EC vs PPM in hydroponics: what the numbers mean

A short guide for home growers.

Every hydroponic feeding chart tells you to aim for a number, and that number is given as EC, as PPM, or both. They sound like two different things, but they are two ways of reporting the same measurement. Understanding how they relate saves you from the most common mistake in nutrient mixing: following a chart written for a different scale than your meter.

What EC measures

EC is electrical conductivity. Dissolved nutrient salts split into charged particles (ions) that carry electric current, so the more nutrient is in the water, the better it conducts. A meter passes a small current between two electrodes and reports the conductivity, normally in millisiemens per centimetre (mS/cm). Plain rainwater or reverse osmosis water reads close to 0, and a typical leafy-green nutrient solution reads somewhere around 1 to 2 mS/cm.

You may also see EC as dS/m (the same number as mS/cm), as µS/cm (1,000 times larger) or as CF, the conductivity factor, which is simply EC × 10.

What PPM is, and why it is only an estimate

PPM means parts per million of dissolved solids. A meter cannot count dissolved solids directly. It measures EC and multiplies by a fixed factor to show a ppm figure. That factor depends on which reference salt the maker chose, so there are three common scales:

• 500 scale: ppm = EC × 500 (often listed for Hanna meters)
• 640 scale: ppm = EC × 640 (often listed for Eutech meters)
• 700 scale: ppm = EC × 700 (often listed for Bluelab meters)

Check your meter's manual to confirm which one yours uses, because brands change and some meters let you switch.

At 1.6 mS/cm, the same water reads 800 ppm on the 500 scale, 1,024 ppm on the 640 scale and 1,120 ppm on the 700 scale. The water has not changed, only the multiplier. That gap is large enough to over- or under-feed plants if you mix up scales.

Which one should you track?

Track EC. It is the actual measurement, it has no scale to get wrong, and most horticulture references use it. If a chart you like gives only ppm, find out which scale it assumes and convert it to EC once, then work in EC from then on. The EC to PPM converter does the conversion on all three scales at once.

Habits that keep readings trustworthy

• Calibrate regularly with calibration solution. Meters drift.
• Measure at a consistent temperature. Conductivity rises with temperature, so use a meter with temperature compensation or measure at about the same temperature each time.
• Rinse the probe between samples and store it as the maker recommends.
• Remember that EC includes your tap water. Hard water can start at 0.3 mS/cm or more before you add any nutrient, so measure your source water first.

A quick example

Your feeding chart says 800 ppm for young lettuce and your meter uses the 700 scale. If the chart was written for the 500 scale, 800 ppm is 1.6 mS/cm, which is 1,120 ppm on your meter. Dialling in 800 ppm on your meter would give only 1.14 mS/cm, noticeably weaker than the chart intended. If the water is too strong or weak, the hydroponic PPM calculator shows how much water or nutrient to add.

Estimates only: always follow your nutrient manufacturer's label and check your readings with a calibrated meter.