Quick answer: EC measures electrical conductivity. TDS is a dissolved-solids quantity that many meters estimate from EC. Salinity describes dissolved salt content and can be expressed as ppt, percent, or through related values such as specific gravity.
Comparison
| Reading | What it represents | Common units |
|---|---|---|
| EC | Ability of water to conduct electrical current | µS/cm, mS/cm |
| TDS | Dissolved-solids amount or an EC-based estimate | mg/L, ppm |
| Salinity | Salt content or a salinity-related estimate | ppt, %, practical salinity |
| Specific gravity | Density ratio relative to a reference | Unitless |
Why readings do not convert universally
The relationship between EC, TDS, salinity, and specific gravity depends on chemical composition and temperature. A fixed conversion may be useful for a defined system, but it should not be presented as universally exact.
U.S. hydroponic PPM scales
Hydroponic users may use different EC-to-PPM factors. For example, 1.4 mS/cm can be displayed as approximately 700 ppm with a 500 factor or approximately 980 ppm with a 700 factor. State the factor and use EC as the primary value when comparing systems.
Which reading should you use?
- Hydroponics: use EC as the primary trend value and document any PPM factor.
- Saltwater and reef systems: use the salinity or specific-gravity convention appropriate to the care protocol and temperature method.
- General water screening: EC or TDS can show change, but neither identifies the dissolved chemicals.
Sources
Reviewed August 1, 2026. Educational content; application-specific targets should be confirmed for the actual system, crop, or aquatic species.