What Is TDS?
TDS (Total Dissolved Solids) measures the combined concentration of all inorganic and organic substances dissolved in water — minerals, salts, metals, and small amounts of organic matter. TDS is measured in parts per million (ppm) or milligrams per liter (mg/L).
A TDS meter works by measuring the electrical conductivity of water. Pure water is a poor conductor; dissolved minerals increase conductivity. The meter converts this reading to a TDS value.
Important Limitation: TDS meters cannot distinguish between harmful contaminants (lead, arsenic) and beneficial minerals (calcium, magnesium). A TDS reading alone tells you HOW MUCH is dissolved, not WHAT is dissolved.
TDS Level Guide
| TDS Level (ppm) | Classification | What It Means |
|---|
| 0-50 | Very low | Distilled or RO water. May taste flat. |
| 50-150 | Low | Carbon filtered water. Good taste. |
| 150-300 | Moderate | Most tap water. Generally acceptable. |
| 300-500 | Hard | Noticeable mineral taste. Some scale. |
| 500-1,000 | Very hard | Strong taste, significant scale. |
| 1,000+ | Unsafe | EPA recommends <500 ppm for drinking. |
Expected TDS Readings by Filter Type
| Water Source | Typical TDS |
|---|
| Distilled water | 0-5 ppm |
| Reverse osmosis | 5-50 ppm |
| Brita/pitcher filtered | Unchanged (removes only chlorine) |
| ZeroWater pitcher | 0-5 ppm |
| Tap water (US average) | 200-400 ppm |
| Well water | 100-800+ ppm |
| Bottled spring water | 50-300 ppm |
| Bottled mineral water | 200-1,000+ ppm |
How to Use a TDS Meter
- Remove the protective cap from the meter
- Turn on the meter (usually a single button)
- Immerse the probe in water up to the indicated line
- Gently stir to remove air bubbles
- Wait for the reading to stabilize (3-10 seconds)
- Record the reading in ppm
- Rinse the probe with clean water and replace the cap
Calibrate your meter monthly using a calibration solution (342 ppm NaCl standard). Cheap meters can drift significantly over time.
When TDS Readings Are Useful
- RO membrane check: Compare input TDS to output TDS. Rejection rate should be >90%. If it drops below 85%, replace the membrane.
- Filter saturation: If filtered water TDS starts rising, carbon filter may be exhausted.
- Water softener check: Hard water has higher TDS than soft water (mostly calcium/magnesium).
When TDS Readings Mislead: A TDS meter will NOT detect: bacteria, viruses, cysts, pesticides, VOCs, chlorine, chloramine, petroleum products, or pharmaceutical residues. For these, you need specific tests or certified filtration.
Frequently Asked Questions
Is low TDS water bad for you?No. Low TDS water (0-50 ppm) is safe to drink. The WHO states that water with TDS <300 ppm is "excellent" and 300-600 ppm is "good." Very low TDS may taste flat but is not harmful.
Can TDS meter detect lead?Indirectly — if lead raises the TDS reading. But a TDS meter cannot distinguish lead from calcium or magnesium. For lead detection, use a specific lead test kit or certified lab analysis.
Why does my TDS meter show 0 after RO?That's normal and good. A properly functioning RO system should reduce TDS by 85-95%. If your tap water is 200 ppm, expect 10-30 ppm after RO.
Do I need an expensive TDS meter?No. A $15-30 TDS meter from HM Digital or ZeroWater is sufficient for home use. Laboratory-grade meters ($100+) offer better accuracy and calibration features but aren't necessary for basic testing.
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