The units converter handles the conversions growers hit daily: EC to ppm on any TDS scale, nutrient dosing between ml/L and ml/gal, plus volume, mass, length, area, temperature, pressure and airflow.
Formula — Units converter
A TDS meter does not measure dissolved solids at all — it measures electrical conductivity and multiplies it by a fixed scale factor. Three different scales are in circulation, so the same nutrient solution can legitimately read 1000, 1280 or 1400 ppm depending on which meter you own. Always record EC in a growlog; it is the only value that never needs a footnote.
- ppm = EC [mS/cm] × scale
- EC [mS/cm] = ppm ÷ scale
- EC [µS/cm] = EC [mS/cm] × 1000
- ml/gal (US) = ml/L × 3.785 · ml/gal (UK) = ml/L × 4.546
- °F = °C × 9/5 + 32
- Scale 500 (NaCl, “ppm 500” or TDS/2): Hanna, Bluelab in ppm500 mode, most cheap pens.
- Scale 640: European meters, Milwaukee, some Adwa models.
- Scale 650: older Eutech and a few hydroponics meters.
- Scale 700 (KCl, “ppm 700”): Truncheon, Oakton, most US nutrient charts.
Worked example — Units converter
A meter shows 1.8 mS/cm (1800 µS/cm) in mid-flower feed. On the 500 scale that is 1.8 × 500 = 900 ppm; on the 700 scale the same solution reads 1.8 × 700 = 1260 ppm. A feeding chart that says “1200 ppm” therefore means EC 2.4 on a 500-scale meter but EC 1.71 on a 700-scale one — a 40% difference in actual salt load, which is how growers accidentally burn a crop by following someone else’s numbers.
Dosing works the same way: 2 ml/L becomes 2 × 3.785 = 7.6 ml per US gallon, and a 40 L reservoir needs 80 ml of that product. Converting the other way, a US chart calling for 5 ml/gal is 5 ÷ 3.785 = 1.3 ml/L.
Typical ranges — Units converter
Coco and hydro run at the lower end of these bands with higher frequency; soil tolerates the upper end. Runoff EC above the input EC means salts are accumulating and the medium needs plain water; runoff well below the input EC means the plant is eating faster than you are feeding it. pH targets stay 5.5–6.2 in coco and hydro, 6.2–6.8 in soil, independent of scale.
| Stage | EC (mS/cm) and ppm on the 500 scale |
|---|---|
| Seedling / clone | 0.4–0.8 · 200–400 ppm |
| Early veg | 0.8–1.2 · 400–600 ppm |
| Late veg | 1.2–1.8 · 600–900 ppm |
| Early flower | 1.6–2.2 · 800–1100 ppm |
| Peak flower | 2.0–2.6 · 1000–1300 ppm |
| Flush / late ripening | 0.4–1.0 · 200–500 ppm |