What is a TDS reading? It is total dissolved solids: the combined amount of everything dissolved in the water, minerals, salts and metals together, expressed as one number. A handheld meter gives it in seconds, which makes it the most commonly quoted water figure in households and also the most commonly misread, because it is a total, not an identification.
How the meter actually works
A TDS meter does not weigh dissolved solids. It passes a small current between two probes and measures electrical conductivity, which rises as more dissolved ions are present, then applies a conversion factor to express that conductivity as a solids figure. That indirect method has two consequences worth knowing. Substances that do not conduct, including many organic compounds, contribute little or nothing to the reading. And the conversion factor is an assumption about the mix of ions present, so two waters with the same true dissolved content can read differently on the same meter.
What contributes to the number
Calcium and magnesium from hardness are usually the largest contributors in a household supply, along with sodium, potassium, bicarbonates, chlorides, sulfates and nitrate. Trace contaminants contribute almost nothing to the total, because their concentrations are minute compared to the common minerals. That is why the reading correlates loosely with hardness and taste but not at all with safety. Mineral-rich water from a limestone aquifer reads high and is perfectly good to drink. Water that has picked up lead from a service line barely moves the number at all.
What a TDS reading cannot tell you
It cannot identify anything. It will not distinguish beneficial minerals from contaminants, and it does not detect lead, arsenic, PFAS, pesticides, bacteria, viruses or most organic compounds in any meaningful way. A low reading is not a safety certificate and a high reading is not proof of a problem. Anyone selling equipment by demonstrating a high number on your tap water and a low one on their filtered sample is showing you that dissolved minerals were removed, which is true, and implying something about contamination, which the instrument cannot support.
Where the number is genuinely useful
It is an excellent trend tool. On a reverse osmosis system, comparing the feed reading to the treated reading gives a rejection percentage that tracks membrane health over time, and a rising treated figure is often the earliest warning that a membrane is failing or a bypass is leaking. It flags brackish or salty supplies that taste unpleasant. It confirms that a softener has changed the ionic content of the water. Log the readings from the day of installation, because the value is in the trend rather than in any single measurement.
What to do instead when the question is safety
Use a laboratory panel that names contaminants. For a private well, that means testing coliform bacteria, nitrate, and the metals and chemistry relevant to your area rather than reading a meter. For a city supply, start with the annual report your utility publishes and test at the tap for what your own plumbing might add. Our TDS meter guide covers using the instrument for what it does well, which is monitoring treatment performance and comparing water before and after a system rather than judging whether it is safe.
Questions people ask about what is a tds
Is a low TDS reading always better?
No. Very low readings mean almost nothing is dissolved, which tastes flat and can make water more aggressive toward metal plumbing. Many well-regarded drinking waters read comfortably high because of natural mineral content.
Can a TDS meter detect lead in my water?
No. Lead occurs at concentrations far too small to move a total dissolved solids reading, so a meter cannot detect it at any level that matters. Lead requires a laboratory test of a properly taken sample.
Why did my TDS go up after installing a softener?
Ion exchange swaps calcium and magnesium for sodium or potassium, and the exchange is not one for one by weight, so the total dissolved figure can rise slightly even though the water is genuinely soft.
What is a good TDS reading for a reverse osmosis system?
Judge it as a comparison rather than an absolute: measure the feed water and the treated water and watch the gap between them. A shrinking gap over time is the signal that the membrane is nearing the end of its life.