TDS (Total Dissolved Solids)
What TDS actually measures
TDS (Total Dissolved Solids) is the concentration of all dissolved substances in water, measured in milligrams per litre (mg/L) or parts per million (ppm). These dissolved substances include:
Beneficial minerals: calcium, magnesium, potassium, bicarbonate — naturally occurring from rock and soil Neutral salts: sodium chloride, sulphates Contaminants: lead, arsenic, nitrates, PFAS compounds, heavy metals
A TDS meter measures the electrical conductivity of water and infers dissolved solid concentration. It cannot tell you what is dissolved — only how much.
Why zero TDS is not the goal
ZeroWater’s marketing centres on achieving 000 ppm TDS. This is technically impressive. But a TDS of zero means the filter removed both the lead (harmful) and the calcium and magnesium (beneficial) from your water. In hard-water areas where tap water has a TDS of 200–400 ppm, those minerals contribute roughly 10–20% of daily calcium and magnesium intake.
The relevant question is not “is my TDS low” but “what is actually in my tap water report, and which of those things are harmful?”
TDS as a filter quality indicator — with caveats
TDS readings are useful for:
- Confirming your RO membrane is working (output should be 90–98% lower than input)
- Detecting filter exhaustion in ZeroWater-style ion exchange systems (TDS creeping above 10 ppm signals resin saturation)
- General water hardness estimation
TDS readings are NOT useful for:
- Confirming PFAS removal (PFAS compounds are not electrically conductive and are not measured by standard TDS meters)
- Confirming bacterial removal
- Confirming NSF certification compliance
Typical TDS ranges
| Water type | Typical TDS (ppm) | Notes |
|---|---|---|
| Distilled water | 0–5 | Near-zero minerals |
| RO filtered (no remineralisation) | 5–20 | Minerals removed |
| UK soft water (e.g., London post-softening) | 50–120 | |
| US average municipal tap | 150–400 | |
| UK hard water (e.g., East Anglia) | 200–450 | |
| WHO upper guideline | 500 | Above this, taste degradation begins |
| EPA secondary standard | 500 | Non-enforceable |
What to do with this information
Read your local Consumer Confidence Report (US) or contact your water company (UK) before buying a filter based on TDS alone. If your report shows elevated lead or PFAS, a TDS meter is irrelevant — buy a filter certified to NSF 53 or NSF 58 for those specific contaminants. If your primary concern is calcium scale on kettles and appliances, a water softener addresses the cause more directly than a TDS-reducing filter.