A TDS meter estimates Total Dissolved Solids from water's electrical conductivity and shows the result in parts per million. It helps compare reverse osmosis feed and treated water. It does not measure hardness or identify specific contaminants, so use a hardness test to check a water softener.
A TDS meter is one of the fastest ways to see what is dissolved in your water. TDS stands for Total Dissolved Solids. That is the combined weight of the minerals, salts, and metals dissolved in the water. The meter gives you one number in parts per million (ppm). It helps a plumber compare feed water with water treated by a reverse osmosis system. To check a softener, use a hardness test: ion exchange replaces calcium and magnesium with sodium or potassium and does not remove the water's overall dissolved-solids load.
What a TDS meter actually measures
A TDS meter does not sniff out specific contaminants. It cannot tell lead from calcium. What it really measures is electrical conductivity. Pure water barely conducts electricity. Dissolved minerals and salts carry a charge, so more dissolved solids means more conductivity. The meter reads that conductivity and converts it to a ppm estimate. The conversion factor depends on the meter and the dissolved ions, so the result is an estimate. The meter is useful for tracking changes, but it does not tell you whether water is safe to drink. Testing for a specific contaminant requires the appropriate laboratory test.
What the numbers mean
The EPA lists a secondary standard of 500 mg/L for TDS, which is about the same as 500 ppm. It is a taste and comfort guideline, not a health limit. Higher TDS can affect taste or leave deposits, depending on which substances are present. TDS and hardness describe different things: hardness measures calcium and magnesium, while TDS includes other dissolved salts. A high TDS reading alone does not identify the substance or establish a health risk. Choose water treatment from the actual water problem and test results.
Checking an RO system with a TDS meter
The meter shines when you test a reverse osmosis membrane system. Dip it in the tap water first, then in the RO water. Compare the reduction with the performance specified for your system and test conditions. Say your tap reads 400 ppm and the RO water reads 40 ppm. That is a 90% drop. It shows dissolved-solids reduction, but does not prove that every contaminant has been removed. If the RO number climbs over time, the membrane or filters may be worn out. That is your cue to check filter replacement.
- US EPA: Secondary Drinking Water Standards, Guidance for Nuisance Chemicals (TDS secondary standard 500 mg/L)
- Water Quality Association: Consumption of Low TDS Water (TDS measured in ppm with a conductivity/TDS meter)
- University of Nebraska-Lincoln Extension: Drinking Water Treatment, Water Softening (G1491)