Convert EC to TDS or TDS to EC using 500, 640, 700 or custom scales in mS/cm, µS/cm, dS/m, ppm and g/L, with ppm/mS factors and a taste guide.
EC to TDS Formula
TDS (ppm) = EC (mS/cm) × k
EC (mS/cm) = TDS (ppm) / k
- TDS — total dissolved solids in ppm (equivalent to mg/L)
- EC — electrical conductivity in mS/cm (equivalent to dS/m); 1 mS/cm = 1000 µS/cm
- k — conversion scale (ppm per mS/cm), set by the meter or reference salt
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 Scales and Typical Ranges
The three common scales used by handheld TDS meters:
| Scale | Reference salt | Factor | ppm per 1 mS/cm |
|---|---|---|---|
| 500 | NaCl | 0.50 | 500 |
| 640 | KCl | 0.64 | 640 |
| 700 | 442 | 0.70 | 700 |
Typical EC and TDS values you can use as a sanity check (640 scale):
| Water type | EC (mS/cm) | TDS (ppm) |
|---|---|---|
| Pure / RO water | 0.00 – 0.05 | 0 – 30 |
| Soft tap water | 0.1 – 0.5 | 65 – 320 |
| Hard tap water | 0.5 – 1.0 | 320 – 640 |
| Hydroponic nutrient | 1.0 – 2.5 | 640 – 1600 |
| Brackish | 3 – 30 | 1900 – 19000 |
| Seawater | ~50 | ~32000 |
Worked Example and FAQ
Example. Your meter reads EC = 1.2 mS/cm and is set to the 640 scale. TDS = 1.2 × 640 = 768 ppm. On the 500 scale, the same EC reads 600 ppm. Same water, different number.
Which scale should I use? Match the scale your meter or test method uses. Hydroponics and pool guides often quote the 500 (NaCl) or 700 (442) scale. Lab and drinking-water references commonly use the 640 (KCl) scale.
Are mS/cm and dS/m the same? Yes. 1 mS/cm = 1 dS/m = 1000 µS/cm.
Does temperature matter? Yes. EC rises about 2% per °C. Use a meter with automatic temperature compensation (ATC) referenced to 25 °C, or your TDS estimate will drift with the water temperature.
Why is the conversion only approximate? Conductivity depends on which ions are present and their mobility. The scale factor assumes a specific reference salt. Real samples deviate from that assumption, so treat TDS from EC as an estimate, not a measured mass.
