This free NPK fertilizer calculator converts a nutrient recommendation into kilograms and bags of urea, DAP and potash. Tell the calculator how much nitrogen, phosphate and potash you want to apply and which fertilizers you have. It works out the kilograms and bags of each, plus the cost. You can also switch to blend grade mode to find the N-P-K analysis of any mix you make.
How to use the NPK Fertilizer Calculator
- Choose what you want to do: plan fertilizer for an N-P-K target, or find the grade of a mix.
- Enter the nitrogen (N), phosphate (P₂O₅) and potash (K₂O) needed, with the rate unit and area.
- Pick the phosphorus, potassium and nitrogen sources, the bag size and, optionally, the price per bag.
- Press Calculate to get kg, bags and cost.
The NPK fertilizer calculator counts the nitrogen already supplied by DAP before topping up with your nitrogen source. For background on soil fertility and nutrient management, see the FAO Soils Portal.
How the fertilizer amounts are calculated
Fertilizer grades show the percentage of N, P₂O₅ and K₂O by weight. A 50 kg bag of DAP (18-46-0) contains 9 kg N and 23 kg P₂O₅. The calculator works in this order:
- Phosphorus first: P source needed = P₂O₅ target ÷ its P₂O₅ %.
- Potash: K source needed = K₂O target ÷ its K₂O %.
- Nitrogen top-up: subtract the N already supplied by the P source (DAP, MAP, NP), then N source needed = remaining N ÷ its N %.
Common fertilizer grades
| Fertilizer | N-P₂O₅-K₂O | Notes |
|---|---|---|
| Urea | 46-0-0 | Cheapest N per kg; loses N as ammonia if left on the surface |
| DAP | 18-46-0 | Most common P source; apply at sowing |
| SSP | 0-18-0 | Also supplies calcium and sulfur |
| MOP (potash) | 0-0-60 | Contains chloride. Avoid on tobacco and some fruit and vegetables |
| SOP | 0-0-50 | Chloride-free; also supplies sulfur |
| CAN | 26-0-0 | Less volatile than urea; good for top dressing |
Base your target rates on a soil test and your crop’s recommendation. Over-applying nitrogen wastes money and can pollute groundwater with nitrate.
Frequently asked questions
How much urea equals 1 kg of nitrogen?
Urea is 46% N, so 1 kg of N needs 1 ÷ 0.46 = 2.17 kg of urea. A 50 kg bag supplies 23 kg of N.
Why does the calculator reduce urea when I use DAP?
DAP contains 18% nitrogen. The nitrogen supplied by DAP counts towards your N target, so less urea is needed.
How do I convert kg per hectare to kg per acre?
1 hectare = 2.471 acres, so divide a per-hectare rate by 2.471 to get the per-acre rate.
What is the difference between P and P₂O₅?
Fertilizer labels and recommendations normally use the oxide forms P₂O₅ and K₂O. Elemental P = P₂O₅ × 0.436 and elemental K = K₂O × 0.830.
How do I calculate urea and DAP for one acre?
DAP is 18-46-0, so 50 kg of DAP supplies 23 kg of P₂O₅ and 9 kg of N. If the crop needs 46 kg N per acre, the other 37 kg N comes from urea (46% N): 37 ÷ 0.46 = 80 kg, or about 1.6 bags of 50 kg.
How do I find the NPK grade of a fertilizer mix?
Add up the kg of N, P₂O₅ and K₂O supplied by each product, divide each by the total weight of the mix and multiply by 100. For example, 50 kg urea + 50 kg DAP (100 kg) holds 23 + 9 = 32 kg N and 23 kg P₂O₅, so the grade is 32-23-0.
Related tools
For guidance only. Follow local agronomic recommendations and product labels. Built by a fertilizer-testing laboratory professional.