What Is TDS in Water? How to Check It at Home and What Your Reading Means (India)

Short answer: TDS (total dissolved solids) is the amount of salts and minerals dissolved in your water, shown in ppm or mg/L. You can check it at home in under a minute with a pen-style TDS meter. The Indian drinking water standard, BIS IS 10500, treats 500 mg/L as the acceptable limit. Below that, a UV or UV+UF purifier is usually enough if your water is otherwise clean. Above that, you need RO. A TDS reading does not tell you whether water has germs, so it is one test, not the whole answer.

What TDS actually measures

Water picks up dissolved material on its way to your tap. That includes calcium and magnesium (the minerals that cause hardness), sodium, chloride, sulphate, bicarbonate, and small amounts of other salts. TDS is the total weight of all of these in one litre of water.

Two things it does not measure:

• Germs. Bacteria, viruses and cysts are not dissolved solids. Water can read 80 ppm and still make you sick.
• Specific harmful chemicals. A low TDS number does not rule out arsenic, fluoride, lead or pesticides. Those need a lab test.

So TDS answers one question well: how much dissolved salt and mineral is in this water? That single number decides whether you need RO.

How to check TDS at home in 4 steps

A digital TDS meter looks like a thick pen with two metal pins at one end. It measures how well the water conducts electricity and converts that into a ppm reading.

1. Take a clean glass. Rinse it with the same water you are testing, not with purified water, so leftover drops do not skew the result.
2. Fill it from the tap you want to test. Let the tap run for a few seconds first if the pipe has been sitting idle.
3. Remove the cap, switch the meter on and dip only the pins. Stir gently once to clear air bubbles.
4. Wait for the number to settle, then press HOLD. Note the reading, rinse the pins and put the cap back.

A basic meter costs a few hundred rupees. We have a Konvio digital TDS meter listed at ₹299.

Mistakes that give a wrong reading

• Testing hot or very cold water. Conductivity changes with temperature. Test water at room temperature.
• Dirty pins. Hard water leaves a white film on the pins over time. Wipe them with a soft cloth.
• Dipping the whole meter. Most cheap meters are not waterproof above the pins.
• Testing only once. Borewell and tanker water can change from week to week. Test a few times across a month before you buy a purifier.
• Reading purified water as "too low = broken purifier". RO output is supposed to be low. Read the section on RO output below before you panic.

What your reading means

Your tap water reading	What it usually means	Purifier type to look at
Under 200 ppm	Low dissolved salts. Common for treated municipal supply	UV or UV+UF. RO not needed
200 to 500 ppm	Within the BIS acceptable limit. Taste may turn slightly salty or heavy towards 500	UV+UF if taste is fine. RO with a TDS controller if taste or hardness bothers you
Above 500 ppm	Above the acceptable limit. Common with borewell and tanker water	RO, ideally with a TDS controller or mineral cartridge
Above about 2,000 ppm	Very high. Many standard RO units are rated only up to a certain input TDS	RO rated for high input TDS. Check the product page

Two caveats:

• TDS alone is not a safety verdict. If your water comes from a borewell in an area known for fluoride or arsenic, get a lab test. Your local water department or an accredited lab can do it.
• Germs need killing at every TDS level. Whatever type you pick, it should have UV or UF, or both, for bacteria and viruses.

RO vs UV vs UF in plain words

• RO (reverse osmosis) pushes water through a very fine membrane that holds back most dissolved salts. It is the only one of the three that lowers TDS. It needs electricity and good water pressure, and it sends part of the input water to the drain as reject water.
• UV shines ultraviolet light on the water to kill bacteria and viruses. It does not change TDS at all.
• UF (ultrafiltration) is a membrane that blocks bacteria, cysts and fine particles. It does not reduce TDS. Many UF stages work without electricity.

Most purifiers sold today combine two or three of these. An "RO+UV+UF" unit does all three jobs. A "UV+UF" unit is for low-TDS water and skips the membrane that lowers TDS.

Why your RO water reads 20 or 40 ppm

People often test their RO output, see a very low number and worry the water has "no minerals". This is where the TDS controller (also called a TDS adjuster or mineraliser) comes in. It lets a small amount of filtered, non-RO water mix back in, so the output reading rises to a level you choose.

If your purifier has one, you can adjust it until the taste suits you. If it does not, a low reading only means the RO membrane is working. Rising output TDS over months is the warning sign: it usually means the membrane is due for service.

Matching your reading to real purifiers

These are live listings on RichDeals right now. Prices change often, so open each one for the current price.

For low-TDS municipal water (under about 200 ppm):

Purifier	Type	Posted price
Copper UV+UF 12L purifier for municipal water	UV+UF	₹2,899
Pureit Copper UV tankless purifier	UV	₹5,998
Havells Aquas Lite 7L	UV+UF	₹6,999

For high-TDS borewell or tanker water (above about 500 ppm):

Purifier	Type	Posted price
Kinsco Aqua Punch 15L with TDS control	RO+UV+UF	₹3,999
Aqua D Pure 12L	RO+UV+UF	₹4,749
Aquaguard Sure Delight NXT	RO+UV	₹8,999

For brand-by-brand picks and what to check in the warranty, read our water purifier buying guide. For today's other price-checked deals, see our live offers page.

When to test again

• Before buying a purifier. Test the tap water a few times over two to four weeks.
• When you move house or change source. Switching from municipal supply to a borewell or tanker can change the reading a lot.
• Every month or two on RO output. A steady climb in output TDS means the membrane or filters need service.
• When the taste changes. A sudden salty or bitter taste is worth a quick check.