Aquarium EI Dosing Calculator
Calculate dry KNO3 and KH2PO4 doses for the Estimative Index (EI) fertilizer method from your planted-tank volume, per dose and per week.
How it works
liters = US gallons × 3.785 · g KNO3 = target NO3 ppm × liters × 0.001 ÷ 0.61 · g KH2PO4 = target PO4 ppm × liters × 0.001 ÷ 0.70 · dose 3× per week
The Estimative Index (EI) is a planted-aquarium fertilizing method that deliberately keeps nutrients in slight excess so plants are never limited, then resets the water column with a large (~50%) weekly water change so nothing accumulates to harmful levels. Instead of testing constantly, you dose a fixed amount several times a week. Nitrate comes from potassium nitrate (KNO3), which is about 61% NO3 by mass, and phosphate from monopotassium phosphate (KH2PO4), which yields about 70% of its mass as PO4 — so the dry grams needed are the target parts-per-million times the water volume in litres, divided by that mass fraction. A common high-light target is roughly 7.5 ppm NO3 and 1.3 ppm PO4 per dose, three times a week; low-light tanks use a leaner schedule. Because 1 ppm equals 1 mg per litre, multiplying ppm by litres and by 0.001 converts to grams.
Worked example
A 40 US gallon tank holds 40 × 3.785 ≈ 151.4 L. For the high-light target of 7.5 ppm NO3: 7.5 × 151.4 × 0.001 ÷ 0.61 ≈ 1.86 g of KNO3 per dose. For 1.3 ppm PO4: 1.3 × 151.4 × 0.001 ÷ 0.70 ≈ 0.28 g of KH2PO4 per dose. Dosed three times a week that is about 5.59 g KNO3 and 0.84 g KH2PO4, with a large water change at week's end.
Frequently asked questions
What is the Estimative Index (EI) method?
EI is a dosing approach for planted tanks that supplies nutrients in modest excess so plant growth is never nutrient-limited, and relies on a big weekly water change (about 50%) to prevent buildup. It trades precise testing for a simple, repeatable dosing routine.
Why divide by 0.61 and 0.70?
Dry salts are compounds, not pure nutrient. Potassium nitrate is roughly 61% nitrate by mass, and monopotassium phosphate delivers about 70% of its mass as phosphate. Dividing the nutrient target by that fraction gives the actual weight of salt to weigh out.
Do I still need water changes with EI?
Yes — the weekly ~50% water change is central to the method, not optional. It clears the accumulated excess nutrients and keeps concentrations bounded, which is what makes deliberately over-dosing safe. EI is a method to adapt, so tune the amounts to your own tank's response.
Related calculators
General guidance — consult your vet for your specific animal.