TDS Calculator — Total Dissolved Solids in Water

Képlet

How this is calculated

TDS (Total Dissolved Solids) is the combined concentration of all dissolved inorganic and organic substances in water — salts, minerals, metals and organic matter — reported in ppm (parts per million) or mg/L, which are numerically equivalent for water. A TDS or EC meter does not measure solids directly; it measures electrical conductivity (EC) in microsiemens per centimetre (µS/cm), because dissolved ions carry electrical charge. TDS is then estimated as TDS ≈ EC × K, where K is an empirically determined conversion factor that accounts for the ionic composition of the water.

K varies with the type of dissolved ions: K ≈ 0.5 is used for sodium chloride (NaCl) calibration solutions, which is the factory default for most consumer meters; K ≈ 0.64 is the standard for general potable water with a typical ion mix; K ≈ 0.7 is used with potassium chloride (KCl) standards and some laboratory instruments. If your meter label says "NaCl calibration", use K = 0.5 or 0.64 depending on your source water. The default 0.64 is a reasonable starting point for most tap and well water.

Quality benchmarks: the US EPA Secondary Drinking Water Regulation recommends a maximum of 500 ppm for aesthetic reasons; the WHO guideline is also 500 ppm. TDS below 50 ppm (e.g., RO or distilled water) may taste flat and lack beneficial calcium and magnesium. Very low TDS does not guarantee safety — bacteria and many pesticides have negligible effect on conductivity. A lab gravimetric test (APHA Method 2540C) is the reference method for TDS; EC meters provide an estimate.