Water Soluble Fertilizer Calculator
Calculate water soluble fertilizer with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Grams/Gallon = (Target ppm N x 3.785) / (Fertilizer %N x 10)
This formula converts the desired nitrogen concentration (in ppm = mg/L) to grams of fertilizer per gallon of water. The 3.785 converts liters to gallons, and dividing by (N% x 10) converts the percentage to a ratio. Phosphorus and potassium ppm are calculated proportionally from the NPK ratio. Multiply grams per gallon by total gallons for the full amount needed.
Worked Examples
Example 1: Greenhouse Tomato Fertigation
Problem:Mix 200 ppm N solution using 20-10-20 fertilizer for 100 gallons of water.
Solution:Grams per gallon = (200 x 3.785) / (20 x 10) = 3.79 g/gal\nTotal fertilizer = 3.79 x 100 = 379 grams (13.4 oz)\nPPM N delivered = 200\nPPM P2O5 delivered = (10/20) x 200 = 100 ppm\nPPM K2O delivered = (20/20) x 200 = 200 ppm\nEstimated EC contribution = 500/700 = 0.71 mS/cm
Result:379 grams (13.4 oz) of 20-10-20 in 100 gallons | EC ~0.71 mS/cm
Example 2: Houseplant Feeding Solution
Problem:Make 5 gallons of 150 ppm N solution with 24-8-16 fertilizer.
Solution:Grams per gallon = (150 x 3.785) / (24 x 10) = 2.37 g/gal\nTotal = 2.37 x 5 = 11.8 grams (~0.8 tablespoons)\nPPM N = 150\nPPM P2O5 = (8/24) x 150 = 50\nPPM K2O = (16/24) x 150 = 100\nTotal dissolved = 300 ppm\nEC estimate = 300/700 = 0.43 mS/cm
Result:11.8 grams (~0.8 tbsp) of 24-8-16 in 5 gallons
Frequently Asked Questions
How do I calculate how much water-soluble fertilizer to use?
The calculation depends on three factors: your target nitrogen (N) concentration in parts per million (ppm), the N percentage in your fertilizer, and the volume of water. The formula is: grams per gallon = (target ppm N x 3.785) / (fertilizer N% x 10). For example, to achieve 200 ppm N with a 20-20-20 fertilizer: (200 x 3.785) / (20 x 10) = 3.79 grams per gallon. Multiply by total gallons for the full amount needed. This formula works because 1 ppm equals 1 mg per liter, and 1 gallon equals 3.785 liters.
What does the N-P-K ratio mean on fertilizer labels?
The three numbers on a fertilizer label represent the percentage by weight of Nitrogen (N), Phosphorus (as P2O5, phosphorus pentoxide), and Potassium (as K2O, potassium oxide). A 20-20-20 fertilizer is 20% N, 20% P2O5, and 20% K2O by weight. The remaining 40% is filler, carriers, and micronutrients. To convert to elemental phosphorus, multiply P2O5 by 0.4364. To convert to elemental potassium, multiply K2O by 0.8302. So a 20-20-20 actually delivers 20% N, 8.7% elemental P, and 16.6% elemental K.
How much water do garden plants need?
Most vegetables need about 1 inch (0.62 gallons per square foot) of water per week from rain plus irrigation. Sandy soil drains faster and may need 2 inches. Clay soil retains moisture longer. Water deeply and less frequently to encourage deep root growth. Morning watering reduces evaporation and disease risk.
How do I calculate fertilizer application rates?
If a soil test recommends 2 pounds of nitrogen per 1,000 square feet and your fertilizer is 10-10-10 (10% N), you need 2 / 0.10 = 20 pounds of fertilizer per 1,000 square feet. For a 200 square foot bed: 20 * (200/1000) = 4 pounds. Split applications into 2-3 feedings during the growing season for best results.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy