Blend Calculator
Enter up to 5 fertilizer components with weights and NPK grades to calculate the resulting blend.
Enter Fertilizer Values
Fill in fertilizer grades and weights above, then click Calculate to see your blend NPK grade, simplified ratio, and nutrient composition chart.
How to Use the NPK Calculator
Choose a Calculator Mode
Select Blend Calculator to mix multiple fertilizers and find the resulting grade, Label Decoder to analyze a single bag, or Blend Optimizer to calculate how much of each product to apply to meet a target nutrient rate. Switch between modes using the tabs above the input panel.
Enter Fertilizer NPK Grades
Type the N%, P₂O₅%, and K₂O% values from your fertilizer bags. Use the quick-select preset buttons to auto-fill common grades like Urea (46-0-0), DAP (18-46-0), MAP (11-52-0), or MOP (0-0-60). You can also toggle between oxide and elemental notation using the P₂O₅/K₂O button.
Add Weights or Target Rates
In Blend mode, enter the weight of each fertilizer component — any consistent unit works (lb, kg, oz) as long as you use the same unit throughout. In Optimizer mode, enter your target nutrient rates in lb/acre or kg/ha from your soil test report. In Decoder mode, optionally enter the bag weight to get per-bag nutrient pounds.
Read Results and Export
The results panel shows your blend grade, simplified N:P:K ratio, named ratio pattern (e.g., 3:1:2 lawn maintenance), elemental equivalents, and a stacked bar chart. Use the Copy Blend Grade button to copy the grade string, Export CSV to download a spreadsheet, or Print to get a printable summary.
Frequently Asked Questions
What is the difference between N-P₂O₅-K₂O and N-P-K notation?
The numbers on fertilizer bags use oxide notation: nitrogen as elemental N, but phosphorus as P₂O₅ (phosphorus pentoxide) and potassium as K₂O (potassium oxide). This is a historical convention from analytical chemistry. The actual elemental phosphorus in a fertilizer is P₂O₅ × 0.4364, and elemental potassium is K₂O × 0.8301. So a 18-46-0 DAP bag contains 18% elemental N, but only 20.1% elemental P (not 46%). Soil test reports sometimes use elemental notation, so knowing which system you are working in is essential for correct fertilizer calculations. Our Label Decoder mode automatically converts between both.
How do I calculate the NPK grade of a fertilizer blend I'm mixing at home?
The blend calculation is a weighted average. For each nutrient, multiply each component's weight by its nutrient percentage, sum all contributions, then divide by the total weight. For example, 100 lb of 46-0-0 (urea) plus 50 lb of 18-46-0 (DAP) totals 150 lb. Nitrogen: (100 × 0.46) + (50 × 0.18) = 55 lb ÷ 150 = 36.7% N. Phosphate: 50 × 0.46 = 23 lb ÷ 150 = 15.3% P₂O₅. Result: 36.7-15.3-0. Our Blend Calculator performs this automatically for up to five components with weights.
What NPK ratio should I use for my lawn, vegetables, or tomatoes?
Lawn maintenance typically benefits from a 3:1:2 nitrogen-to-phosphorus-to-potassium ratio — high nitrogen for leaf growth, moderate potassium for stress resistance, and relatively little phosphorus unless establishing from seed. Lawn establishment and transplanting favor 1:2:1 for root development. General vegetable gardens do well with 2:1:1. Tomatoes at transplanting use high phosphorus (1:2:1), then shift to high potassium during fruiting (1:2:3 or 4:1:2). Leafy greens like lettuce prefer heavy nitrogen (3:1:1 or 4:1:1). Use the Crop Ratio Reference panel in the calculator for a full table of crop-specific recommendations.
Why does my fertilizer bag show only 30% total nutrients — what is the rest?
A 10-10-10 fertilizer contains 10% nitrogen, 10% P₂O₅, and 10% K₂O by weight — totaling only 30%. The remaining 70% is carrier or filler material, typically crushed limestone, clay, sand, or other inert bulking agents. Filler serves important practical purposes: it gives the product enough bulk and weight to be spreadable uniformly, prevents caking and clumping, allows manufacturers to blend granules of different sizes evenly, and sometimes provides a pH buffer. Higher-analysis fertilizers like 46-0-0 urea have very little filler because nearly half the molecule is nitrogen. Our Label Decoder shows filler percentage for any grade you enter.
What is a balanced fertilizer and when should I use one?
A balanced fertilizer has roughly equal amounts of nitrogen, phosphorus, and potassium — like 10-10-10 or 20-20-20 — producing a simplified ratio of 1:1:1. Balanced fertilizers are useful when you have no soil test data and want a safe, general-purpose product, or when growing crops that need all three macronutrients in similar amounts such as ornamentals and houseplants. However, for most specialized applications — lawns, vegetables, fruiting crops — a balanced fertilizer over-applies one or more nutrients relative to crop need. If your soil test shows adequate phosphorus, using 10-10-10 wastes money and risks phosphorus runoff. A nutrient-specific blend is almost always a better agronomic choice.
How do I know if I am applying too much or too little of one nutrient?
The most reliable way is a certified soil test, conducted through your state university cooperative extension service or a commercial lab. Soil tests measure existing nutrient levels and generate crop-specific recommendations in lb/acre or kg/ha. Use these as your targets in our Blend Optimizer. Visual symptoms can also indicate deficiency: yellowing of older leaves suggests nitrogen shortage; purple tinting on leaf undersides suggests phosphorus deficiency; leaf-edge scorch on older leaves indicates potassium deficiency. However, symptoms are often ambiguous and appear only after significant stress has already occurred. Tissue testing — analyzing plant sap or leaf tissue — is an even more direct measurement of what the plant is actually taking up.