A salinometer (salinity meter) is a device designed to measure the salinity, or dissolved salt content, of a solution. Since dissolved ionized solids like salts and minerals contribute to the conductivity of a solution, a TDS meter measures the conductivity of the solution then converts that figure to an estimated TDS reading.

A TDS meter typically displays the total weight of dissolved solids as a proportion of 1 litre of water. The unit used by most TDS meters is parts per million (ppm). For example, a TDS reading of 1 ppm would indicate there is 1 milligram of dissolved inorganic solids in each 1 litre of water.