A TDS meter estimates total dissolved solids by measuring how well water conducts electricity. It is useful for quick comparisons and filter checks, but it cannot identify contaminants or prove that water is safe to drink.
Quick answer
- Rinse the probe, test a clean sample, wait for the number to stabilize, and record the temperature if your meter does not compensate automatically.
- Use TDS to compare sources, track trends, or check reverse osmosis performance.
- Do not use TDS alone for wells, springs, or safety decisions.
- If the result matters for health, use contaminant-specific lab testing.
Safety note: A TDS meter does not test for bacteria, lead, nitrate, arsenic, PFAS, pesticides, or many other contaminants.
What a TDS meter measures
A handheld TDS meter usually measures electrical conductivity, then converts that conductivity into an estimated TDS value. The display is often shown in ppm, which is treated similarly to mg/L for everyday water discussions.
The meter responds mostly to dissolved ions. Calcium, magnesium, sodium, chloride, sulfate, bicarbonate, and other charged dissolved substances can all affect the reading. Non-ionized substances may not show up clearly. Microbes are not measured by a standard TDS meter.
How to use a TDS meter
Use a clean glass or sample cup. Rinse the cup with the water you plan to test, then fill it with a fresh sample. Remove the meter cap, rinse the probe with distilled or low-TDS water if available, and gently place the probe into the sample without touching the sides or bottom.
Wait for the reading to stabilize. Some meters have a hold button. Record the result, source, date, and any recent filter change or plumbing change. Rinse the probe after testing and let it dry before storing.
For comparisons, test the same way each time. Use a similar sample temperature, a clean container, and the same meter. Small differences can come from temperature, calibration, sample handling, and the meter conversion factor.
What counts as a high reading?
EPA lists 500 mg/L as a secondary guideline for total dissolved solids in public drinking water. Secondary standards focus on taste, odor, color, staining, scale, and other aesthetic or practical issues. They are not the same as health-based contaminant limits.
That means 500 mg/L is a useful reference point, not a universal safety line. Some mineral waters are intentionally high in dissolved minerals. Some low-TDS waters can still contain contaminants that require specific tests.
Best uses for a TDS meter
A TDS meter is good for checking whether reverse osmosis is reducing dissolved ions. Test the water before and after the RO system. A large reduction usually means the membrane is doing something useful, although the exact target depends on the system and source water.
It is also helpful for comparing bottled waters, tap water, spring water samples, and filtered water by mineral character. If a familiar source suddenly changes, TDS can document that change and help decide whether more testing is needed.
Common mistakes
The biggest mistake is treating a TDS number as a safety score. A low number does not mean "clean." A high number does not identify the problem. The meter cannot tell whether the dissolved solids are mostly harmless minerals or a substance that needs attention.
Another mistake is expecting a carbon filter to lower TDS. Carbon filters can improve taste and reduce some chemicals, but they usually do not remove most dissolved minerals. Reverse osmosis, distillation, and deionization are more likely to reduce TDS.
It is also easy to compare numbers from different meters as if they are exact. Consumer meters can use different conversion factors. Treat the result as a practical estimate unless you have lab data.
When a TDS meter is not enough
Use lab testing when water comes from a private well, a spring, an unknown outdoor source, old plumbing, or an area affected by flooding or nearby contamination. Use lab testing if water suddenly changes color, smell, taste, or sediment. Use lab testing when infants, pregnant people, immunocompromised people, or medically vulnerable people may drink the water.
TDS can tell you that something changed. It usually cannot tell you what changed.
What to do next
Use a TDS meter as one small tool in a larger testing plan. Pair it with pH, hardness, chlorine, and source-specific lab tests when needed. For a deeper explanation of the number, read What Is TDS in Water?. For choosing tests by situation, use How To Test Drinking Water at Home.
