Fertilizer dosing calculator (dry salts)
Dry salts are the cheapest way to fertilize a planted tank, but only if you know what a gram does. This calculator converts between grams, ppm and millilitres of stock solution, using exact molecular weights.
EI-style planner
The Estimative Index (EI) gives a planted tank more than it can use, dosed 2–3 times a week, then resets the excess with a large weekly water change (Barr). Enter the rise you want per dose. The planner works out each salt, counting the potassium that KNO₃ and KH₂PO₄ already supply. For a leaner, PPS-style routine, enter smaller daily rises and set doses per week to 7.
How it works
A salt contributes each nutrient in proportion to its share of the molecular weight. Potassium nitrate (KNO₃, 101.10 g/mol) is 61.33% nitrate (NO₃) and 38.67% potassium (K). Dissolve 1 g (1,000 mg) of it in 100 L and you add 1,000 × 0.6133 ÷ 100 = 6.13 mg/L of nitrate. In water, mg/L and ppm are effectively the same.
| Salt | Molar mass | Supplies |
|---|---|---|
| KNO₃ | 101.10 g/mol | NO₃ 61.33% · N 13.85% · K 38.67% |
| KH₂PO₄ | 136.08 g/mol | PO₄ 69.79% · P 22.76% · K 28.73% |
| K₂SO₄ | 174.25 g/mol | K 44.88% · SO₄ 55.12% |
| MgSO₄·7H₂O | 246.47 g/mol | Mg 9.86% · SO₄ 38.97% |
Watch the "as N" vs "as NO₃" trap. Some test kits report nitrate as nitrogen (NO₃-N). 1 ppm NO₃-N equals 4.43 ppm NO₃. Make sure you compare like with like. The same applies to phosphate: PO₄-P × 3.07 = PO₄.
Worked examples
1 g KNO₃ in a 100 L tank
1,000 × 0.6133 ÷ 100 = 6.13 ppm NO₃, plus 1,000 × 0.3867 ÷ 100 = 3.87 ppm K.
Raise nitrate by 5 ppm in a 30 US gal tank
30 US gal = 113.6 L. 5 × 113.6 ÷ 0.6133 = 926 mg ≈ 0.93 g KNO₃.
A stock solution
20 g KNO₃ made up to 500 mL. A 10 mL dose contains 20,000 × 10 ÷ 500 = 400 mg of salt. In 200 L that's 400 × 0.6133 ÷ 200 = 1.23 ppm NO₃.
EI worst case
Dosing 7.5 ppm NO₃ three times a week (22.5 ppm a week) with a 50% weekly change can never build past 22.5 ÷ 0.5 = 45 ppm, even if plants take up nothing. That ceiling is what makes EI "estimative".
Practical notes
- Use a scale that reads to 0.01 g for small tanks. Teaspoon measures vary by product and how tightly the powder packs.
- Make stock solutions up to a final volume (add salt, then top up to the mark), not "20 g plus 500 mL of water".
- KH₂PO₄ and trace mixes with chelated iron can react in one bottle. Most keepers keep macros and micros in separate solutions.
- Fertilizer-grade salts vary in purity. These numbers assume pure, dry compounds.
- Shrimp and snails can be sensitive to sudden changes. Increase doses gradually.
References
- Barr, T. The Estimative Index of dosing, or no need for test kits. Barr Report
- Barr, T. (2006) EI "light" for the less technical aspects of the Estimative Index (target ranges: NO₃ 5–30, PO₄ 1–2, K 10–30, Fe 0.2–0.5 ppm)
- IUPAC Commission on Isotopic Abundances and Atomic Weights: Standard atomic weights
- PubChem compound records: potassium nitrate, monopotassium phosphate, potassium sulfate, magnesium sulfate heptahydrate
Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.
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