Calculate how much fertilizer product to apply based on the N-P-K ratio, the nitrogen requirement and the area.
NPK fertilizers are labelled with three numbers representing the percentage by weight of nitrogen (N), phosphorus (as P₂O₅) and potassium (as K₂O). A 10-10-10 bag contains 10% of each: 100 g of product supplies 10 g of N, 10 g of P₂O₅ and 10 g of K₂O.
This calculator works backwards from a nitrogen requirement: if you need 5 g/m² of N and you have a 10-10-10 fertilizer, you need 50 g/m² of product (5 / 0.10). The tool also shows how much P and K that dose delivers, which helps you check for over- or under-application of the other nutrients.
The nitrogen dose is usually the controlling factor because N is the most commonly deficient nutrient and the most responsive to application. A soil test tells you what your soil actually needs — without one, these calculations are educated guesses.
Product (kg) = N needed (g/m²) / (N% / 100) × area (m²) / 1000. P supplied = product × P%. K supplied = product × K%.
The percentage by weight of N, P₂O₅ and K₂O. A 20-5-10 fertilizer is 20% nitrogen, 5% phosphorus pentoxide, 10% potassium oxide.
Ideally, yes. Without one, you are guessing the dose. Over-fertilising wastes money and can burn plants; under-fertilising limits growth.
Yes — calculate the dose for each nutrient separately and blend to match. Or use single-nutrient fertilisers for precise control.
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