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NPK Calculator
Convert your target N, P₂O₅ and K₂O rates into actual bagged fertilizer amounts using DAP (18-46-0), urea (46-0-0) and muriate of potash (0-0-60).
Results
How to Use This Calculator
Enter target N, P₂O₅ and K₂O in kg per hectare, field area, and current prices for urea, DAP and muriate of potash. Click Calculate to see the split by product, total fertilizer weight and estimated cost.
About This Tool
This NPK calculator uses the standard extension decomposition: Fertilizer amount = target nutrient ÷ (nutrient fraction in fertilizer ÷ 100). It assumes DAP supplies both P₂O₅ (46%) and a small amount of N (18%), with urea making up the remaining N and MOP supplying all K₂O (60%).
Applicability: Applicable globally, with particular relevance for North American and European cropping systems. Adapt rates to your soil test results.
How the Calculation Works
DAP = target P₂O₅ ÷ 0.46. Nitrogen from DAP = DAP × 0.18. Urea needed = (target N − N from DAP) ÷ 0.46. Muriate of potash = target K₂O ÷ 0.60. Multiply each by field area and local price for cost.
Real-World Example
Targets 150 N, 60 P₂O₅, 100 K₂O kg/ha. DAP = 60 ÷ 0.46 = 130 kg/ha, supplying 23 kg N. Remaining N = 127 kg → 276 kg urea/ha. MOP = 100 ÷ 0.60 = 167 kg/ha. Total = 573 kg/ha.
Common Mistakes & Tips
Common mistakes: confusing P₂O₅ with elemental P (multiply by 0.436), ignoring the nitrogen in DAP and over-application N, buying compound fertilizers that force over-application, and not accounting for road moisture or caking. Always weigh out a sample bag.
Reviewed by Agricalc Editorial Team — Agricultural engineers and agronomists with 10+ years of field experience. Our team verifies every calculator formula against peer-reviewed extension publications. For site-specific advice, consult your local agronomist.
References
- USDA NRCS — Nutrient Management Planning
- University of Minnesota Extension — Nitrogen & Fertilizer Recommendations
- FAO — Fertilizer and Plant Nutrition
Frequently Asked Questions
What do N-P₂O₅-K₂O numbers mean?
Fertilizer labels express nutrients as oxides by convention: N is elemental nitrogen, P₂O₅ is phosphorus pentoxide (elemental P = P₂O₅ × 0.436), K₂O is potassium oxide (elemental K = K₂O × 0.83). A 15-15-15 bag is 15% N, 15% P₂O₅ and 15% K₂O by weight.
Which fertilizer source is best for phosphorus?
Diammonium phosphate (DAP, 18-46-0) and monoammonium phosphate (MAP, 11-52-0) are the most common high-P sources. Rock phosphate is slow-release and only effective on acidic soils. Superphosphate is less concentrated and rarely used now.
How much urea equals 100 kg N?
Urea is 46% N by weight, so 100 kg N ÷ 0.46 = 217 kg urea. Ammonium sulfate (21% N) would require 476 kg, and calcium ammonium nitrate (27% N) requires 370 kg.
Should I use compound fertilizer or blend single nutrients?
Blending single-nutrient sources (DAP + urea + MOP) is cheaper and more flexible when you have a soil test. Compound fertilizers (15-15-15, 20-10-10) are convenient for uniform blends but force you to over-apply nutrients where ratios do not match crop demand.
How do I convert kg/ha to lb/acre?
Multiply kg/ha by 0.892 to get lb/acre. Conversely, lb/acre × 1.12 = kg/ha. A target of 150 kg N/ha equals about 134 lb N/acre.
