The unit of TDS is milligrams per liter (mg/L), which is the same as parts per million (ppm) . TDS stands for total dissolved solids, which is the total concentration of dissolved substances in water. TDS is essentially a measure of anything dissolved in water that is not an H2O molecule. TDS can be measured using a TDS meter, which is a small hand-held device used to indicate the total dissolved solids in drinking water. The normal TDS level ranges from 50 ppm to 1,000 ppm. The Environmental Protection Agency (EPA) has identified TDS as a secondary standard, meaning that it is a voluntary guideline in the United States.