About this tool
The Planted Tank Fertilizer Dosing calculator is a free, in-browser tool that turns a target nutrient concentration into the exact amount of dry fertilizer salt to weigh out per dose. It supports the salts used in Estimative Index (EI) and PPS-Pro dosing: potassium nitrate (KNO3), monopotassium phosphate (KH2PO4), potassium sulfate (K2SO4), potassium chloride (KCl), Epsom salt (MgSO4·7H2O), and CSM+B trace mix. Pick the salt and the element or radical you are targeting, enter the ppm you want to raise and your tank volume, and it returns grams and teaspoons per dose. All math runs locally in your browser.
Because 1 ppm = 1 mg/L, the milligrams of the target element are target_ppm × effective_volume_L, where the effective volume is the gross tank volume × 0.9 to account for substrate, rock, and displacement. Grams of the compound are then (mg ÷ 1000) ÷ element_mass_fraction, using each salt's mass fraction (for example KNO3 is 13.85% N, 38.67% K, and 61.32% NO3; KH2PO4 is 22.76% P, 69.79% PO4). You can target the element (N, P) or the radical (NO3, PO4) — NO3 = N × 4.427 and PO4 = P × 3.066 — by choosing the matching option. Teaspoon output uses typical dry densities (KNO3 ≈ 5.9 g/tsp, KH2PO4 ≈ 5.5 g/tsp).
As a worked example, raising NO3 by 7.5 ppm in a 100 L tank with KNO3: effective volume = 90 L, so NO3 needed = 7.5 × 90 = 675 mg; grams KNO3 = (675 ÷ 1000) ÷ 0.6132 ≈ 1.10 g per dose, about 0.19 tsp. Multiply by your doses per week for the weekly total. Weigh dry salts on a milligram scale when possible; teaspoon figures are approximate because bulk density varies with humidity and grind.