Utility July 13, 2026 · 8 Min Read

Water Soluble Fertilizer Calculator – Guide & Formulas

Calculate fertilizer concentration in PPM and EC from water-soluble fertilizers. Free online calculator for hydroponics, fertigation, and greenhouse growing.

Calculate the exact amount of water-soluble fertilizer needed to achieve your target PPM or EC. Supports all NPK ratios and injector dilution ratios for hydroponics and fertigation.

Key Takeaway

Use the free Water Soluble Fertilizer Calculator to calculate fertilizer concentration in ppm and ec from water-soluble fertilizers. free online calculator for hydroponics, fertigation, and greenhouse growing. Get instant results with step-by-step explanations.

How to Use the Water Soluble Fertilizer Calculator

  1. Enter the target PPM (parts per million) or EC (mS/cm) for your nutrient solution.
  2. Input the fertilizer NPK analysis from the label (e.g., 20-10-20).
  3. Enter the volume of water (gallons or liters) to be mixed.
  4. Review fertilizer amount needed in grams, ounces, and EC estimate.

The Formula

Fertilizer (g) = (Target PPM × Volume in Liters) ÷ (N% / 100). For ounces: grams ÷ 28.35. EC = PPM ÷ PPM Scale (500 for most US meters).

Variable Definitions

  • Target PPM: Desired concentration of nitrogen in mg/L (ppm)
  • N%: Nitrogen percentage from fertilizer label (e.g., 20 for 20-10-20)
  • Volume (L): Total water volume in liters (gallons × 3.785)
  • PPM Scale: Conversion factor: 500 for Hanna/most US meters, 700 for Bluelab/EU
  • EC: Electrical Conductivity in mS/cm — measures total dissolved salts

Mixing 20-10-20 Fertilizer for 100 Gallons at 200 PPM N

Calculate fertilizer amount and companion nutrient PPMs for a fertigation system.

  1. Step 1: Convert volume: 100 gal × 3.785 = 378.5 L.
  2. Step 2: Fertilizer grams = (200 × 378.5) ÷ 0.20 = 378,500 mg = 378.5 g.
  3. Step 3: Convert to ounces: 378.5 ÷ 28.35 = 13.35 oz.
  4. Step 4: Companion nutrients: P₂O₅ = 200 × (10/20) = 100 ppm. K₂O = 200 × (20/20) = 200 ppm.
  5. Step 5: EC estimate (500 scale): 200 ÷ 500 = 0.40 mS/cm.

Frequently Asked Questions

What is the difference between PPM and EC?

PPM (parts per million) measures the mass of dissolved nutrients per unit volume. EC (electrical conductivity) measures how well the solution conducts electricity. They are related: EC × 500 ≈ PPM on most US meters. EC is more stable and temperature-independent.

How do I convert fertilizer label numbers to elemental nutrients?

NPK labels show: Nitrogen as N (elemental), Phosphorus as P₂O₅ (not elemental), Potassium as K₂O (not elemental). To convert: Elemental P = P₂O₅ ÷ 2.29. Elemental K = K₂O ÷ 1.20. For calculations, use the label numbers directly.

What PPM should I target for hydroponic lettuce?

Lettuce thrives at 560–840 PPM (EC 1.0–1.6 mS/cm). Start at the lower end and increase as plants grow. For seedlings, use half-strength (280–420 PPM). For fruiting crops like tomatoes, target 1,400–2,800 PPM (EC 2.8–5.6).

How do I calculate stock solution for an injector?

For a 1:100 injector: Oz per gallon stock = (Target PPM × Injector Ratio) ÷ (N% × 75). For 200 PPM N with 20-10-20 at 1:100: (200 × 100) ÷ (20 × 75) = 13.33 oz per gallon of stock solution.

What is the PPM scale difference between meters?

Different meters use different conversion factors: Hanna and most US meters use 500. Bluelab (EU standard) uses 700. Truncheon uses 640. Always check which scale your meter uses to interpret PPM readings correctly.

How often should I change my nutrient solution?

In hydroponics, completely change the nutrient solution every 1–2 weeks. In soil or coco, feed every other watering. Monitor EC and pH daily — rising EC indicates salt buildup, falling EC indicates nutrient uptake or evaporation.

Can I mix different fertilizers together?

Yes, but use fertilizers from the same product line to avoid nutrient conflicts. Always dissolve one fertilizer completely before adding another. Calcium-containing fertilizers should not be mixed with sulfates or phosphates in concentrated form — they will precipitate.

How does water temperature affect fertilizer concentration?

Warmer water dissolves fertilizers more quickly but also promotes algae and bacterial growth. Cold water (<60°F) dissolves some fertilizers poorly. Ideal mixing temperature is 65–75°F. Always adjust nutrient solution temperature before use.