Convert between fertilizer material weight and actual nutrient units (N, P₂O₅, K₂O). This calculator works in both directions: enter a fertilizer weight to see how much of each nutrient it supplies, or enter a desired nitrogen amount to find how much fertilizer you need.
Select a fertilizer grade (or enter a custom N-P-K), then enter a weight to see the nutrient units it contains. You can also enter a desired amount of nitrogen to calculate how much of that fertilizer is required — and see how much phosphorus comes along with it.
Nutrient Units (lb) = Fertilizer Weight (lb) × Nutrient % ÷ 100
Fertilizer grades are expressed as N-P₂O₅-K₂O percentages by weight. A 46-0-0 urea means 46% of the bag's weight is actual nitrogen.
Fertilizer Needed (lb) = Desired Nutrient (lb) ÷ (Nutrient % ÷ 100)
When using a multi-nutrient fertilizer like DAP (18-46-0), applying enough to meet your nitrogen requirement will also supply phosphorus — sometimes more than you need. Always check the accompanying nutrients.
100 lb of urea (46-0-0): N = 100 × 46 ÷ 100 = 46 lb N, P₂O₅ = 0, K₂O = 0.
100 lb of DAP (18-46-0): N = 18 lb, P₂O₅ = 46 lb, K₂O = 0.
To supply 50 lb of N using DAP: Fertilizer needed = 50 ÷ 0.18 = 277.8 lb. This also supplies 277.8 × 0.46 = 127.8 lb P₂O₅ — a significant amount that must be accounted for in your nutrient plan.
1. Select your fertilizer type from the dropdown, or choose Custom and enter the N-P-K percentages. 2. Enter a fertilizer weight to see how many nutrient units it contains. 3. Enter a desired nitrogen amount to reverse-calculate how much fertilizer is needed. 4. Review the accompanying phosphorus and potassium that come with that fertilizer amount. Click Calculate.
Fertilizer grades follow the N-P-K convention: the three numbers represent the percentage by weight of nitrogen (N), phosphate (P₂O₅), and potash (K₂O). Note that phosphorus and potassium are expressed as their oxide forms, not elemental P and K. To convert P₂O₅ to elemental P, multiply by 0.436; to convert K₂O to elemental K, multiply by 0.830. Understanding these conversions is essential for accurate nutrient management planning and avoiding over-application of nutrients.
Common mistakes: Confusing P₂O₅ with elemental phosphorus — P₂O₅ contains only 43.6% actual P. Ignoring the accompanying nutrients when using complex fertilizers — DAP applied for N can over-apply P. Using volume (bags) instead of weight — fertilizer density varies, always weigh. Forgetting that slow-release or controlled-release fertilizers have different availability factors than the grade suggests.
✓ Expert Reviewed: This calculator and its content have been reviewed by agricultural experts. Formulas are based on standard extension service recommendations.