What is a good TDS reading for drinking water?
The EPA secondary guideline is 500 ppm (taste-based, not health-based). Most US tap water reads 100–400 ppm. RO-filtered water typically reads under 50 ppm when the system is healthy.
A $15 pen that tells you when your filters are done. Here's how to read it.
TDS = total dissolved solids, measured in ppm (parts per million). It's the weight of dissolved minerals and salts in your water — calcium, magnesium, sodium, and similar. It is not a safety test: a low TDS doesn't mean water is safe, and a high TDS doesn't mean it's dangerous. The EPA's secondary (non-enforceable) guideline for TDS is 500 ppm, mostly about taste.
| Reading | Typical meaning |
|---|---|
| Tap water 100–400 ppm | Normal for most US municipal supplies |
| RO product water < 50 ppm | Membrane working well |
| RO water climbing toward tap level | Pre-filters spent or membrane aging — investigate |
Don't obsess over absolute numbers. Measure tap and filtered water, then compute rejection: (tap − filtered) ÷ tap × 100. A healthy RO rejects 90–98%. When your rejection rate drops noticeably from its baseline, something needs changing — usually a pre-filter first, the membrane last.
Baseline it when everything is new. That number is your reference for the life of the system.
The EPA secondary guideline is 500 ppm (taste-based, not health-based). Most US tap water reads 100–400 ppm. RO-filtered water typically reads under 50 ppm when the system is healthy.
No. TDS measures dissolved minerals, not contaminants like bacteria, lead, or pesticides — some of which don't move the TDS needle much. TDS is a filter-health indicator, not a safety certificate.
Usually a spent sediment or carbon pre-filter letting chlorine through (which damages the membrane), or an aging membrane. Change pre-filters first — it's the cheap fix — then retest before blaming the membrane.
Monthly is plenty. Test tap and filtered water back-to-back and track the rejection rate over time.
Cheap pens drift. Rinse probes after each use, and replace the pen if readings become erratic. For filter-change decisions, trend matters more than lab accuracy.