Short answer. Every "ppm" or TDS meter actually measures electrical conductivity (EC) and then multiplies by an assumed factor [1]. On the 500 scale (NaCl) the factor is 0.5; on the 700 scale (often called 442 or KCl-style) it is 0.7; some agricultural sources use 0.64 [1][2]. So the same solution at 1.0 mS/cm reads 500 ppm on one meter and 700 ppm on another. Talk in EC (mS/cm), or always state the scale.
What's happening
Dissolved salts conduct electricity; more ions, more conductivity. EC is measured in siemens per distance and corrected to 25 °C. It does not tell you which ions are present, only the total charge-carrying load.
"PPM" (TDS, total dissolved solids, mg/L) is an estimate: different salts conduct differently per gram, so no single factor is right for every solution. NDSU notes the true factor for natural waters rises with conductivity, from about 0.65 at 1,000 µS/cm to about 0.82 at 10,000 µS/cm [1]. Meter makers just pick one:
- 500 scale (NaCl, factor 0.5): common in North America.
- 700 scale (factor 0.7): common in Europe and Australia; often based on a "442" reference mix of sodium sulfate, sodium bicarbonate and sodium chloride.being reviewed
- 640: used in some agricultural and extension material [2].
None is the "true" ppm of your nutrients. A nutrient solution's ppm-by-element (e.g. N 180 mg/L) is a different thing again.
Units
| Unit | Equals |
|---|---|
| 1 mS/cm | 1,000 µS/cm |
| 1 mS/cm | 1 dS/m |
| 1 mS/cm | 1 mmho/cm |
| CF (conductivity factor) | EC (mS/cm) × 10 |
Conversion table
| EC (mS/cm) | CF | ppm 500 scale | ppm 640 | ppm 700 scale |
|---|---|---|---|---|
| 0.2 | 2 | 100 | 128 | 140 |
| 0.4 | 4 | 200 | 256 | 280 |
| 0.6 | 6 | 300 | 384 | 420 |
| 0.8 | 8 | 400 | 512 | 560 |
| 1.0 | 10 | 500 | 640 | 700 |
| 1.2 | 12 | 600 | 768 | 840 |
| 1.4 | 14 | 700 | 896 | 980 |
| 1.6 | 16 | 800 | 1,024 | 1,120 |
| 1.8 | 18 | 900 | 1,152 | 1,260 |
| 2.0 | 20 | 1,000 | 1,280 | 1,400 |
| 2.2 | 22 | 1,100 | 1,408 | 1,540 |
| 2.5 | 25 | 1,250 | 1,600 | 1,750 |
| 3.0 | 30 | 1,500 | 1,920 | 2,100 |
Formulas: ppm₅₀₀ = EC × 500; ppm₇₀₀ = EC × 700; ppm₇₀₀ = ppm₅₀₀ × 1.4; ppm₅₀₀ = ppm₇₀₀ × 0.714.
How strong should a cannabis feed be?
Typical working ranges are 0.6–1.0 mS/cm for seedlings and clones, 1.2–1.8 in veg and 1.6–2.2 in flower (see building a feed by stage); these are grower practice. The research supports not going higher: in deep-flow hydro, doubling input from 2 to 4 mS/cm raised solution nutrients but not yield or quality [3]; in rockwool, yield did not differ between 1.8 and 3 mS/cm, while 12 mS/cm cut yield about threefold [4].
How to tell which scale your meter uses
- Check the manual or the meter's setting menu (many let you choose).
- Test a calibration standard: 1.413 mS/cm (1,413 µS/cm) solution should read ~707 ppm on the 500 scale and ~990 ppm on the 700 scale.
- If the meter can display EC, switch it to EC and forget ppm.
Common mistakes
- Following a "1,000 ppm" feed chart written on the 700 scale with a 500-scale meter, and overfeeding by 40%.
- Forgetting that tap water already has EC; feed charts usually mean total EC, but some mean "added". Read the chart notes.
- Comparing EC readings taken at different temperatures on a meter without automatic temperature compensation.
- Thinking EC tells you nutrient balance. It doesn't; sodium chloride has EC too.
