Dry fertilizer dosing calculator: ppm per gram, with the chemistry

NO3: 62.004 = 62.004 g/mol → 61.33 %
N: 14.007 = 14.007 g/mol → 13.85 %
K: 39.098 = 39.098 g/mol → 38.67 %

NO3: 1 g × 1000 mg/g × 0.6133 ÷ 100 L = 6.13 mg/L

What the calculator does

Pick a salt or a label-percentage product, enter the water volume, and either enter the grams you plan to add or the concentration you want to reach. The result lists every part of the salt that ends up in the water, in mg/L, and the “Show the chemistry” panel prints the molar mass and the arithmetic so you can check it by hand.

The salts are the ones dry-dosing methods are built on: potassium nitrate (KNO3), monopotassium phosphate (KH2PO4), potassium sulfate (K2SO4) and magnesium sulfate heptahydrate (Epsom salt, MgSO4·7H2O), plus calcium sulfate dihydrate (gypsum) and the two bicarbonates used to raise KH ^{[6, 7]}. For calcium, magnesium and bicarbonate the result also shows the change in GH or KH in German degrees.

The formula

Every result comes from one line:

mg/L of a part = grams × 1000 × (mass of that part in one formula unit ÷ molar mass of the salt) ÷ liters

Molar masses are not typed in from a table. They are added up from the IUPAC abridged standard atomic weights ^[1], and the totals agree with the PubChem records to within 0.02 g/mol: 101.10 g/mol for KNO3, 136.08 for KH2PO4, 174.25 for K2SO4 and 246.47 for MgSO4·7H2O ^{[2, 3, 4, 5]}.

Worked example: 1 g of each macro salt in 100 L

Salt	Part reported	Share by mass	mg/L from 1 g in 100 L
KNO3	NO3 / N / K	61.33 % / 13.85 % / 38.67 %	6.13 / 1.39 / 3.87
KH2PO4	PO4 / P / K	69.79 % / 22.76 % / 28.73 %	6.98 / 2.28 / 2.87
K2SO4	K / SO4	44.88 % / 55.12 %	4.49 / 5.51
MgSO4·7H2O	Mg / SO4	9.86 % / 38.97 %	0.99 / 3.90

Two things stand out. Epsom salt is mostly water of crystallization, so a gram of it adds under 1 mg/L of magnesium to 100 L. And potassium arrives from three different salts, so a method that doses KNO3 and KH2PO4 is already adding potassium before any K2SO4 goes in. The EI and PPS-Pro planner accounts for that overlap when it sizes the K2SO4 dose.

Ions or elements: read your test kit first

Hobby test kits for nitrogen and phosphorus usually read the ion (NO3 and PO4), while fertilizer labels and scientific papers usually report the element (N and P). The conversion is fixed by the atomic weights: NO3 = N × 4.427 and PO4 = P × 3.066. A target of “10 ppm nitrate” and “10 ppm nitrogen” differ by more than a factor of four, which is the most common reason two dosing calculators seem to disagree. This calculator lists both forms so the comparison is explicit. Labels that state P2O5 and K2O are a third convention; see reading fertilizer labels.

Trace mixes and chelated iron

Trace-element products are mixtures, so they have no single formula. The calculator uses the guaranteed analysis printed by the manufacturer instead. The presets are taken from Master Plant-Prod’s own product pages:

• Plantex NutriTrace CSM: Fe 7 %, Mn 2 %, Zn 0.4 %, Cu 0.1 %, Mo 0.06 % and Mg 1.5 %, chelated with EDTA, with no boron listed ^[8]. The “CSM+B” sold in the aquarium hobby is a different blend that includes boron, so use the custom option with the numbers on your bag.
• Plant-Prod Chelated Micronutrient Mix: Fe 7 %, Mn 2 %, Zn 0.4 %, Cu 0.1 %, B 1.3 % and Mo 0.06 %, chelated with a mix of EDTA and DTPA ^[9].
• Plant-Prod Chelated Iron 13.2 % (EDTA) and Chelated Iron EDDHA 6 % ^{[10, 11]}.

A guaranteed analysis is a minimum, so the real content can be slightly higher than the figure printed. For any other product, choose “Custom product” and type the percentage from its label.

What the result does not tell you

• Where the level goes next. Plants take nutrients up, water changes dilute them and fish food adds more. The number here is the step change from one dose.
• Whether the salt is pure. The arithmetic assumes the named compound. Garden-grade products can carry fillers or a different hydrate; the label or safety data sheet tells you which.
• Solubility in a stock bottle. Dissolving salts into a small volume of water is a different calculation, with solubility limits. Calcium sulfate, for example, dissolves at only about 0.2 g per 100 mL ^[6]. Use the stock-solution calculator for that.
• Safety. Fertilizer salts are chemicals. Follow the label and the manufacturer’s safety data sheet.