TDS stands for total dissolved solids: the combined weight of everything dissolved in your water, including minerals, salts and metals. A TDS meter estimates that total by measuring how easily the water conducts electricity. It is a useful single number for tracking a system, and a poor number for judging whether water is safe, because it says nothing about what the dissolved material is.
What the meter is really measuring
A handheld TDS meter does not weigh anything. It measures electrical conductivity, which rises with dissolved ions, and converts that reading into an estimate using a fixed factor. That means it responds strongly to things that ionize, such as calcium, magnesium, sodium, chloride and sulfate, and hardly at all to things that do not, including many organic compounds. Two waters with the same reading can have completely different content, and a substance present in trace amounts moves the number so little that the meter is blind to it.
A high reading is not automatically bad
Most of what makes up dissolved solids in ordinary tap water is harmless mineral content. Hardness is a large part of it, and the USGS defines that hardness as dissolved calcium and magnesium, sorting it into bands measured as calcium carbonate: soft is 0 to 60 mg/L, moderately hard is 61 to 120, hard is 121 to 180, and very hard is above 180 mg/L. Spring water that many people pay for reads high. In the United States, total dissolved solids sits under the secondary standards, which address taste and appearance rather than health.
What a low reading does not prove
This is the trap. Water can read very low and still carry something you care about, because a trace contaminant contributes almost nothing to conductivity. Bacteria contribute nothing at all. Reverse osmosis output reads low precisely because the membrane removed the harmless minerals along with everything else, and a distilled sample reads near zero while telling you nothing about a volatile compound that carried over with the steam. Safety is a question for a laboratory panel, never for a meter.
Where TDS is genuinely useful
Its real job is monitoring equipment you already own. Take a reading of your feed water and of your reverse osmosis output, and the ratio between them tells you how well the membrane is performing. When that ratio starts sliding, the membrane is on its way out, and that is far more informative than tasting the water. It also flags a sudden change in a supply worth investigating. Read our water softener cost guide alongside a hardness test if scale rather than taste is what brought you here.
Questions people ask about what does tds mean
What is a good TDS reading for drinking water?
There is no single good number, because the reading says nothing about what is dissolved. Ordinary tap water and bottled spring water both read well above reverse osmosis output.
Does a low TDS reading mean water is safe?
No. Trace contaminants and bacteria barely affect conductivity, so water can read low and still carry something a laboratory panel would find.
Does TDS measure hardness?
It includes hardness, since calcium and magnesium are dissolved ions, but it does not separate them out. A hardness test is what you need for sizing a softener.
Why did my TDS reading go up after I installed a filter?
Some media, such as neutralizing calcite, dissolve minerals into the water deliberately. Carbon filters barely move the reading, because they do not remove dissolved salts.