NPK Fertilizer Calculator

NPK stands for Nitrogen (N), Phosphorus (P), and Potassium (K) — the three primary macronutrients plants need. The numbers on a fertilizer label (e.g., 20-20-20) represent the percentage by weight of each nutrient. Nitrogen promotes leafy growth and chlorophyll production. Phosphorus supports root development, flowering, and fruiting. Potassium enhances overall plant health, disease resistance, and water regulation. Note that P is expressed as P₂O₅ (phosphate) and K as K₂O (potash) per industry convention.

Use the formula: Fertilizer amount (grams) = (Target ppm × Water volume in liters) / (NPK% × 10). For example, to achieve 150 ppm of Nitrogen using a 20-20-20 fertilizer in 1 gallon (3.785 L) of water: (150 × 3.785) / (20 × 10) = 567.75 / 200 = 2.84 grams. Our Dilution Calculator above automates this for you — just enter your fertilizer NPK, target ppm, and water volume.

Leafy vegetables (lettuce, kale, spinach): Higher N, such as 20-10-10 or 30-10-10, to promote lush foliage.
Fruiting vegetables (tomatoes, peppers, cucumbers): Balanced or slightly higher P-K during fruiting, like 10-20-20 or 5-10-10.
Flowering plants (roses, petunias, orchids): Higher P for blooms, such as 10-30-20 or 5-10-5.
Root crops (carrots, potatoes, beets): Higher K, like 5-10-20 or 10-10-20.
Lawns: High N formulas like 30-0-0 or 20-5-5 for green growth.

Yes! By blending two fertilizers with different NPK profiles, you can create a custom nutrient mix. For example, mixing a high-nitrogen fertilizer like 30-10-10 with a bloom booster like 10-30-20 in a 2:3 ratio yields approximately 18-22-16. Use our Blend Two Fertilizers tab to find the optimal mixing ratio for your target NPK. The calculator uses numerical optimization to minimize the difference between your target and the achievable blend.

NPK percentage tells you the concentration of nutrients in the fertilizer itself (by weight). PPM (parts per million) tells you the concentration in the final solution after mixing with water. For context: 1 ppm = 1 milligram of nutrient per liter of water. A 20% N fertilizer contains 200,000 ppm of nitrogen in its pure form. When diluted at 1 gram per liter, it contributes 200 ppm N to the solution (1g × 20% / 1L = 200mg/L = 200ppm).

Organic fertilizer NPK values are generally less precise than synthetic fertilizers because nutrient content can vary based on source material, processing, and batch. Common organic NPK ranges: Fish emulsion (5-1-1), Bone meal (3-15-0), Blood meal (12-0-0), Kelp meal (1-0-4). Organic nutrients also release more slowly and may require soil microbial activity for full availability. Always check the label and consider organic fertilizers as providing a range rather than an exact percentage.

Over-fertilization causes nutrient burn — symptoms include brown or yellow leaf tips/edges, wilting despite moist soil, white crust on soil surface (salt buildup), stunted growth, and in severe cases, plant death. High salt concentrations from excess fertilizer draw water out of roots through osmosis (reverse osmosis effect). If you suspect over-fertilization, flush the soil with plain water (2-3× the pot volume) and reduce future applications. For hydroponics, drain and replace the reservoir with a weaker solution.

Single-nutrient or two-nutrient fertilizers are designed for targeted supplementation. Examples: Urea (46-0-0) provides pure nitrogen for leafy growth; DAP (18-46-0) supplies nitrogen and phosphorus without potassium; MOP (0-0-60) delivers potassium only. These are useful when soil tests show a deficiency in one specific nutrient, or when you want to customize your blend precisely without adding unnecessary nutrients that could build up in the soil.

1 US gallon = 3.785 liters. If a recommendation says "1 tablespoon per gallon," that's approximately 1 tablespoon per 3.785 liters (or about 0.26 tablespoons per liter). For metric applications, fertilizer rates are often given in grams per liter or milliliters per liter. Our calculator handles both unit systems — simply toggle between gallons and liters in the input section. For dry fertilizers: 1 teaspoon ≈ 5 grams, 1 tablespoon ≈ 15 grams (approximate, varies by density).

Hydroponic nutrient solutions typically target specific ppm ranges rather than NPK ratios alone. Common targets: Leafy greens: 150-200 ppm N, 30-50 ppm P, 150-200 ppm K. Tomatoes/peppers (vegetative): 180-220 ppm N, 40-60 ppm P, 200-250 ppm K. Tomatoes/peppers (fruiting): 150-180 ppm N, 50-80 ppm P, 250-350 ppm K. Seedlings/clones: 80-120 ppm N, 20-40 ppm P, 80-120 ppm K. Use our Dilution Calculator to achieve precise ppm targets from your chosen fertilizer.