TDS water testing measures total dissolved solids: everything dissolved in the water added together, read by a meter that measures how well the water conducts electricity. It is quick, cheap and genuinely useful for one job in particular, and badly misunderstood for almost every other job people try to use it for.
What the meter is actually measuring
A handheld TDS meter does not identify anything. It passes a small current between two probes, measures conductivity, and converts that to an estimated dissolved solids figure using a fixed factor. Dissolved minerals and salts conduct, so they show up. The result is a single lumped number describing quantity, not identity. Two samples with the same reading can be completely different water: one full of harmless calcium and bicarbonate, the other carrying sodium, nitrate or dissolved metals. The USGS defines hardness as dissolved calcium and magnesium, with the soft to very hard scale running from 0 to 60 mg/L as calcium carbonate up past 180, and that hardness is part of what a TDS meter is reading, but the meter cannot separate it out.
What TDS cannot tell you
Most of the contaminants that actually frighten people conduct poorly or occur at concentrations far too small to move a TDS reading. Bacteria do not register. PFAS do not register. Most pesticides and solvents do not register. Lead at levels of concern will not move the number meaningfully. So a low TDS reading is not a clean bill of health, and a high one is not proof of danger: some excellent tasting spring water reads high because it is rich in harmless minerals. The EPA notes that private wells are not federally regulated under the Safe Drinking Water Act, which makes proper testing the well owner's own job, and a TDS meter is not a substitute for the panel that job requires.
The job TDS testing is genuinely good at
Where the meter earns its keep is monitoring a reverse osmosis system. RO works by removing dissolved solids, so comparing a reading taken on the feed water with one taken on the treated water tells you directly how much the membrane is still removing. Take both readings on the day the system is commissioned and write them down, because the baseline is the whole point. When the treated reading starts climbing toward the feed reading, the membrane is tiring, or a bypass or seal is leaking untreated water past it. The same before-and-after comparison works for tracking a change in the incoming water, and for checking whether a deionizing cartridge is exhausted.
Using it sensibly alongside a real test
Treat TDS as a trend instrument, not a verdict. Take readings on the same tap, at the same point in the treatment train, with the water run for the same length of time, because the number moves with temperature and with how long the water has sat. Calibrate the meter or replace it when readings drift, since cheap meters wander. Then use it for what it does: watching change over time and verifying that equipment is still performing. For anything that decides a purchase or a health question, use a laboratory panel that names contaminants and reports each one individually, and keep the reports so you can compare year on year.
Questions people ask about tds water testing
Is a low TDS reading proof my water is safe?
No. Bacteria, PFAS, most pesticides and lead at levels of concern do not move a TDS reading, so a low number says nothing about those contaminants.
What is a good TDS reading for drinking water?
There is no single right figure, because the number lumps harmless minerals in with everything else. Taste preference and a proper contaminant test matter more.
Why is my TDS higher after the softener?
Ion exchange swaps calcium and magnesium for sodium rather than removing solids, so the total dissolved figure often stays similar or rises slightly.
How do I use a TDS meter to check an RO system?
Measure the feed water and the treated water on the same day and compare. Record both at commissioning, then watch the treated reading for a climb over time.