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ppm, EC, and the PPM scales in hydroponics
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.
Crop converts and accounts for all of this
Work in ppm (either scale) or EC, set your tap-water baseline, and Crop gives the exact salt weights to hit your target — for raw salts or branded kits (MasterBlend, Jack's, General Hydroponics). EC/pH logging with drift warnings built in. Pay once, no subscription, works offline.
Sources
- Penn State Extension — Hydroponics nutrient solutions
- Cornell Controlled Environment Agriculture — hydroponic recipes
General guidance for vegetable and herb growing — calibrate your meter regularly and verify against your own system.