EC to PPM Conversion Formulas
The conversion between EC (mS/cm) and PPM is linear, but the multiplier depends on your meter's calibration standard:
PPM (500 scale) = EC × 500 — Hanna Instruments, USAPPM (640 scale) = EC × 640 — Milwaukee, EuropePPM (700 scale) = EC × 700 — Bluelab, TruncheonExample: EC 2.0 mS/cm = 1000 PPM on the 500 scale, 1280 PPM on the 640 scale, or 1400 PPM on the 700 scale. This is why specifying the scale matters when sharing readings.
Frequently Asked Questions
What is EC in hydroponics?
EC (Electrical Conductivity) measures the concentration of dissolved ions in a nutrient solution, expressed in mS/cm (millisiemens per centimeter). Higher EC means more nutrients are dissolved. Most plants prefer EC between 0.8 and 2.5 mS/cm depending on growth stage.
What is the difference between the 500, 640, and 700 PPM scales?
PPM meters use different conversion factors: the 500 scale (used by Hanna Instruments and common in the USA) multiplies EC by 500; the 640 scale (used by Milwaukee and common in Europe) multiplies by 640; the 700 scale (used by Bluelab and Truncheon) multiplies by 700. Always check which scale your meter uses before comparing readings.
Is EC or PPM better for measuring nutrient solution?
EC is universally comparable because it uses a standardized unit (mS/cm). PPM depends on the conversion factor used by your meter, making it potentially confusing when communicating with other growers. For technical accuracy, use EC. For quick reference, PPM is fine as long as you know your scale.
What EC should I use for seedlings?
Seedlings are sensitive to high nutrient concentrations. Start with EC 0.4–0.8 mS/cm and increase gradually as plants develop their root system. Starting too high can cause nutrient burn and stunted growth.
How do I raise or lower EC in my reservoir?
To raise EC: add more nutrient concentrate. To lower EC: add plain water or RO water to dilute the solution. Always adjust gradually and re-test after mixing. pH drift often accompanies EC adjustments in recirculating systems.