Work out ec and tds for hydroponics instantly with clear inputs, formula shown and shareable results.
TDS meters do not measure dissolved solids; they measure conductivity and multiply by a scale factor. The three common scales — 500, 640 and 700 ppm per mS/cm — give readings up to 40% apart for the same solution, which is why EC in mS/cm is the safer unit to specify a recipe in.
EC to TDS
TDS (ppm) = EC (mS/cm) x scale factor (500, 640 or 700)
Dilution
Water to add = volume x (EC now - EC target) / EC target
Lettuce and leafy greens 1.2-1.8 mS/cm, tomato and pepper 2.2-3.5, strawberry 1.4-2.0. Raise EC to firm up fruit quality and lower it in hot weather when transpiration is high.
Plants take up water faster than salts in hot weather, concentrating the remainder. Top up with plain water, not feed, and check EC daily.