TDS, in water treatment, is the inorganic residue left after the filtration of colloidal and suspended solids and then the evaporation of a known volume of water. TDS is reported as ppm or mg/l. TDS, in RO design projections, is determined by calculation using the sum of the cations, anions and silica ions (with the ion reported "as such", not "as calcium carbonate"). Feed or permeate TDS, in RO design projections, can also be estimated by applying a conversion factor to the conductivity of the solution. TDS can also be determined in the field by use of a TDS meter.

            TDS meters measure the conductivity of the water and then apply a conversion factor that reports TDS to a known reference solution (e.g. ppm sodium chloride or ppm potassium chloride). The user is cautioned that TDS levels for waters with a mixture of ions and determined from conductivity measurements may not agree with TDS calculated as a sum of the ions. As a rough rule of thumb, one ppm of TDS (when referenced to a NaCl solution) correlates to a conductivity of two micromhos/cm (microSiemens/cm).