Convert EC to PPM Instantly: 500, 640, and 700 Scales
This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.
Hydroponic feed charts describe nutrient strength as either EC or PPM, and mixing the two up leads to overfeeding or underfeeding.
This calculator converts between them.
The catch with PPM is that meters use different conversion scales, so the same solution reads as a different PPM number on different meters.
All the common scales are shown, so you can match your meter to your feed chart.
EC: mS/cm
PPM (500 scale):
PPM (640 scale):
PPM (700 scale):
Match the scale to your own meter or your feed chart. When in doubt, work in EC, which reads the same on every meter.
How this works
EC, electrical conductivity, is the direct measurement, and it reads the same on any calibrated meter.
PPM is worked out from EC by multiplying by a scale factor set by the meter maker.
The three common scales are 500, used by Hanna, Milwaukee, and most US meters, 640, a European standard, and 700, used by Bluelab and Truncheon.
The calculator applies whichever you need and shows the others for comparison.
Understanding your result
Match the scale to the meter you own, or to the one your feed chart was written for.
If a chart lists PPM but not the scale, that is a common problem, and it is why many growers work in EC instead, since EC avoids the confusion.
As a rough guide, many leafy crops sit around 1 to 2 mS/cm, and fruiting crops higher, but follow the range for your own plant.
Important note
This is an exact conversion for the scale you pick.
Even so, PPM itself is an estimate of total dissolved solids, not a precise particle count, and the scale factors are meter-maker conventions rather than a single scientific constant.
That is why the same solution gives different PPM numbers on different meters.
When in doubt, work in EC.
Scientific basis
The role of EC in hydroponic nutrient management, and typical crop ranges, come from university extension research, while the 500, 640, and 700 scale factors are meter-manufacturer conventions:
- Oklahoma State University Extension: Electrical Conductivity and pH Guide for Hydroponics
- Oklahoma State University Extension: EC and pH Guide (full fact sheet PDF)
Keep Planning With These Calculators
I recommend these calculators alongside this one because reading the strength correctly is only the first step in managing a feed.
After a reading shows a reservoir is too strong, the Reservoir EC Dilution Calculator works out how much water brings it back to target.
To mix a feed from a label dose, the Fertiliser Dilution Calculator gives the exact amount for any volume.
For indoor setups, the DLI Calculator turns a light-meter reading into a daily light integral.
Related reading
My guide on hydroponic nutrient solutions explains how to mix and manage a feed.
My guide to hydroponic pH and EC meters covers getting a reading you can trust.
This calculator is for educational purposes and general guidance only. It is not professional horticultural advice. Follow the feeding guidance for your specific crop and nutrient product.
