NPK Fertilizer Calculator
Product needed = nutrient rate ÷ (grade % ÷ 100). For 1 lb N per 1,000 sq ft with 21-7-14, apply 100 ÷ 21 = 4.76 lb of product per 1,000 sq ft. The tool also blends urea, DAP and potash and estimates nutrients removed by corn, soybean and wheat grain.
Use the NPK Fertilizer Calculator
How much fertilizer to apply
Product needed = nutrient rate ÷ (grade % ÷ 100). The three numbers on the bag are N, P₂O₅ and K₂O in percent by weight.
Apply
— lb per 1,000 sq ft
—
| Nutrient | Target | Product to meet it | Supplied |
|---|
Blend from urea, DAP and potash
DAP (18-46-0) covers P₂O₅, muriate of potash (0-0-60) covers K₂O, and urea (46-0-0) tops up the N that DAP does not supply.
| Product | Rate | Total for area |
|---|
Crop nutrient removal estimator
Crop & Yield Inputs
Calculates total nutrient export mass across the entire field.
Select a grain crop, enter your yield goal and field area, and click Calculate to view harvested nutrient removal.
Harvest Removal Breakdown
| Nutrient | Removal Coeff. | Rate / Area | Field Total |
|---|
Primary Source Provenance & Limitations
- Nitrogen Coefficients: 0.69 lb/bu corn, 3.44 lb/bu soybean, 1.05 lb/bu wheat. Source: University of Delaware Cooperative Extension (Nitrogen Removal in Delaware Crops).
- P₂O₅ & K₂O Coefficients: Corn (0.32 P₂O₅ / 0.22 K₂O lb/bu), Soybean (0.72 P₂O₅ / 1.20 K₂O lb/bu), Wheat (0.55 P₂O₅ / 0.27 K₂O lb/bu). Source: Iowa State University Extension & Outreach (crop removal table).
- Removal ≠ Fertilizer Prescription: Nutrient removal measures grain export only. Fertilizer application must be guided by calibrated soil tests (e.g., Bray-1, Mehlich-3, Olsen), soil pH, buffer capacity, subsoil fertility, and local Extension recommendations.
Standard Test Weights & Chemical Relationships
Soybean Grain: 60 lb/bushel (0.0272 t/bu)
Winter Wheat: 60 lb/bushel (0.0272 t/bu)
Elemental P = P₂O₅ × 0.4364
K₂O = Elemental K × 1.2046
Elemental K = K₂O × 0.8302
Metric conversion rule: kg nutrient / metric tonne = (lb nutrient per bushel / standard lb grain per bushel) × 1000.
Fertilizer needed per pound of nutrient
Product = 100 ÷ grade %. The same ratio works in kilograms: 100 kg/ha of N from urea takes 217.4 kg/ha of product.
| Fertilizer | Grade | lb product per 1 lb N | per 1 lb P₂O₅ | per 1 lb K₂O |
|---|---|---|---|---|
| Urea | 46-0-0 | 2.17 | — | — |
| Ammonium nitrate | 34-0-0 | 2.94 | — | — |
| Ammonium sulfate | 21-0-0 | 4.76 | — | — |
| Diammonium phosphate (DAP) | 18-46-0 | 5.56 | 2.17 | — |
| Monoammonium phosphate (MAP) | 11-52-0 | 9.09 | 1.92 | — |
| Triple superphosphate (TSP) | 0-46-0 | — | 2.17 | — |
| Muriate of potash (MOP) | 0-0-60 | — | — | 1.67 |
| Sulfate of potash (SOP) | 0-0-50 | — | — | 2.00 |
| Balanced garden | 10-10-10 | 10.00 | 10.00 | 10.00 |
| Balanced | 13-13-13 | 7.69 | 7.69 | 7.69 |
| Lawn | 21-7-14 | 4.76 | 14.29 | 7.14 |
| Lawn | 29-0-4 | 3.45 | — | 25.00 |
| Starter | 5-10-10 | 20.00 | 10.00 | 10.00 |
1 acre = 43,560 sq ft, so a rate per 1,000 sq ft × 43.56 = rate per acre. 1 lb/acre = 1.121 kg/ha.
