Every fertilizer calculator does the same simple sum. Once you know it, you can work out how much urea, DAP or potash to buy for any field, on paper or on your phone. The numbers below are examples to show the method, not recommendations: your crop, soil test and local advice decide the real targets. Skip the arithmetic with our free fertilizer calculator.
In short
- Fertilizer needed = nutrient needed ÷ nutrient % in the bag × 100.
- P and K on a bag are shown as P₂O₅ and K₂O, so calculate with those figures.
- DAP also contains nitrogen. Count it before you add urea.
- 1 hectare = 2.47 acres. Divide kg per hectare by 2.47 for kg per acre.
The formula
Fertilizer (kg) = Nutrient needed (kg) ÷ Nutrient % × 100
A bag marked 46-0-0 (urea) holds 46% nitrogen, so 46 kg of nitrogen needs 100 kg of urea.
Worked example (per hectare, illustrative targets)
A crop needs 120 kg N, 60 kg P₂O₅ and 40 kg K₂O per hectare.
| Step | Calculation | Result |
|---|---|---|
| Phosphorus from DAP (18-46-0) | 60 ÷ 46 × 100 | 130 kg DAP |
| Nitrogen already in that DAP | 130 × 0.18 | 23 kg N |
| Nitrogen still needed | 120 − 23 | 97 kg N |
| Urea (46% N) | 97 ÷ 46 × 100 | 211 kg urea |
| Potassium from MOP (0-0-60) | 40 ÷ 60 × 100 | 67 kg MOP |
For an acre, divide each by 2.47: about 53 kg DAP, 85 kg urea and 27 kg MOP. A calculator that keeps the decimals shows about 210 kg of urea; the small difference is rounding.
Split the nitrogen
Nitrogen is easily lost to water and air. Applying it in two or three doses matched to growth stages usually works better than one big dose. Phosphorus and potassium are often applied before or at planting.
Common mistakes
- Using P and K percentages as if they were pure P and K. The label gives oxide forms.
- Forgetting the nitrogen in DAP or NPK compounds.
- Ignoring the soil test. If the soil already has plenty of phosphorus, adding more wastes money.
- Copying a neighbour's rate. Soils and crops differ.
- Ignoring organic matter. Compost and manure supply nutrients and improve the soil.
How Famo AI helps
Famo AI shows your nitrogen, phosphorus and potassium results against target ranges and marks them low, in range or high, so you know which nutrients need adding before you calculate. It keeps test history for each field and adds the follow-up tasks.
Get Famo AI on Google Play or try the live demo, no sign-up needed.
Frequently asked questions
How do I calculate urea per acre?
Work out the nitrogen your crop needs per acre, divide by 46 and multiply by 100. For 50 kg N per acre that is 109 kg of urea.
How many kg of DAP per hectare?
Divide the phosphorus target in kg P₂O₅ by 46 and multiply by 100. For 60 kg P₂O₅ per hectare that is about 130 kg of DAP.
Does a soil test change the calculation?
Yes. It tells you which nutrients are low, so you add only what the soil lacks.
