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PPM (Parts Per Million)
Definition: PPM, or parts per million, expresses how concentrated dissolved solids are in water or nutrient solution. Most grow meters don't measure ppm directly; they measure electrical conductivity and convert it using a 500 or 700 scale, so the same solution can show two very different ppm numbers on two different meters.
How PPM (Parts Per Million) works
PPM is popular in the US because it sounds intuitive, but it hides a catch. A handheld “ppm meter” is an EC meter with a built-in multiplier. Manufacturers pick different multipliers, so a feed chart written for one scale can lead you to overfeed or underfeed on the other.
Conversion table
| EC (mS/cm) | 500 scale (ppm) | 700 scale (ppm) |
|---|---|---|
| 0.5 | 250 | 350 |
| 1.0 | 500 | 700 |
| 1.5 | 750 | 1,050 |
| 2.0 | 1,000 | 1,400 |
Check your meter’s manual or settings to see which scale it uses, then check which scale your nutrient company’s chart assumes.
What your water reads
City tap water in the US often lands between 50 and 300 ppm on the 500 scale, depending on hardness. Reverse osmosis water reads close to zero, which is a clean slate but also means you’ll need to add calcium and magnesium, usually with a Cal-Mag supplement.
Another ppm in the grow room
The same unit shows up for gases. When growers talk about 1,200 ppm of CO2, they mean the concentration of carbon dioxide in the air, which has nothing to do with nutrient strength. Context tells you which one is meant.
If you share numbers on forums or compare with other growers, quoting EC avoids the scale confusion entirely.