Worked examples
- Lawn, 1 lb N per 1,000 sq ft with 21-7-14. 1 ÷ 0.21 = 4.76 lb of product per 1,000 sq ft. A 5,000 sq ft lawn needs 23.8 lb, about half of a 50-lb bag, and also receives 0.33 lb P₂O₅ and 0.67 lb K₂O per 1,000 sq ft.
- Sports turf, 0.9 lb N with 20-5-10. 0.9 ÷ 0.20 = 4.5 lb per 1,000 sq ft, or 4.5 × 43.56 = 196 lb per acre. One 50-lb bag covers 50 ÷ 4.5 × 1,000 = 11,111 sq ft.
- Field blend, 150-60-90 lb per acre. DAP = 60 ÷ 0.46 = 130.4 lb, which also supplies 23.5 lb N. Potash = 90 ÷ 0.60 = 150 lb. Urea = (150 − 23.5) ÷ 0.46 = 275.0 lb. Total 555.5 lb of blend per acre.
- Elemental P on a lab report. 20 lb P per acre × 2.2914 = 45.8 lb P₂O₅, which takes 45.8 ÷ 0.46 = 99.6 lb of triple superphosphate (0-46-0).
- Crop removal. 180 bu/acre corn removes 180 × 0.69 = 124.2 lb N, 180 × 0.32 = 57.6 lb P₂O₅ and 180 × 0.22 = 39.6 lb K₂O per acre in the grain.
Summary
This NPK fertilizer calculator converts a nutrient recommendation into an amount of fertilizer product. Fertilizer grades list nitrogen (N), phosphate (P2O5) and potash (K2O) as percentages by weight, so the product needed is the nutrient rate divided by the grade percentage. Enter the rate per 1,000 sq ft, per acre or per hectare, pick a grade, and the tool shows the product rate, the total for your area, the number of bags and what the other two nutrients add up to. It can also build a blend from urea, DAP and potash, and it estimates the N, P2O5 and K2O removed in harvested corn, soybean and wheat grain from University Extension coefficients. Base the rate itself on a soil test and local Extension guidance.
How it works
- Enter the recommended N, P2O5 and K2O rate and choose its unit: lb per 1,000 sq ft, lb per acre or kg per hectare. If your soil report gives elemental P and K, switch the basis and the tool converts them (P2O5 = P × 2.2914, K2O = K × 1.2046).
- Pick a fertilizer grade or type the three numbers from the bag. The product needed for each nutrient is rate ÷ (grade ÷ 100).
- By default the rate is set by the nutrient that needs the least product, so no nutrient goes over its target; you can base it on N, P2O5 or K2O instead. The tool shows what the other nutrients add up to.
- Enter the area and bag size to get the total product and number of bags. The blend panel shows how much urea (46-0-0), DAP (18-46-0) and muriate of potash (0-0-60) meet all three targets.
- The crop removal estimator at the top multiplies yield by grain removal coefficients (lb per bushel or kg per tonne) to show the nutrients leaving the field. "Use crop removal" copies that figure into the fertilizer calculator as a maintenance target.
Use cases
- Work out how many pounds of 10-10-10, 21-7-14 or 29-0-4 to spread per 1,000 sq ft of lawn or garden.
- Convert a soil-test recommendation in lb per acre or kg per hectare into product and bags for a field.
- Build a urea, DAP and potash blend that meets an N-P2O5-K2O target.
- Convert elemental P and K on a lab report to the P2O5 and K2O basis used on fertilizer bags.
- Quantify the nitrogen, phosphate and potash leaving the field in harvested corn, soybean or wheat grain.
- Compare crop removal with a fertilizer plan to track long-term soil nutrient balance.