Quick answer: Total dissolved solids (TDS) refers to dissolved material in water and is often reported in milligrams per liter or parts per million for dilute water solutions. Many electronic meters do not directly weigh dissolved solids; they estimate a TDS value from electrical conductivity.
TDS is not a list of chemicals
A TDS number does not identify the individual minerals, nutrients, salts, or contaminants present. Two samples can show similar TDS values while having very different chemical compositions.
Conductivity conversion
Because the relationship between conductivity and dissolved-solids concentration depends on water chemistry, electronic conversions use a factor. If the factor is not stated, readings from different meters can disagree even when their conductivity measurements are similar.
How to use TDS responsibly
- Use TDS to follow trends in the same system with the same instrument and settings.
- Record the unit and, when available, the conversion factor.
- Use EC as the primary value in hydroponics when users may apply different PPM scales.
- Use laboratory analysis when you need to know which dissolved substances are present.
Common causes of change
Evaporation, dilution, nutrient additions, mineral accumulation, source-water changes, and water replacement can change TDS. Probe contamination, temperature differences, bubbles, and an incorrect conversion setting can also create apparent changes.
Important limitation
TDS alone does not determine whether water is safe to drink and does not detect many microbiological or chemical hazards.
Sources
- U.S. Geological Survey Water Science Glossary
- U.S. Geological Survey: Conductance and Dissolved-Solids Relationship
Reviewed August 1, 2026. Educational content; application-specific targets should be confirmed for the actual system, crop, or aquatic species.