A TDS test measures total dissolved solids, and the handheld meters that do it actually measure electrical conductivity and convert. That distinction is the whole point: the reading responds to every dissolved ion in the water without distinguishing between them, so a single number is being asked to summarize a great many different substances.
What the number contains
Calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate and nitrate all contribute, along with whatever else is in solution. Beneficial minerals count exactly the same as unwanted ones, which is why a mineral rich spring water reads higher than many municipal supplies while being perfectly desirable. There is no health based standard for TDS: it carries a secondary, aesthetic recommendation concerning taste. A high reading is a prompt to find out what is in the water, not a finding in itself.
The job it genuinely does
Checking a reverse osmosis membrane. Measure the feed water and the permeate, and the percentage difference is the rejection rate, which is the single most useful indicator of membrane health. A falling rejection rate means the membrane is failing or bypassing, and it shows up long before taste changes. Take a baseline when the system is commissioned and repeat periodically, since the trend matters more than any single figure. This is what TDS meters are for and they do it well.
What people wrongly use it for
Hardness, most often. The USGS defines hardness as dissolved calcium and magnesium, soft being 0 to 60 mg/L as calcium carbonate through to very hard above 180 mg/L, and a conductivity meter cannot isolate those two ions from everything else. Softened water reads roughly the same as it did before treatment, because sodium replaced calcium ion for ion, which regularly convinces owners a working softener has failed. Use a titration kit or strips for hardness and leave TDS to the membrane.
Testing it usefully
Take samples from a cold tap with the aerator removed, after running until the water is cold and steady, and at the same point each time so readings are comparable. Temperature affects conductivity, so compare like with like or use a meter with temperature compensation. Calibrate against a known solution occasionally, since these meters drift. And treat any surprising result as a reason to commission a full analysis rather than a conclusion. Our well water testing guide covers what a proper panel includes.
Questions people ask about tds test
Is a high TDS reading dangerous?
Not in itself. TDS carries an aesthetic recommendation about taste, not a health standard, and beneficial minerals count towards the number.
What is a TDS meter genuinely good for?
Checking reverse osmosis membrane health. Compare feed and permeate readings to get the rejection rate and track it over time.
Does TDS tell me my water hardness?
No. It cannot isolate calcium and magnesium from other ions, and softened water reads much the same as it did before treatment.
How should I take the reading?
From a cold tap with the aerator off, after running until cold and steady, at the same point each time, with temperature compensation.