The full form of TDS is Total Dissolved Solids. It is the total amount of everything dissolved in water that you cannot see with the naked eye.
TDS is mostly minerals and salts, plus small amounts of organic matter. Because these are dissolved, perfectly clear water can still have a high TDS.
Units: TDS is reported in milligrams per litre (mg/L) or parts per million (ppm). For drinking water the two are the same, so 300 mg/L and 300 ppm mean exactly the same thing.
What makes up TDS?
Most of the dissolved solids in water are common minerals and salts:
- Calcium and magnesium: the main causes of water hardness.
- Sodium and potassium: common dissolved salts in groundwater.
- Bicarbonate, chloride and sulphate: the ions that largely decide alkalinity and taste.
These minerals come from rock and soil as water travels underground. Most are harmless in normal amounts. Water can also pick up unwanted substances such as nitrate. TDS measures everything together and does not tell you how much of each one is present.
How is TDS measured?
Most TDS meters do not measure dissolved solids directly. They measure how easily electricity passes through the water (its electrical conductivity) and convert that into an estimated TDS figure. Because it is an estimate:
- Different meters can show slightly different readings for the same water.
- Water temperature and meter calibration affect the result.
- The meter cannot tell you which minerals are present, only the overall total.
What a TDS meter can't tell you
Germs and bacteria. A low TDS reading says nothing about germs. Water with low TDS can still carry bacteria because of poor handling, plumbing or storage.
Specific contaminants. It cannot detect small but harmful amounts of substances such as lead, arsenic or pesticides.
Safety. TDS alone does not tell you whether water is safe. A high reading does not automatically mean unsafe water; it mostly means a lot of dissolved minerals, which affects taste.
What does the World Health Organization say?
The World Health Organization (WHO) has no health-based limit for TDS in drinking water. It does describe how TDS affects taste:
| TDS (mg/L or ppm) | Taste |
|---|---|
| Below 600 | Generally good |
| 600 – 1,000 | Increasingly noticeable mineral taste |
| Above 1,000 | Significantly and increasingly unpleasant |
Source: WHO, Total dissolved solids in drinking-water, background document.
Want to know what your own reading means? Our TDS chart for drinking water lists each range with Pakistan's official limits for drinking and bottled water.
How RO plants use TDS
At an RO plant, TDS is most useful as a comparison. Take one reading of the feed water going into the RO system and one of the product water coming out of the membrane. The difference shows how well the membrane is working:
Rejection (%) = (Feed TDS − Product TDS) ÷ Feed TDS × 100
For example, if your feed water stays the same but rejection keeps falling week after week, the membrane probably needs cleaning or replacing. Our RO membrane and filters guide covers the other signs to watch, and how an RO plant works explains where each reading comes from.
Is a lower TDS reading better?
No. Reverse osmosis can bring TDS down very low, but water with too few minerals tastes flat and empty and is slightly acidic. That is why professional plants add a controlled amount of minerals back after the membrane: calcium and magnesium for taste and hardness, and sodium bicarbonate to raise alkalinity and pH.
With more plants opening across Pakistan, especially in cities like Lahore, finding reliable, genuine food-grade minerals has become a real problem for plant owners. Minerals by Mr. RO was started to solve it.
Need minerals for your plant?
Calcium chloride, magnesium sulphate and sodium bicarbonate at Rs 800 per kg, with certificates.
Getting more from your TDS meter
- Use the same meter whenever you compare readings.
- Rinse the probe with clean water, and test samples at a similar temperature.
- Check calibration regularly against a calibration solution.
- Look at the whole picture. Treat TDS as one reading alongside pH and microbiological tests.
If you don't have a meter yet, see our TDS meter price guide for Pakistan, with step-by-step instructions for using one.
Frequently asked questions
Is zero TDS best for drinking water?
No. Water with almost no dissolved minerals tastes flat and can be slightly acidic. A low reading does not by itself mean better water.
Is a high TDS level bad?
Not automatically. It depends on what is dissolved. Only a laboratory test can tell you which substances are present and in what amounts.
Does boiling water lower TDS?
No. Boiling kills germs but does not remove dissolved minerals. As some water evaporates, the TDS reading can actually rise slightly.
Does a TDS meter tell me if water is safe?
No. TDS is one useful indicator for taste and membrane performance, but it cannot detect bacteria or specific harmful substances such as lead or arsenic.
What is the full form of TDS?
Total Dissolved Solids. It's the total amount of minerals, salts and other matter dissolved in water.
What is TDS measured in?
Milligrams per litre (mg/L) or parts per million (ppm). For drinking water they're the same number, so 250 ppm is 250 mg/L.
How can I check TDS at home?
With a digital TDS meter, often called a TDS pen. Basic pens cost about Rs 1,500 to Rs 1,800 in Pakistan. Our TDS meter guide shows how to get a reading you can trust.
Related: TDS of drinking water: chart for Pakistan · TDS meter price in Pakistan · RO water remineralisation guide · Sodium, calcium & magnesium in water treatment