How ppm from dry fertilizer salts is calculated
Short answer
mg/L added = grams × 1000 × (mass of the nutrient in the formula ÷ molar mass of the salt) ÷ liters. Potassium nitrate is 61.3 % nitrate by mass, so 1 g in 100 L adds 6.13 mg/L of NO3 [1, 2]. The two traps are ion versus element (NO3 is 4.43 times N) and hydrated salts, such as Epsom salt, that are mostly water.
One formula behind every dosing chart
Every dry-salt dosing table, calculator and method write-up comes down to the same calculation. Dissolve a known mass of a salt in a known volume of water; the concentration of any part of that salt is the mass of the salt, times the fraction of it that is the part you care about, divided by the volume:
mg/L = grams × 1000 × (mass of the part in one formula unit ÷ molar mass of the salt) ÷ liters
The factor of 1000 turns grams into milligrams. The fraction in brackets is a fixed property of the compound, and it is where most dosing mistakes start.
Step 1: molar masses from atomic weights
A molar mass is the sum of the atomic weights in the formula. Using the IUPAC abridged standard atomic weights (K 39.098, N 14.007, O 15.999, H 1.008, P 30.974, S 32.06, Mg 24.305) [1]:
| Salt | Sum | Molar mass (g/mol) | PubChem |
|---|---|---|---|
| KNO3 | 39.098 + 14.007 + 3 × 15.999 | 101.102 | 101.103 [2] |
| KH2PO4 | 39.098 + 2 × 1.008 + 30.974 + 4 × 15.999 | 136.084 | 136.086 [3] |
| K2SO4 | 2 × 39.098 + 32.06 + 4 × 15.999 | 174.252 | 174.26 [4] |
| MgSO4·7H2O | 24.305 + 32.06 + 4 × 15.999 + 7 × 18.015 | 246.466 | 246.48 [5] |
The small differences from PubChem come from rounding in the atomic weights and do not matter for dosing.
Step 2: the share of each nutrient
Divide the mass of the part by the molar mass of the whole:
- Nitrate in KNO3: NO3 = 14.007 + 3 × 15.999 = 62.004, so 62.004 ÷ 101.102 = 61.33 %. Potassium is the rest of the cation: 39.098 ÷ 101.102 = 38.67 %.
- Phosphate in KH2PO4: PO4 = 94.970, so 94.970 ÷ 136.084 = 69.79 %; potassium 28.73 %.
- Potassium in K2SO4: two atoms, 78.196 ÷ 174.252 = 44.88 %.
- Magnesium in Epsom salt: 24.305 ÷ 246.466 = 9.86 %.
Step 3: put the numbers in
Example 1. 1 g of KNO3 in a 100 L tank: 1 × 1000 × 0.6133 ÷ 100 = 6.13 mg/L of nitrate, and 1 × 1000 × 0.3867 ÷ 100 = 3.87 mg/L of potassium from the same gram.
Example 2, backwards. How much KH2PO4 raises phosphate by 1 mg/L in 60 L? Rearrange: grams = 1 × 60 ÷ (0.6979 × 1000) = 0.086 g. Small doses like this are why many people dissolve salts in a bottle first; see mixing stock solutions.
Example 3, a mixture by label. Trace mixes have no single formula, so use the label percentage directly. Plantex NutriTrace CSM guarantees 7 % iron [6]. 0.1 g in 100 L adds 0.1 × 1000 × 0.07 ÷ 100 = 0.07 mg/L of iron, plus 0.02 mg/L manganese from its 2 % Mn, and so on down the label.
The dry fertilizer dosing calculator does all three, and prints the working.
Trap 1: ion or element?
Test kits for nitrate and phosphate usually read the ion (NO3, PO4). Fertilizer labels and scientific papers usually report the element (N, P). The ratio is fixed by atomic weights:
- NO3 ÷ N = 62.004 ÷ 14.007 = 4.427
- PO4 ÷ P = 94.970 ÷ 30.974 = 3.066
So “10 ppm nitrate” is only 2.26 ppm nitrogen. When two sources seem to disagree by a factor of three or four, this is usually why.
US fertilizer labels add a third convention: they state phosphorus as P2O5 and potassium as K2O. Montana State University Extension gives the conversions as P = P2O5 × 0.44 and K = K2O × 0.83 [7]. See reading fertilizer labels.
Trap 2: water of crystallization
Epsom salt is MgSO4·7H2O. The seven water molecules make up 126.1 of its 246.5 g/mol, more than half its mass, which is why it is only 9.86 % magnesium. If you buy anhydrous magnesium sulfate (MgSO4, 120.36 g/mol), the same mass delivers about twice as much magnesium. Check the label or the safety data sheet for which form you have, and pick the matching entry in the calculator.
