Water Soluble Fertilizer Calculator
Calculate the exact amount of water soluble fertilizer to dissolve per gallon or per tank. Enter fertilizer grade, target nutrient ppm, and tank size to get grams per gallon, ounces per gallon, and total quantity needed.
Water Soluble Fertilizer Calculator Tool
Why use this free water soluble fertilizer calculator?
Built with the features most competitors miss — deeper inputs, benchmark data, and actionable guidance alongside the core calculation.
How to use this water soluble fertilizer calculator
Common water soluble fertilizer grades and uses
| Grade | N-P-K | Typical use | Target N ppm |
|---|---|---|---|
| 20-20-20 | Balanced NPK | Seedling to vegetative | 100 to 150 |
| 30-10-10 | High N | Fast vegetative growth | 150 to 200 |
| 13-2-13 | Low P | Flowering / fruiting | 100 to 150 |
| 0-45-45 | No N | Bloom booster addition | 0 (K/P boost) |
| 15-5-15 | Moderate balanced | All-purpose vegetative | 100 to 200 |
| 18-6-18 | Cal-Mag blend | Tomatoes, peppers | 150 to 200 |
How this calculator compares
LazyTools fills the gaps most competing tools leave open — deeper analysis, benchmark context, and actionable guidance alongside the core calculation.
| Feature | LazyTools | OmniCalculator | Hanna Instruments | General Hydroponics |
|---|---|---|---|---|
| Any NPK grade input | ✓ Yes | ✓ | ✗ | ✗ |
| All 3 nutrient ppm output | ✓ Yes | ✓ | Partial | ✗ |
| Multiple tank sizes | ✓ Yes | ✓ | ✗ | ✗ |
| Grams and ounces | ✓ Yes | ✓ | ✗ | ✗ |
| Litres and gallons auto convert | ✓ Yes | ✓ | ✗ | ✗ |
| Free, no registration | ✓ Yes | ✓ | ✓ | ✓ |
Water Soluble Fertilizer Calculator: Complete Guide
Calculating the exact amount of water soluble fertilizer to dissolve per gallon or per tank ensures you deliver the correct nutrient concentration to your plants, whether for hydroponics, fertigation, or foliar feeding. This calculator converts between any NPK grade and target ppm for any tank volume.
The mix rate formula
Grams per litre = Target ppm / (Nutrient % x 10). This formula works because: 1 ppm = 1 mg/L = 0.001 g/L. If your fertilizer is 20% N, then 1 g of fertilizer dissolved in 1 L delivers 0.20 g N per L = 200 mg N per L = 200 ppm N. So to hit 150 ppm N: g/L = 150/200 = 0.75 g/L. For a 100 L tank: 75 grams total. Scale this formula to any grade or target.
P and K delivered alongside target N
When you target a specific nitrogen ppm, the phosphorus and potassium concentrations are set by the fertilizer grade. A 20-20-20 grade at 150 ppm N also delivers 150 ppm P2O5 and 150 ppm K2O. A 13-2-13 grade at 150 ppm N delivers only 23 ppm P2O5 and 150 ppm K2O. Choosing the right grade for your growth stage requires understanding that you cannot adjust N independently of P and K in a fixed-grade product — this is why multi-part nutrient systems are used in hydroponics.
Nutrient targets by crop and growth stage
Leafy greens (lettuce, spinach, herbs): target 100 to 150 ppm N. Fruiting crops (tomato, pepper, cucumber) vegetative: 150 to 200 ppm N. Fruiting crops in flower and fruit: 100 to 150 ppm N with higher K (200 to 300 ppm K2O). Cannabis vegetative: 150 to 200 ppm N. Cannabis flowering: 50 to 100 ppm N, 100 to 200 ppm K2O. Always use EC as a cross-check on total dissolved solids alongside individual nutrient targets.
Foliar feeding rates
Foliar applications use much lower concentrations than soil or hydroponic fertigation: typically 100 to 200 ppm total nutrients (EC 0.2 to 0.4 mS/cm). At 20-20-20 fertilizer: target 50 to 75 ppm N for foliar application = 0.25 to 0.375 g/L. Apply in the early morning or evening to avoid burning. Foliar feeding is not a replacement for root zone nutrition but supplements micronutrients and corrects deficiencies rapidly.