TDS - Total Dissolved Solids

TDS is a measure of all dissolved substances in water that can pass through a very small filter (a sieve measuring 2 µm). aqion calculates TDS as the sum of all ions: 1

(1) TDS   =   Σ cations + Σ anions

The calculated TDS value is displayed in in the output table in units of mg/L (or ppm). Since TDS is made up of inorganic salts it is often referred to salinity.

The three so-called sum parameters (TDS, EC, and ionic strength) depend on all ions dissolved in the aqueous solution and, in this way, they are related to each other. For example, TDS and EC are related by the empirical linear relationship:

(2) TDS (mg/L)   =   conversion factor × EC (µS/cm)

where the conversion factor ranges between 0.55 and 0.70. On the other hand, a simple rule of thumb is:

(3) 2 µS/cm ≈ 1 ppm (mg/L)

Another empirical formula relates ionic strength I with TDS:

(4) I (mol/L)   ≈   2.5 10-5 × TDS (mg/L)

Last but not least, TDS is a helpful parameter to check the plausibility of a water sample analysis.


  1. Organic compounds and suspended particles are ignored here. 

[last modified: 2014-11-30]