TDS meters: what they can't tell you

A TDS meter estimates how much dissolved mineral and salt is in water from how well the water conducts electricity. It cannot tell you which substances those are, whether germs are present, or whether the water is safe to drink, and lead or PFAS at EPA's limits are far too dilute to move the reading.

What the meter actually measures

Very pure water, such as distilled water, conducts electricity poorly. Dissolved salts split into charged particles called ions, and those let current flow (USGS). A conductivity reading is an indirect measure of all the dissolved ions together, and it is commonly used to estimate total dissolved solids (USGS).

The conversion depends on which minerals are in the water. USGS gives a typical ratio of 0.5 to 0.9 between dissolved solids and conductance (same source). So the number on a TDS meter is an estimate, in milligrams per liter, which is the same as ppm (EPA).

What it can't tell you

• What is dissolved. CDC describes TDS as how much "stuff", such as calcium or sodium, is mixed in your water (CDC). Two glasses with the same reading can hold very different minerals.
• Whether germs are present. A conductivity reading counts dissolved ions (USGS), and bacteria and viruses are not ions. CDC lists coliform bacteria as a separate yearly test for well owners (CDC).
• Lead, even at EPA's action level. EPA's health goal for lead is zero (EPA). Its lead action level is 15 parts per billion, which is 0.015 ppm, and EPA's 2024 lead rule lowers it to 10 ppb (EPA). In water that reads 250 ppm, 15 ppb of lead would be 0.006% of the total, far too little to notice.
• PFAS. EPA's limit for PFOA and for PFOS is 4.0 parts per trillion (EPA). That is 0.000004 ppm.
• Whether the water is safe. EPA's guideline for TDS, 500 mg/L, is a secondary standard for taste, staining and scale. EPA does not enforce it, and it does not consider the substances on that list a health risk at those levels (EPA).

High and low readings

Above 500 mg/L, EPA lists hardness, deposits, colored water, staining and a salty taste as effects people may notice (EPA). If hardness is your worry, test for hardness itself and use that number in our softener size calculator.

A low reading does not clear the water either. It only means there are few dissolved ions, and it still says nothing about germs or trace contaminants.

Where a meter helps

• Checking a reverse osmosis system. Reverse osmosis systems can be certified to NSF/ANSI 58 for TDS reduction. For example, the listing for the AquaTru Classic includes a TDS claim (IAPMO W-10194). In our product data, every filter with a certified TDS claim is a reverse osmosis system (how we label claims). Comparing readings from the tap and from the filter is a quick spot check that the membrane is still working. Replace parts on the schedule in the manual.
• Noticing change in a well. EPA and CDC include TDS in the yearly well test (EPA, CDC). Between lab tests, a meter can show that something changed. It cannot show what changed.

What to use instead

• On public water, your utility's water quality report lists measured contaminants next to their limits. See how to read your water quality report.
• For lead, follow EPA's testing advice in lead pipes and lead filters.
• For a private well, use a state-certified laboratory (CDC). See well water testing.