PPM (Parts Per Million)

PPM (parts per million) is a measurement of dissolved solids in nutrient solution, originally adapted from water quality testing for agricultural use. PPM is mathematically derived from EC (electrical conductivity) using one of two conversion factors: the "500 scale" (also called NaCl or PPM-500, where PPM = EC × 500) used by Hanna and most American meters, or the "700 scale" (also called KCl or PPM-700, where PPM = EC × 700) used by Bluelab and most international meters. This ambiguity is why serious growers prefer EC — a reading of "1500 PPM" is meaningless without specifying the scale, and the two scales differ by 40%. Standard PPM ranges for cannabis (using 500 scale): Seedlings 200-400 PPM, Early veg 500-700 PPM, Late veg 700-900 PPM, Early flower 900-1100 PPM, Mid-flower 1100-1300 PPM, Late flower 1000-1200 PPM, Flush 0-200 PPM. To convert to 700 scale, multiply these numbers by 1.4. Many beginner growers make the mistake of using PPM ranges from one source without verifying scale, causing massive overfeeding (when intended numbers were 700 scale but their meter reads 500 scale, plants receive 40% more nutrients than intended). PPM is also affected by source water quality — RO water has near-zero starting PPM, while tap water can carry 200-400 PPM of background minerals that should be subtracted from the target feeding strength. The cannabis industry is gradually transitioning fully to EC for clarity, but PPM persists in the United States and remains common in older nutrient brand labeling.