Hydroponic Nutrient EC / PPM & Dosing Calculator

How it works

ppm = EC (mS/cm) × scale factor (500, 640 or 700); nutrient (mL) = (EC_target − EC_current) ÷ (EC per mL/L) × litres ÷ parts; EC after evaporation = EC × V ÷ (V − lost)

Hydroponic feed strength is measured as electrical conductivity because dissolved fertiliser salts conduct electricity in proportion to their concentration; "ppm" meters simply multiply EC by a conversion factor, and because different manufacturers chose 500, 640 or 700 as that factor, a ppm number is meaningless without its scale. Dosing works backwards from the target: the EC gap times the reservoir volume divided by how much EC each millilitre of concentrate produces. Plants take up water faster than nutrients, so EC drifts upward as the reservoir shrinks and the correct response is to top up with plain water first, adding nutrient only if the reading is still low; regular full changes prevent the imbalances that a meter cannot see.

Worked example

Raising a 100 L reservoir from tap-water EC 0.4 to a lettuce target of 1.2 mS/cm (600 ppm on the 500 scale, 840 on the 700) with a two-part concentrate that adds 1.1 mS/cm per mL/L takes 36 mL of each part. After 15 L evaporates the remaining 85 L reads about 1.4; topping up with 15 L of plain water restores 1.2.

Frequently asked questions