TDS Calculator

Total dissolved solids is the combined mass of inorganic salts and dissolved organic matter in water, reported in mg/L. Measuring it directly means evaporating a filtered sample to dryness at 180 degrees C and weighing the residue, which takes hours, so field practice infers it from electrical conductivity instead. Dissolved ions carry current, and over the ranges found in natural water the conductivity rises nearly in proportion to the ion load, giving TDS = EC * k. The conversion factor k is not universal: it runs from about 0.50 for water dominated by sodium chloride to about 0.70 for water rich in calcium and sulfate, with 0.65 a common default for mixed freshwater. Conductivity is also strongly temperature dependent, gaining roughly 2 percent per degree C, so readings are normalized to the reference temperature of 25 degrees C using EC25 = EC / (1 + 0.02 * (T - 25)) before the factor is applied. Useful reference points: distilled water sits below 10 mg/L, typical municipal tap water between 100 and 400 mg/L, and seawater near 35,000 mg/L. The World Health Organization describes palatability as excellent below 300 mg/L, good to 600, fair to 900, poor to 1200 and unacceptable above that, while the United States Environmental Protection Agency sets 500 mg/L as a secondary, non-enforceable standard based on taste rather than health. The method's limitation is fundamental: conductivity sees only ionized species, so dissolved sugars, silica, alcohols and most non-ionic organic contaminants contribute to true gravimetric TDS but register nothing on a meter. A conductivity-derived figure is therefore an estimate, typically within about 10 percent of the gravimetric value for ordinary freshwater and much worse outside it.

Quantitative conductivity measurement of solutions was established by Friedrich Kohlrausch in Germany during the 1870s. By using alternating current and platinized platinum electrodes he eliminated the electrode polarization that had defeated earlier attempts, and in 1875 he formulated the law of independent migration of ions, showing that each ion contributes its own share to the total conductance. That result, reinforced by Svante Arrhenius's theory of electrolytic dissociation in 1884, is precisely why a single bulk conductivity reading can be translated into an approximate total ion concentration. Reproducibility across laboratories came with standard potassium chloride reference solutions, refined to high precision by Grinnell Jones and Bassett Bradshaw in 1933 and still the basis of conductivity cell calibration. The gravimetric definition of total dissolved solids was codified by the American Public Health Association in Standard Methods for the Examination of Water and Wastewater, first issued in 1905, whose residue-on-evaporation procedure remains the reference against which conductivity estimates are judged. Regulatory limits followed in the second half of the century: the Environmental Protection Agency published TDS among its National Secondary Drinking Water Regulations in 1979, treating it as an aesthetic parameter affecting taste, scaling and corrosion, and the World Health Organization included palatability ratings in its Guidelines for Drinking-water Quality from 1984 onward. Inexpensive handheld meters that display an inferred TDS value rather than raw conductivity spread widely from the 1990s, which is why the conversion factor and its assumptions now matter to consumers as well as laboratories.