NPK Fertilizer Mixing Calculator
Calculate how much fertilizer to apply, or blend up to three products toward a target ratio or actual final grade. Fertilizer labels give percentages by weight of nitrogen (N), phosphate (P2O5), and potash (K2O)—not elemental P and K.
1. Choose calculation
Results
| Source | Weight | % batch | N | P₂O₅ | K₂O | Bags | Cost |
|---|
| Nutrient | Target | Supplied | Surplus / deficit |
|---|
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How to use the calculator
- Choose Application rate for a soil-test recommendation, or Blend design for a batch.
- Enter rates or a target. Ratio mode matches relative nutrient proportions; final-grade mode targets actual label percentages and fills the remainder with carrier.
- Select common products or enter custom grades.
- Review required product, nutrient surplus/deficit, bags, and cost before checking the product label.
Ratio, grade, and application formulas
A 4:2:1 ratio describes relative nutrient proportions; a 4-2-1 grade contains 4%, 2%, and 1% by total product weight. Product needed for one nutrient is target nutrient ÷ label decimal. For example, 100 lb N ÷ 0.46 = 217.39 lb of 46-0-0. Blend nutrients are the sum of each source weight × its label percentage. The best-fit search prohibits negative weights and minimizes normalized nutrient error.
Worked examples
4:2:1 blend
A 50 lb batch using 46-0-0, 11-52-0, and 0-0-60 can preserve a 4:2:1 proportion, but its resulting grade is stronger than 4-2-1. Choose final-grade mode when the percentages themselves matter.
Replacing unavailable 46-0-0
For 100 lb N/acre, 46-0-0 requires 100 ÷ .46 = 217.4 lb/acre. If only 21-0-0 ammonium sulfate is available, 100 ÷ .21 = 476.2 lb/acre; it also supplies sulfur, so confirm that amount is suitable.
Lawn bags
A 10,000 ft² lawn at 1 lb N per 1,000 ft² needs 10 lb N. A 30-0-4 product requires 10 ÷ .30 = 33.3 lb, so one 50 lb bag is enough, with product left over.
Crop blend
For an N-P₂O₅-K₂O target, urea supplies N, MAP supplies N and phosphate, and potash supplies K₂O. The result table exposes MAP’s extra N and any surplus rather than hiding it.
FAQs
How do I calculate fertilizer from an N-P-K label?
Divide the required nutrient weight by its label percentage as a decimal, then check nutrients supplied by the resulting product amount.
Can I mix fertilizers to make 10-10-10?
Sometimes. Select final-grade mode; if the sources cannot reach all three percentages without negative weights, the calculator shows the closest non-negative blend and required filler.
What is the difference between N-P-K ratio and fertilizer grade?
A ratio is relative, while a grade is the percentage by total product weight. A 20-10-5 grade has a 4:2:1 ratio but is not a 4-2-1 grade.
How much 46-0-0 do I need per acre?
Divide the target N rate by 0.46. A 100 lb N/acre target needs 217.4 lb of 46-0-0 per acre.
Why can phosphorus or potassium be too high?
Excess may waste money, raise loss risk, or create nutrient imbalance. Follow soil-test and local guidance.
Can dry fertilizers always be mixed?
No. Products may be chemically or physically incompatible, absorb moisture, cake, or segregate. Check labels and compatibility guidance before blending or storage.
How do I convert P to P₂O₅ or K to K₂O?
Approximate P₂O₅ = P × 2.29 and K₂O = K × 1.20. Convert the other way with P = P₂O₅ × 0.436 and K = K₂O × 0.830.
Practical fertilizer decisions
Common grades
46-0-0 urea is concentrated N; 11-52-0 MAP and 18-46-0 DAP supply N plus phosphate; 0-0-60 potash supplies K₂O; 21-0-0 ammonium sulfate also supplies sulfur.
When exact blends are impossible
A target outside the nutrient proportions available from your products cannot be exact. Use the best-fit result, change products, or accept a documented surplus.
Compatibility
Do not assume dry products can be combined or stored together. Check labels for moisture, reaction, caking, segregation, and application restrictions.
Soil-test workflow
Use the laboratory recommendation, confirm whether it reports P/K or P₂O₅/K₂O, calculate product, calibrate equipment, and record the application.
Organic and synthetic labels
Both use guaranteed-analysis percentages, but nutrient availability and release timing differ. Do not treat equal label grades as equal release curves.
Safety
Wear label-required PPE, prevent dust exposure and runoff, keep products dry and secured, and never exceed label rates.
Review and calculation assumptions
Prepared and reviewed by: Starlight Robotics calculator editorial team · Last reviewed: 12 July 2026
Calculations use guaranteed-analysis percentages, assume uniform dry product, and treat P and K label values as P₂O₅ and K₂O. Hand-check validation: 100 lb N ÷ 46% = 217.39 lb 46-0-0, matching the application result. Consult your soil-test laboratory, fertilizer label, and local university extension guidance for agronomic decisions.
Reference starting points: University of Georgia fertilizer calculator and University of Minnesota Extension nutrient guidance.
Disclaimer
Always follow product labels, soil test results, and local agronomy guidance. This calculator provides a mathematical blend and does not account for micronutrients, soil pH, or crop-specific nutrient timing.