Calculator
Fertiliser Dosing Calculator
Turn dry salts into ppm, or a target level into grams to weigh out.
By Mike Elmira
Enter your tank’s water volume and how much salt you're adding. Use the realistic water volume, not the full glass size.
Rough weekly targets (EI)
As a starting point, many planted tanks aim for roughly 10 to 30 ppm NO3, 1 to 3 ppm PO4, 10 to 30 ppm K and 5 to 10 ppm Mg across a week, split over several doses with a big water change at week’s end. These are ballpark figures, adjust to what your plants and algae tell you.
Dry dosing is cheap and precise once you know the numbers, and this does the numbers for you. Pick a salt, enter your tank's water volume, and either say how many grams you are adding to see the ppm it lifts each nutrient, or set a target and get the grams to weigh out. The yields come straight from the molar masses, so potassium nitrate reads as 61% nitrate and 39% potassium by weight, and so on. Rough weekly targets are shown to aim at.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
How do I calculate dry fertiliser dosing for a planted tank?
Each salt yields a fixed amount of each nutrient by weight. Potassium nitrate, for example, is about 613 mg of nitrate and 387 mg of potassium per gram, so the ppm rise is that figure times the grams added, divided by your tank litres. This calculator runs both directions for the common salts.
What are the EI dosing targets?
Estimated Index aims for generous, non-limiting nutrients, roughly 10 to 30 ppm nitrate, 1 to 3 ppm phosphate, 10 to 30 ppm potassium and 5 to 10 ppm magnesium across a week, split over several doses with a large water change at the end. They are starting points, not gospel, so tune to your tank.