Trap 3: ppm is not quite mg/L
Strictly, ppm is a mass ratio. A liter of fresh water at 25 °C weighs 997 g, not 1000 g [8], so 1 mg/L is 1.003 ppm by mass. The difference is far below the accuracy of any hobby test kit, which is why dosing methods use the two interchangeably.
What the arithmetic cannot tell you
The formula gives the step change at the moment the salt dissolves. Where the concentration goes afterward depends on plant uptake, water changes and other inputs such as fish food and tap water. The water-change calculator models the build-up between water changes; the EI and PPS-Pro guides show how two dosing methods use the same numbers in opposite ways.
Questions
Is ppm the same as mg/L?
For dilute fresh water, effectively yes: a liter of water has a mass of about 1 kg (997 g at 25 °C) [8], so 1 mg per liter is about one part per million by mass.
Why does my dosing chart give a different number?
Check which form it reports. A chart in ppm N gives numbers 4.43 times smaller than one in ppm NO3 for the same dose, and a label in P2O5 differs from ppm P by a factor of 2.29 [7].
Calculators for this
- Dry fertilizer dosingConvert grams of KNO3, KH2PO4, K2SO4, Epsom salt or a trace mix into mg/L for your tank, or work back from a target ppm. Every molar mass is shown.
- EI and PPS-Pro plannerTurn the Estimative Index and PPS-Pro, as their authors published them, into grams or milliliters for your tank, with the build-up each schedule produces.
- Stock solutionPlan a bottle of dissolved fertilizer salts: what each milliliter adds to your tank, how long the bottle lasts, and whether the salts will dissolve.
Related guides
- FertilizersEstimative Index (EI) dosing: what the method prescribesEI as Tom Barr wrote it up: dose a surplus three times a week, change 50 % of the water weekly, and why that caps build-up at twice the weekly dose.
- FertilizersNitrogen, phosphorus and potassium in planted tanksWhat N, P and K do in plants, the forms plants take up, how aquarium dosing expresses them, and what growers and dosing methods say about each.
- FertilizersPPS-Pro: the lean daily dosing method and its targetsPPS-Pro as its author published it: two 500 mL bottles, 1 mL per 40 L a day, low water-column targets, and how it differs from EI in intent and arithmetic.
- FertilizersReading fertilizer labels: P2O5 and K2O to elemental P and KWhy fertilizer labels list phosphorus as P2O5 and potassium as K2O, the exact conversion factors, and worked examples using real aquarium product labels.
- FertilizersMixing dry-salt stock solutions: solubility, separation and storageHow to turn dry fertilizer salts into a dosing bottle: solubility limits from PubChem, why phosphate and iron go in separate bottles, and storage and safety.
Sources
- Abridged standard atomic weights. IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW). H 1.0080, N 14.007, O 15.999, Mg 24.305, P 30.974, S 32.06, K 39.098. Accessed October 12, 2026.
- Potassium nitrate (CID 24434). PubChem, National Library of Medicine. Molecular weight 101.103 g/mol. Accessed October 12, 2026.
- Potassium dihydrogen phosphate (CID 516951). PubChem, National Library of Medicine. Molecular weight 136.086 g/mol. Accessed October 12, 2026.
- Potassium sulfate (CID 24507). PubChem, National Library of Medicine. Molecular weight 174.26 g/mol. Accessed October 12, 2026.
- Magnesium sulfate heptahydrate (CID 24843). PubChem, National Library of Medicine. Molecular weight 246.48 g/mol; formula H14MgO11S. Accessed October 12, 2026.
- Plantex NutriTrace CSM Fertilizer. Master Plant-Prod Inc.. Guaranteed minimum analysis: Fe 7 %, Mn 2 %, Zn 0.4 %, Cu 0.1 %, Mo 0.06 %, Mg 1.5 %. Accessed October 12, 2026.
- Nutrient Management Module No. 10: Commercial Fertilizers and Soil Amendments (4449-10). Montana State University Extension. Fertilizer grade as % N, % P2O5, % K2O; conversion factors P = P2O5 × 0.44, K = K2O × 0.83. Accessed October 12, 2026.
- Recommended table for the density of water between 0 °C and 40 °C based on recent experimental reports. Metrologia 38: 301–309. Tanaka, M., Girard, G., Davis, R., Peuto, A. and Bignell, N. (2001). Density of pure water 997.05 kg/m³ at 25 °C. Accessed October 12, 2026.