TDS stands for total dissolved solids, and it is the combined weight of everything dissolved in the water: minerals, salts and ions of every kind added into one figure. That aggregation is both what makes it a convenient single number and what makes it far less informative than people assume when they read one off a meter.
What the number contains
Calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate and nitrate all contribute, alongside anything else in solution. Beneficial minerals count exactly as much as unwanted ones, so a mineral rich spring water reads higher than many municipal supplies while being entirely desirable, and a supply reading low could still carry a contaminant that matters at concentrations far too small to move the total. The number says how much, never what.
Whether high is bad
Not in itself. TDS carries a secondary, aesthetic recommendation concerned with taste rather than a health based standard, which is exactly because the figure does not identify what is present. Very high readings often mean high mineral content that affects taste and may indicate hardness, and very low readings mean water that tastes flat. Neither tells you the water is safe or unsafe, and treating a TDS figure as a safety measure is the most common misuse of it.
How the meter gets there
It does not weigh anything. A handheld meter measures electrical conductivity, since dissolved ions carry current, and multiplies by a conversion factor to display a dissolved solids estimate. Manufacturers use slightly different factors, which is why two pens disagree on the same sample and why tracking one meter over time is more meaningful than comparing between them. Temperature affects conductivity, so compensation matters.
The one job it does well
Checking a reverse osmosis membrane. Compare the feed water reading with the permeate reading and the proportional difference is the rejection rate, which is the best available indicator of membrane condition. Take a baseline at commissioning and track it, since a falling rate reveals a failing membrane long before the water tastes different. It cannot give hardness, since the USGS defines that as calcium and magnesium specifically. Our softener cost guide covers what hardness decides.
Questions people ask about what does tds mean in water
What does TDS stand for?
Total dissolved solids: the combined amount of everything in solution, beneficial minerals and unwanted ions alike.
Is a high TDS reading dangerous?
Not in itself. It carries an aesthetic recommendation about taste, not a health standard, since it does not identify what is present.
How does a TDS pen measure it?
It measures electrical conductivity and multiplies by a conversion factor, which varies between manufacturers.
Can it tell me my water hardness?
No. Hardness is calcium and magnesium specifically, and conductivity cannot isolate two ions from everything else dissolved.