A TDS meter costs a few hundred rupees and gives you the most important number for choosing a water purifier. Hereâs how to use it properly and what the result means.
What TDS actually measures
TDS (total dissolved solids) is the total amount of minerals, salts and other substances dissolved in water: calcium, magnesium, sodium, chlorides, sulphates, bicarbonates and so on.
A handheld meter doesnât measure these directly. It measures electrical conductivity (dissolved salts let water carry current) and converts that into an estimated TDS using a fixed factor. Thatâs why itâs an estimate, and why it canât tell you what is dissolved.
How to test your water
- Use a clean glass or steel tumbler. Rinse it with the water youâre testing.
- Run the tap for 10â20 seconds before filling it, to flush out water that has been sitting in the pipe.
- Let the water reach room temperature. Most meters correct for temperature (look for âATCâ on the box), but very hot or cold water still gives less reliable readings.
- Remove the cap and switch the meter on.
- Dip the electrodes to the depth marked on the meter, usually 2â5 cm. Donât submerge the whole meter.
- Stir gently to remove air bubbles from the electrodes.
- Wait until the number stops changing, then press âholdâ if your meter has it, and read the result.
- Rinse the tip with clean water, shake it dry, and put the cap back on.
Test two or three times and take the average. Test each source separately: your municipal line, borewell, tanker water, and the output of any purifier you already have.
What the reading means
| TDS (mg/L) | What it usually means |
|---|---|
| Below 50 | Very low. Typical of RO output without a mineraliser. Safe, but may taste flat. |
| 50â300 | Typical of treated municipal water and well-adjusted purifier output. Usually tastes good. |
| 300â500 | Within the BIS acceptable limit. May taste slightly salty or hard. |
| 500â2,000 | Above the acceptable limit. BIS permits this only if thereâs no other source. An RO purifier is usually recommended. |
| Above 2,000 | Above the permissible limit. Donât drink it untreated. Get a lab test and use RO. |
Source for the limits: BIS IS 10500 (drinking water specification), which gives 500 mg/L as the acceptable limit and 2,000 mg/L as the permissible limit where no alternative source exists.
Using TDS to choose a purifier
- Below about 300 mg/L, from a treated municipal supply: UV + UF is usually enough, and it wastes no water.
- Above about 500 mg/L, or borewell or tanker water: RO + UV.
- In between: it depends on the source and taste. A lab test helps.
Our RO vs UV vs UF guide explains each technology in detail.
Checking your RO purifier is working
Test the input water (the tap feeding the purifier) and the output water. An RO purifier typically removes most dissolved solids, so the output should be much lower than the input. If the output TDS creeps up over a few months, the RO membrane may be wearing out, or the TDS controller may need adjusting. Compare your readings with the brandâs specification, and call service if theyâre well outside it.
What a TDS meter canât tell you
This matters most. A low TDS reading doesnât mean the water is safe. A TDS meter canât detect:
- bacteria, viruses or cysts (which UV and UF deal with)
- specific harmful substances, such as arsenic, fluoride, lead, nitrates or pesticides, at levels that matter for health
- hardness on its own, since TDS includes many substances besides calcium and magnesium
If you use borewell water, live in an area known for fluoride or arsenic, or notice a smell, colour or stomach problems, get a proper lab test from a recognised lab or your stateâs water department.
Keeping the meter accurate
- Keep the cap on when youâre not using it, and keep the electrodes clean and dry.
- Replace the batteries when the display dims or readings jump around.
- Some meters can be calibrated with a standard solution. Check the manual.
- For a quick sanity check, compare your reading with a friendâs meter or the reading a purifier technician gets.
Frequently asked questions
What is a good TDS level for drinking water?
The BIS drinking water standard (IS 10500) sets an acceptable limit of 500 mg/L, and allows up to 2,000 mg/L only where no other source is available. Below 500 mg/L is within the acceptable limit. Many people find water with a TDS of roughly 100 to 300 mg/L tastes best.
Is very low TDS water bad?
IS 10500 sets a maximum, not a minimum. Very low TDS water, below about 50 mg/L, often tastes flat. Many RO purifiers let you raise the TDS with a TDS controller or mineraliser. You get most minerals from food, not water.
Can a TDS meter detect bacteria, lead or arsenic?
No. A TDS meter measures how well the water conducts electricity and estimates the total dissolved solids from that. It can't detect bacteria or viruses, and it can't tell you which substances are dissolved. For borewell water or suspected contamination, get a lab test.
Are ppm and mg/L the same?
For drinking water, effectively yes. 1 ppm (part per million) is about 1 mg/L, so a meter reading of 250 ppm means about 250 mg/L.
