Water Soluble Fertilizer Calculator
Calculate water soluble fertilizer with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
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.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.
Is Water Soluble Fertilizer Calculator free to use?
Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.