Total Dissolved Solids
Everything dissolved in the water added together, a rough proxy for taste and mineral content.
- Safe range
- 0 – 1000 mg/L
- Ideal range
- 300 – 500 mg/L
- Measured in
- mg/L
What it is
TDS is the combined weight of everything dissolved in the water: minerals, salts and metals. Cheap TDS meters are common and give a single number, which is both their appeal and their weakness.
Why it matters
TDS is worth understanding mainly so it is not over-read, because it is the number most often misused to sell filtration. It says how much is dissolved, never what. Mineral-rich spring water can read 400 mg/L and taste excellent; water at 400 mg/L carrying the wrong things is a different matter entirely. A low TDS reading is not proof of purity, and a high one is not proof of contamination. Treat it as context, and let the specific parameters carry the verdict.
How to measure it
A TDS pen is cheap, quick, and accurate at what it does, which is measuring HOW MUCH is dissolved in the water. It cannot tell you what, and no TDS reading is a safety verdict.
Digital meter or pen
- How trustworthy
- Dependable
- Cost
- Mid range
- Effort
- Seconds to read, minutes to calibrate
A probe on a handheld body that reads the water electrically and prints a number. No colour to judge, which removes the commonest source of error and introduces a new one: a meter that has drifted looks exactly as confident as one that has not.
Dip it in a glass and read the number. It is the one home instrument in this section that needs no reagents and almost no technique.
Watch out: A TDS pen measures conductivity and multiplies. It cannot see lead at the concentrations that matter, it cannot tell harmless dissolved minerals from anything else, and a low reading after a filter is evidence that minerals were removed rather than evidence that the water is safe. The in-store demonstration that makes tap water look alarming is this instrument being misused.
Where to find it: Online and in pool and aquarium shops. Buy the calibration solution at the same time; a meter without it is a guess with a decimal point.
Certified laboratory, mail-in kit
- How trustworthy
- Reference grade
- Cost
- Most expensive
- Effort
- Careful sampling, then days or weeks of waiting
You are posted bottles and instructions, you fill them exactly as told, and you post them back. Weeks later a report arrives with figures nobody has to interpret by eye. This is what a result you intend to act on should be built from.
Answers the question a TDS pen raises. If the number is high, a panel says whether it is calcium and magnesium, which is hardness, or sodium, sulphate, or something worth acting on.
Where to find it: State-certified drinking water laboratories; your state's certification officer keeps the list, and many public health departments will point you at one.
Your utility's annual water quality report
- How trustworthy
- Reference grade
- Cost
- Free
- Effort
- A few minutes of reading
Every community water system in the US has to publish one each year, free, listing what it found and what the limits are. It is real laboratory data about the water leaving the plant and travelling the mains. What it cannot describe is your own plumbing.
Published by most systems, and a reasonable thing to check your new pen against the first time you use it.
Where to find it: Your water bill, your supplier's website, or the EPA's index of reports. It is also called a Consumer Confidence Report or CCR.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Take a reading at the tap and a reading after any filter or softener you own, on the same day with the same pen. TDS is most useful as a comparison; a single number with nothing to compare it against says very little.
- • Let the sample reach room temperature. Most pens compensate for temperature, and few do it well at the extremes.
Further reading: US EPA: Consumer Confidence Reports, the annual report every community water system publishes
If it is too low
Very low mineral content. Water can taste flat or empty. Distilled and reverse-osmosis water reads near zero by design.
- • Not a health issue. Some people remineralise RO water purely for taste.
If it is too high
Water may taste salty, bitter or metallic, and scale is more likely. Worth identifying which specific parameters are responsible.
- • Test the individual parameters (hardness, iron, manganese, nitrate) to find out what is actually in there.
- • Reverse osmosis reduces TDS broadly if the specifics turn out to warrant it.
Common questions
What is a good TDS level for drinking water?
Most palatable drinking water falls between 100 and 500 mg/L. The US EPA secondary standard is 500 mg/L, set for taste rather than health. Below about 50 mg/L water tends to taste flat; above 1000 mg/L it usually tastes noticeably salty or bitter.
Does a low TDS reading mean my water is pure?
No, and this is the most common misreading of the number. TDS measures dissolved solids in aggregate and tells you nothing about which ones. Many of the contaminants people worry most about are dangerous at concentrations far too small to move a TDS meter.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.