Total dissolved solids (TDS) and hardness are two water quality parameters. They are usually mistaken to mean the same thing, but are they?

TDS and water hardness involve several physical and chemical parameters that determine water quality.

For chemical parameters, there are total solids, hardness, electrical conductivity, pH, and total dissolved solids. For physical parameters, there are odor, color, etc.

Difference Between TDS vs Hardness

While total dissolved solids (TDS) are defined as the overall concentration of organic compounds or non-organic salts dissolved in water, hard water is simply water with a high concentration of minerals.

Although we will explain the difference in detail below, the summary is that total dissolved solids comprise inorganic and organic substances which can’t be eliminated from your water supply by a filter paper. Water hardness entails the presence of calcium and magnesium cations, chloride, and sulfate.

TDS vs Hardness Differences


TDS stands for Total Dissolved Solids, whereas hardness stands for a high concentration of minerals in the water.


Total dissolved solids or TDS in water are caused by dissolved organic matter and inorganic salts such as calcium salts.

Water hardness is caused by the presence of magnesium and calcium carbonates. Alternatively, it can also be caused by magnesium and calcium chlorides or sulfates.

Unit of Measurement

TDS is measured through ppm (parts per million) units.

Units such as mol/L (molar concentration per liter), mmol/L (millimole concentration per liter), GPG (grains per gallon) are used to quantify or measure water hardness.


TDS measurements are used for testing bodies of freshwater, the maintenance of spas, pools, aquariums, hydroponic and agriculture cultivations, etc.

Before water treatment, people test water hardness to determine the quantity of hardness minerals in the water. After treatment, testing helps you know if the water treatment is working or not.


Since TDS doesn’t refer to just one particular element, it refers to a wide variety of elements, especially those ions or elements that are soluble in water, such as calcium, potassium, magnesium, sodium, etc. They aren’t crucial for the survival of terrestrial beings. They are just a mix of different mineral elements.

However, for aquatic beings to survive, the mineral value of the water has to remain constant. This means the fluctuation of the water’s mineral content or balance can be detrimental to the survival of aquatic beings. TDS is similar to water hardness because they are both a parameter for mineral content in water. However, the scope of what makes up TDS is broader than what constitutes water hardness minerals. The higher the TDS value, the harder the water.

TDS also affects water turbidity. Turbidity is simply a measure of your water’s clarity. A strong TDS value in water means low turbidity water, indicating the water won’t be clear. In summary, a high TDS value is a negative for water quality.

Water hardness triggers scale buildup on surfaces and in your pipes. It changes your water taste and forms soap scum on your bathroom fixtures and calcites on your showerheads. It also causes spotting on your kitchenware.


Due to the tiny size of some dissolved solids, a conventional water filter system won’t be able to eliminate them from your drinking water. To get rid of TDS in your home water supply, use a residential reverse osmosis system.

To get rid of water hardness, use a salt-based water softener and RO filtration.

The Concept of TDS

TDS is an abbreviation for total dissolved solids. In contrast, dissolved solids refer to any metal, anion, cation, salt, or mineral such as calcium carbonate dissolved in your water supply.

TDS comprises dissolved solids and small quantities of organic matter in your water. Basically, it’s a chemical parameter to judge your water quality.

TDS is a broad term, covering everything in your water supply aside from the water itself. The unit for TDS measurements is parts per million or ppm.

Ways to Measure TDS

There are two ways to measure TDS: Electrical Conductivity Method and Gravimetric Method.

Electrical Conductivity Method

The quantity of ions in a water sample is directly proportional to its electrical conductivity. This method has to do with electrical conductivity analysis through the water. Measuring TDS through relative conductivity measurement can be carried out by using a TDS meter or conductivity meter.

Gravimetric Method

This method is carried out by boiling water, so the liquid evaporates, then you measure the residue left behind to determine the TDS value. Measuring TDS through this method involves weighing the residue. This method may take time, but it’s a reliable and accurate measurement method.

TDS measurement is carried out for several reasons, such as maintenance of aquariums, swimming pools, hydroponic cultivations, freshwater testing, etc.

The Concept of Water Hardness

As explained earlier, water hardness refers to the concentration of minerals in your water sample. These minerals form carbonate.

Water hardness builds up in the water when water comes in contact with matter such as chalk or limestone. These rocks comprise mainly calcium and magnesium salts. Therefore, it is of two types: permanent hardness and temporary hardness.

Temporary Hardness

Temporary water hardness is caused by the concentration of bicarbonate compounds in water. This is because the mineral content makes up calcium and magnesium cations coupled with carbonate and bicarbonate anions when they enter and dissolve in the water supply. One can eliminate this hardness by adding lime or heating the water.

Permanent Hardness

This type of water hardness is caused by magnesium and calcium salts (chlorides and sulfates). You can’t remove them through evaporation or boiling. Instead, you’ll need a water softener or an ion-exchange column to remove this hardness.

Water Hardness Measurement

You’ll have to calculate the total concentration of mineral cations in your drinking water for measuring water hardness. The units for calculating are either mmol/L or mol/L. A very strong concentration of mineral content means the water will have a very high hardness degree.

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