# Hydroponic Nutrient Concentration Calculator

> Find the grams of fertiliser to dissolve for a target ppm of nitrogen, phosphorus, potassium or another nutrient, with P₂O₅ and K₂O label conversion.

Category: Agriculture · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/hydroponic-nutrient-concentration-calculator

One ppm in water is one milligram per litre. Multiply the target ppm by the reservoir volume, then divide by the nutrient share of the fertiliser.

Also called: Nutrient solution calculator, Fertigation ppm calculator.

## Inputs
- Target concentration of the nutrient (unit ppm (mg/L); default 200)
- Nutrient content on the fertiliser label (unit %; default 15.5)
- Label states the nutrient as (options: Element (N, P, K, Ca, Mg…), P₂O₅ (phosphate), K₂O (potash); default element)
- Reservoir or tank volume (default 100)
- Volume in (options: Litres, US gallons; default l)
- Stock solution concentration (unit × (e.g. 100 for 1:100); default 100; optional)

## Results
- Fertiliser to dissolve
- Fertiliser per litre
- Fertiliser per US gallon
- Elemental nutrient in the fertiliser
- Fertiliser per litre of stock solution

## Formula

```
Grams = ppm × Litres ÷ (Elemental% × 10);  Elemental% = Label% × Factor (P₂O₅ → P: 0.4364; K₂O → K: 0.8301)
```

Where:
- ppm: mg of the nutrient per litre of solution
- Factor: mass share of the element in its oxide, from atomic weights
- Stock ratio: how many times stronger the stock is than the final solution

## Worked example
1. Nitrogen needed = 200 mg/L × 100 L = 20,000 mg = 20 g.
2. Fertiliser = 20 g ÷ 0.155 = 129.03 g.
3. For a 100× stock: 1.2903 g/L × 100 = 129.03 g per litre of stock.

**Result:** To reach 200 ppm nitrogen in 100 L with a 15.5% N fertiliser, dissolve about 129 g.

## What the result means

A fertiliser supplies several nutrients at once, so calculate the main one and then check what the others come to (ppm of the other nutrient = grams per litre × its % × 10). Confirm the final solution with an EC meter.

## Assumptions and limits
- The fertiliser dissolves fully and the volume change from dissolving it is negligible.
- Nutrients already in the source water are not subtracted.
- Compatibility of fertilisers in one stock tank (calcium with sulphates or phosphates) is not checked.

## Questions

### Why convert P₂O₅ and K₂O?

Labels in many countries give phosphate and potash as oxides, while recipes give elemental P and K. 1% P₂O₅ is 0.436% P and 1% K₂O is 0.830% K.

### What does a 100× stock mean?

The stock is 100 times stronger than the final solution, so an injector set to 1:100 dilutes it to the target.

## Related calculators
- [Aquaponics Water Flow Calculator](https://calculators.geekmidst.com/calculators/aquaponics-water-flow-calculator)
- [Plant Nursery Tray Requirement Calculator](https://calculators.geekmidst.com/calculators/plant-nursery-tray-requirement-calculator)
- [Percentage Calculator](https://calculators.geekmidst.com/calculators/percentage-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/hydroponic-nutrient-concentration-calculator