Total dissolved solids (TDS) represents the total amount of dissolved solids in water, measured in parts per million. The electrical conductivity of water is directly proportional to and can be used to calculate the amount of impurities like TDS present, since pure water has zero conductivity. The formula to calculate TDS from conductivity is: TDS= 0.65× Conductivity, where TDS is measured in parts per million and conductivity is measured in micro siemens per centimeter.