What is TDS?

In my past blog, I have touch on TDS, Total Dissolved Solids. Today, I would like to explain what the TDS is.

 TDS stands for “Total Dissolved Solids”, meaning how many parts are in water and the unit is represented as PPM, Part Per Million.

Let’s see the example. There are 1000 liters of water and 17g of calcium is dissolved in the water. 1000 litters are equal to 1,000,000 mili litter, so the water which has 17g of calcium is 17ppm.


A device that can detect TDS, called a TDS meter, is selling online like amazon and so on for approximately $12.00. The TDS meter detects the electric conductivity in water and detects how many parts are in the water. Today, I measure my apartment’s public water and TDS showed 323ppm. If the parts are all calcium and magnesium, the hardness is 18.86GPG. GPG is Grain per Gallon, and 1GPG is equal to 17.12ppm. So it is 323/17.12 = 18.86.

TDS City Water

However. TDS meter cannot detect materials which do not conduct electricity. So, TDS meter can detect only inorganic ion substances such as calcium, magnesium, sodium, potassium, iron manganese and so on. On the other hand, since pesticide, trichloroethane, chlorine, bacterium and virus do not conduct electricity, TDS meter cannot detect them. In San Antonio, the city adds chlorine and fluorine in its public water system but since these materials do not conduct electricity, TDS meter does not sense them. If you want to know it, you need to get other special tools and reagent kit.


I have Reverse Osmosis System in my kitchen. I measured TDS of the RO water. It was 12ppm, so almost 300ppm has been removed with the RO system. The RO system can remove other materials not conducting electricity, so the RO is one of the good solutions to get very purified water.

TDS RO Water