The two principal methods of measuring total dissolved solids are gravimetric analysis and conductivity. Gravimetric methods are the most accurate and involve evaporating the liquid solvent and measuring the mass of residues left. This method is generally the best, although it is time-consuming. If inorganic salts comprise the great majority of TDS, conductivity-based methods are appropriate.
Conductivity of water is directly related to the concentration of dissolved ionized solids. These ions allow the water to conduct electric current. This electric current can be measured using a conventional conductivity meter or TDS meter. When correlated with laboratory TDS measurements, conductivity provides an approximate value for the TDS concentration, with around 10% accuracy.
The relationship of TDS and specific conductance of groundwater can be approximated by the following equation:
:TDS = kEC
where TDS is expressed in mg/L and EC is the electrical conductivity in microsiemens per centimeter at 25 °C. The conversion factor k varies between 0.55 and 0.8.
Some TDS meters use an electrical conductivity measurement to the ppm using the above formula. Regarding units, 1 ppm indicates 1 mg of dissolved solids per 1000 g of water.