EC Meters Explained: µS/cm, mS/cm, dS/m and PPM
1 mS/cm = 1000 µS/cm = 1 dS/m. Converting to PPM is where meters disagree, because it depends on a conversion factor, not a unit.
24 August 2026 · 6 min read · Science
Electrical conductivity (EC) is the quickest way to measure how much dissolved salt is in water. Aquarium keepers, hydroponic growers, farmers and water treatment plants all use it, but in different units.
The unit ladder
| Unit | Equal to |
|---|---|
| 1 S/m | 10 mS/cm = 10,000 µS/cm |
| 1 dS/m | 1 mS/cm |
| 1 mS/cm | 1,000 µS/cm |
| 1 µS/cm | 0.001 mS/cm |
dS/m and mS/cm are identical, which is why soil reports (dS/m) and hydroponic meters (mS/cm) show the same number.
Typical readings
| Water | Approximate EC |
|---|---|
| Distilled or RO water | under 10 µS/cm |
| Rainwater | 20–100 µS/cm |
| Typical tap water | 50–800 µS/cm |
| Hydroponic nutrient solution | 1.0–2.5 mS/cm |
| Seawater | about 50–55 mS/cm |
These are broad ranges. Local water varies widely.
EC to PPM (TDS) is not a unit conversion
Many meters also display PPM or TDS (total dissolved solids). A meter cannot measure dissolved solids directly. It measures EC and multiplies by an assumed factor:
| Scale | Factor | 1.0 mS/cm reads as |
|---|---|---|
| NaCl (often "500 scale") | 0.5 | 500 ppm |
| "640 scale" | 0.64 | 640 ppm |
| KCl / "442" (often "700 scale") | 0.7 | 700 ppm |
The same water can therefore read 500, 640 or 700 ppm on different meters. When sharing a reading, quote EC or name the scale.
Temperature matters
Conductivity rises about 2% per °C. Good meters compensate automatically to 25 °C. If yours does not, measure at a consistent temperature.
CF and hydroponics
Some growers, particularly in Australia and New Zealand, use CF (conductivity factor):
CF = EC in mS/cm × 10
So an EC of 1.8 mS/cm is CF 18.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy