What Is TDS? Total Dissolved Solids, Explained

TDS is one of those three-letter terms that everyone in water uses and few people define out loud. The full form is Total Dissolved Solids, and the phrase is refreshingly literal: it is the total of everything dissolved in the water, weighed and reported as a single number, usually in milligrams per litre (mg/L), which for practical purposes is the same as parts per million (ppm).

The word doing the work is dissolved. TDS does not count the sand, silt or particles you could filter out — those are suspended solids, a different measurement. TDS counts only what has gone into true solution, invisibly, and would remain if you filtered the water and then boiled it dry. Whatever is left in the dish is the dissolved solids.

What actually makes up TDS

In most natural water, the bulk of TDS is a handful of common ions the water picked up from the rock and soil it passed through:

• Calcium and magnesium — the two that also define hardness.
• Sodium and potassium — the alkali metals; sodium is what climbs in saline and coastal water.
• Bicarbonates, chlorides and sulphates — the main negative ions that balance the metals.

To those, depending on the source, you can add nitrates (often from agricultural runoff or sewage), fluoride, silica, and trace dissolved metals like iron, arsenic or manganese. All of it, harmless and harmful alike, gets summed into the one TDS figure. That is worth remembering: TDS is a quantity, not an identity. It tells you how much is dissolved, not what.

How TDS is measured

There are two ways, and they are not equal.

The gravimetric method is the reference: a lab filters a known volume of water, evaporates it, and weighs the residue directly. It is accurate and it is what a proper water analysis reports.

The handheld TDS meter everyone actually uses does something cleverer and looser. It does not weigh anything. It measures the water's electrical conductivity — dissolved ions carry current, and more ions mean more conductivity — and then multiplies by a conversion factor (commonly around 0.5–0.7) to estimate TDS. This works because most dissolved solids in ordinary water are ionic. But it is an estimate built on an assumption about what is dissolved, which is why two meters can disagree, and why a meter reading is a screen, not a lab result.

What is a typical TDS?

Rough landmarks help calibrate the number:

• Rainwater / distilled / RO permeate: near zero to ~50 mg/L.
• Good municipal supply: often 100–400 mg/L.
• Hard borewell water: 500–1,500 mg/L and up.
• Brackish groundwater: a few thousand mg/L.
• Seawater: roughly 35,000 mg/L.

India's drinking-water standard treats 500 mg/L as the acceptable ceiling and 2,000 mg/L as the permissible limit where no better source exists — the detail we cover in TDS in drinking water.

TDS is not hardness (a common mix-up)

Because calcium and magnesium show up in both, people conflate the two. They are related but distinct. Hardness counts only the scaling ions (calcium and magnesium). TDS counts those plus everything else dissolved. A water can be high in TDS but soft — lots of sodium and chloride, little calcium — or moderately hard with modest TDS. If your problem is scale in boilers and geysers, hardness is your number; if it is salinity, taste, or membrane performance, TDS is.

How to reduce TDS

Because TDS is dissolved matter, ordinary filtration does not touch it — you cannot strain out something in solution. Lowering TDS means one of a few technologies that separate ions from water:

• Reverse osmosis (RO) — the workhorse; a semi-permeable membrane that passes water and rejects the dissolved ions, cutting TDS by 90%+ in one pass.
• Ion exchange / deionisation — resins that swap the dissolved ions for harmless ones; the basis of softening and of demineralised (DM) water.
• Electrodeionisation (EDI) and distillation — for very low-TDS, high-purity applications.

For industry the same logic scales up: reducing the dissolved load in a process or wastewater stream is usually an RO or evaporation problem, and it is rarely free — every ion you remove leaves in a reject or a solid you then have to manage.

The single most useful habit with TDS is to treat it as the opening question, not the closing one. It tells you how loaded your water is and flags when a source has drifted. What is in that load — and whether any of it is harmful — is a separate test, and the one that should actually drive what you do next. If you want help reading a water analysis and choosing the right treatment for it, we are glad to help — [email protected] or +91-98100 00233.