How to use the ec to ppm conversion calculator
- Enter the reading and choose its unit or scale: conductivity in mS/cm, uS/cm or dS/m, or ppm on the 500, 640 or 700 scale.
- Read the conductivity in all three units and the ppm on every scale.
- Use the conductivity factor if your nutrient chart is written in CF.
- Compare readings only on the same scale.
Formula
1 mS/cm = 1,000 uS/cm = 1 dS/m
ppm = EC in uS/cm x scale factor, with the factor 0.5, 0.64 or 0.7
EC in uS/cm = ppm / scale factor
Conductivity factor CF = EC in mS/cm x 10
The three scales come from different assumptions about the dissolved salt: 0.5 for sodium chloride, 0.64 for a mix of salts and 0.7 for potassium chloride. A meter simply multiplies by one of them.
Common readings on three scales
An EC of 1.4 mS/cm is 1,400 uS/cm. On the 500 scale that reads 700 ppm, on the 640 scale 896 ppm and on the 700 scale 980 ppm. Its conductivity factor is 14.
A meter showing 1,000 ppm on the 500 scale stands for 2 mS/cm, which would show as 1,280 ppm on the 640 scale and 1,400 ppm on the 700 scale. A reading of 2,000 uS/cm is the same as 2 mS/cm.
Why one EC has three ppm values
A conductivity meter measures how well the water carries current, not what is dissolved in it. Turning that into ppm needs an assumption about the salts, and meter makers chose different ones. That is why two meters can show different ppm for the same water, while both agree on the EC.
To mix a stronger solution down to the strength you want see the solution dilution calculator, and for acidity the pH and weak acid calculator. Growing tools that go with this one are the VPD calculator and the daily light integral calculator.
Temperature and calibration
Conductivity rises with temperature, so most meters correct to 25 C. Calibrate with a standard solution and rinse the probe. The page converts numbers only; it cannot correct a drifting meter or say what strength a crop should have.
Assumptions and limits
- The scale factors 0.5, 0.64 and 0.7 are the common conventions, not measurements of your water.
- The conversion is linear, which is a fair approximation for dilute solutions and weaker for very strong ones.
- Readings are taken at, or corrected to, the same temperature.
- The page does not say what strength a plant needs or recommend a nutrient mix.
Frequently asked questions
How do I convert EC to ppm?
Multiply the EC in microsiemens per centimeter by the scale factor: 0.5, 0.64 or 0.7. An EC of 1,400 uS/cm is 700 ppm on the 500 scale.
Which ppm scale should I use?
Use the one your meter is set to, and keep it the same when you compare readings. The EC itself is the same on any scale.
What is the conductivity factor?
CF is the EC in mS/cm multiplied by 10, so 1.4 mS/cm is a CF of 14. Some growing charts use it in place of EC.
Is 1 mS/cm the same as 1 dS/m?
Yes. Both equal 1,000 uS/cm, so the number does not change between those two units.