Convert EC to TDS in ppm, or TDS back to EC, using the 500, 640 or 700 scale or your meter’s own factor, with µS/cm, mS/cm and dS/m units.
EC to TDS Formula
- TDS — estimated mass concentration in mg/L; ppm by mass is mg/kg and is numerically similar only for dilute water near 1 kg/L density
- EC — electrical conductivity in mS/cm (equivalent to dS/m); 1 mS/cm = 1000 µS/cm
- k — chosen empirical scale in mg/L per mS/cm; k = 1000 × f, where f is mg/L per µS/cm
The relationship is approximate. Real water contains a mix of ions, and each contributes differently to conductivity. The scale (k) is the assumption your meter uses to translate one into the other. Always check which scale your TDS meter applies before comparing readings to a target.
Conversion Factors and Arithmetic Examples
Three numeric factor scenarios; match the actual meter or sample specification rather than assuming a universal salt identity:
| Scale | Reference basis | Factor f | mg/L per 1 mS/cm |
|---|---|---|---|
| 500 | Meter/sample specific | 0.50 | 500 |
| 640 | Meter/sample specific | 0.64 | 640 |
| 700 | Meter/sample specific | 0.70 | 700 |
Hypothetical arithmetic examples using f = 0.64. These are not water-type classifications, acceptance limits, crop targets or safety recommendations:
| Scenario | EC (mS/cm) | TDS estimate (mg/L) |
|---|---|---|
| Example 1 | 0 | 0 |
| Example 2 | 0.1 | 64 |
| Example 3 | 0.5 | 320 |
| Example 4 | 1 | 640 |
| Example 5 | 1.2 | 768 |
| Example 6 | 2 | 1280 |
Worked Example and FAQ
Example. Your meter reports EC = 1.2 mS/cm on the stated reference-temperature basis. With f = 0.64, estimated TDS = 1200 × 0.64 = 768 mg/L. With f = 0.50 it is 600 mg/L; with f = 0.70 it is 840 mg/L. Approximate ppm values additionally assume dilute water near 1 kg/L density.
Which scale should I use? Match the factor documented for your actual meter, reference or sample. A factor is an empirical assumption; the numeric scale does not by itself identify a universal reference salt or establish suitability.
Are mS/cm and dS/m the same? Yes. 1 mS/cm = 1 dS/m = 1000 µS/cm.
Does temperature matter? Yes. Conductivity depends on temperature and sample composition; no universal 2% per °C coefficient applies. Supply EC on the reference-temperature basis used by your factor or instrument, commonly 25 °C. This calculator does not perform temperature compensation.
Why is the conversion only approximate? Conductivity depends on ion concentrations, mobility and temperature. A constant empirical factor is a simplified sample/meter assumption, not measured gravimetric TDS. Concentrated or saline water may require density and a different relation.
