What a TDS Reading Actually Tells You

A handheld meter makes water testing look simple: dip the probe into a glass and read a number. The number can be useful. It is not a purity score.

What the meter estimates

TDS stands for total dissolved solids, a measure of dissolved material usually reported in milligrams per liter. A typical handheld “TDS meter” measures electrical conductivity and converts that reading into an estimated TDS value.

The conversion depends on the meter and the water’s ionic composition. The instrument does not identify individual substances or directly weigh everything dissolved in the glass. USGS explains how dissolved ions contribute to conductivity.

Laboratory TDS methods are different. Comparing readings requires attention to the method and units, not just the displayed number.

Equal numbers can describe different waters

Two samples can have similar total dissolved material but different individual concentrations. The total does not tell you how much calcium, sodium or a particular contaminant is present.

Nor is a low reading evidence that trace contaminants or microorganisms are absent. Those questions need suitable analyses with their own reporting limits. A handheld meter is not a substitute for those tests.

What EPA’s 500 mg/L guideline means

EPA lists 500 mg/L as a secondary drinking-water standard for TDS. It is a federal guideline addressing aesthetic concerns rather than a general health threshold. States may adopt their own requirements. EPA’s secondary-standards guidance

Crossing that number alone does not establish that a water is unsafe. Staying below it does not establish safety either. Individual contaminants and applicable standards still need to be considered.

Read our records by method and date

The preliminary Eurofins record, job 380-188240-1 generated December 29, 2025, reports total dissolved solids of 410 mg/L using SM 2540C, with a 20 mg/L reporting limit, for its December 10 sample.

A separate Gangwon report on a July 24, 2025 sample lists total solids of 457 mg/L. We retain that analyte name; “total solids” should not silently be relabeled TDS. These records involve different samples and measurements.

For individual minerals, the composition section identifies the manufacturer’s August 5, 2025 inspection. None of these values is a measurement of every bottle now on sale.

Use the number to ask a better question

For tap water, begin with your utility’s current water-quality report and any local notices. For bottled water, look for product-specific records with dates and methods.

A meter may help track a change under consistent conditions. To understand that change, ask what was measured and what analysis would answer the concern. “Higher” and “lower” are only the start of that conversation.