Fertilizer NPK & hydroponic reservoir calculator
Label analysis and explicit targets → product mass, co-delivered nutrients and EC conversions.
Open full calculator · Calculation schema v2 · Sources checked 2026-10-28
Formula and worked example
Product g = target addition mg/L × reservoir L ÷ (1,000 × nutrient mass fraction). A 20%-N product delivering 100 mg/L N into 100 L requires 50 g. For a 20–10–20 oxide label it also adds 21.82 mg/L elemental P and 83.01 mg/L elemental K. Meeting one nutrient target does not establish a balanced formula.
P ≈ 0.4364 × P₂O₅; K ≈ 0.8301 × K₂O. Iowa State fertilizer-rate guidance. EC 2 mS/cm = 2,000 µS/cm = 1,000 ppm on a 500-scale meter or 1,400 ppm on a 700-scale meter.
Added water = initial volume × (initial concentration − target) ÷ (target − water concentration). 10 L × (2 − 1.5) ÷ (1.5 − 0.2) = 3.846 L. A target at or below the added-water concentration cannot be reached by finite dilution.
pH is not an acid-dose calculator
pH alone does not determine acid/base demand. Do not average pH values or infer nutrient grams from EC. Water alkalinity, buffering and reagent concentration matter; see Penn State irrigation-water interpretation. Read the nutrient calculation guide.
