A cheap TDS pen gives you one number in parts per million, and people treat it as a verdict on their water. It is not. The meter measures electrical conductivity and converts it to an estimate of total dissolved solids, so it tells you how much dissolved mineral is present, never which minerals or whether any of them matter.
What the meter is doing
Dissolved salts make water conduct electricity, and the pen measures that conductivity between two probes, then multiplies by a conversion factor to report total dissolved solids. For water, parts per million and milligrams per liter are used interchangeably. That conversion factor is an assumption about the mixture in the water, which is why two meters can disagree on the same glass and why a reading is an estimate rather than an analysis.
Why a high reading is not a health verdict
Calcium and magnesium raise the reading and are harmless to drink, though they scale your kettle. Sodium raises it. So does bicarbonate. Meanwhile lead, arsenic, PFAS, nitrate at typical concentrations, pesticides and bacteria barely move the needle at all, because they are present in tiny quantities or carry no charge. A low reading is therefore not proof of safety, and a high reading on a mineral-rich supply is not proof of danger. The meter cannot distinguish.
Where the meter is genuinely useful
It is a good trend tool. Take a reading on your feed water and on the product water of a reverse osmosis system, and the difference tells you the membrane is still rejecting. Watch that gap over months and a rising product reading is an early warning that the membrane or the check valve is tired. It is also useful for confirming a deionizing cartridge is spent. In all those cases you are comparing a number with itself, which is exactly what an estimate is good for.
The test to run when you actually want to know
For hardness, a titration test or a laboratory result in grains per gallon or as calcium carbonate is what sizes a softener; the USGS classifies hardness as dissolved calcium and magnesium with soft at 0 to 60 mg/L, moderately hard 61 to 120, hard 121 to 180 and very hard more than 180 mg/L. For health-relevant contaminants, order a certified laboratory panel. Our well water testing guide covers which panel matches which worry, and a TDS pen replaces none of it.
Questions people ask about water ppm levels
What is a good ppm level for drinking water?
There is no single healthy number. Dissolved minerals are mostly harmless, so judge water by a laboratory panel rather than by a conductivity reading.
Does a TDS meter detect lead or bacteria?
No. Both are present in quantities far too small to shift a conductivity reading, and bacteria are not dissolved solids at all.
Why does my RO water read almost zero?
Because reverse osmosis removes most dissolved minerals. A near-zero reading confirms the membrane is working; it says nothing about the carbon stages.
Is ppm the same as mg/L?
For drinking water the two are used interchangeably, since a liter of water weighs close enough to a kilogram for the conversion to hold.