ppm, EC, and the PPM scales in hydroponics
Why two nutrient meters can read the same reservoir and show different numbers.
If your meter says "1,200" and a recipe says "EC 1.8," they may be describing the same nutrient strength. Hydroponic nutrient concentration is reported several ways, and mixing them up is one of the most common sources of confusion for vegetable and herb growers.
EC is the real measurement
A meter doesn't actually measure nutrient ppm — it measures electrical conductivity (EC), how well the solution conducts current, which rises with dissolved salts. EC (in mS/cm or µS/cm) is the honest, direct reading. "ppm" is a conversion from EC, and that's where it gets messy.
The two ppm scales
There's no single ppm standard — meters use one of two conversion factors from EC:
| Scale | Factor | EC 1.0 mS/cm = |
|---|---|---|
| 500 scale (TDS / "Hanna") | × 500 | 500 ppm |
| 700 scale ("Eutech" / European) | × 700 | 700 ppm |
So a reservoir at EC 2.0 reads "1,000 ppm" on a 500-scale meter and "1,400 ppm" on a 700-scale meter — same solution, different number. Always know which scale your meter uses before following a ppm-based recipe.
Tip: when in doubt, work in EC. It's unambiguous. The common hobby conversion factor (the "CF" or 0.7 factor) maps EC to the 700 scale; if a recipe gives ppm without saying which scale, EC is the safer language to compare against.
Don't forget your tap water
Most tap water already carries some EC from dissolved minerals. If your source water is EC 0.4 and your target is EC 1.8, you're only adding 1.4 of nutrients. Growers who ignore the baseline often over- or under-feed and chase phantom deficiencies.
Gear this guide uses
Reading EC honestly — and knowing which scale your meter uses — starts with the meter itself.
- Measure conductivity directly with an EC / TDS meter and note whether it reads the 500 or 700 scale.
- Keep it honest with EC calibration solution.
- Mix to a known EC with hydroponic nutrients.
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Sources
General guidance for vegetable and herb growing — calibrate your meter regularly and verify against your own system.