usually fine if:
TDS meters are cheap ($10-20) and easy to use. Just dip the probe in water and read the number. However, remember that the number alone doesn't tell you if water is safe—just how much is dissolved in it.
Reverse osmosis is the primary way to reduce TDS. RO systems remove 90-99% of dissolved solids. Carbon filters do NOT significantly reduce TDS—they remove different contaminants.
Distillation also reduces TDS to near zero, but is impractical for home use.
My TDS meter shows 200 ppm—do I need an RO system?
Probably not. 200 ppm is normal and likely consists of harmless minerals. Unless your water tastes bad, you have specific contamination concerns, or you need very pure water, a standard carbon filter is probably sufficient.
Why does my RO water have TDS of 20-30 instead of 0?
RO membranes aren't 100% effective, and this is normal. Some minerals pass through. TDS under 50 from an RO system is typical and fine.
Is zero TDS water safe to drink?
Yes, but it may taste flat. Some prefer to add a remineralization stage to RO water for better taste and to add back beneficial minerals.