Fertiliser Calculator for N, P and K Application Rates
Estimate indicative nutrient requirements and convert NPK fertiliser analysis into kg/ha product rates for crop, field and lawn applications.
Quick answer
Enter nutrient needs or use the illustrative crop section for rough kg/ha figures, then switch to NPK product rate to convert analysis to product kg/ha. Outputs are an indicative planning guide. Check soil analysis, product labels and local agronomy advice before applying fertiliser.
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Grassland and lawn use fixed illustrative totals (scaled differently from combinable crops). "Other" needs your own targets only.
Optional: override N, P₂O₅ or K₂O (kg/ha)
Leave blank to use illustrative values from the crop and yield. Fill any field to mix manual and table figures (yield may then be required).
Enter a target yield (t/ha), or set your own indicative N, P₂O₅ or K₂O targets below.
Product required to supply a nutrient target
Product rate kg/ha = nutrient target kg/ha ÷ nutrient percentage × 100.
Use the column matching the nutrient percentage in the fertiliser analysis. For example, 46% nitrogen for urea.
| Target (kg/ha) | 10% | 20% | 25% | 34.5% | 46% | 60% |
|---|---|---|---|---|---|---|
| 25 | 250.0 | 125.0 | 100.0 | 72.5 | 54.3 | 41.7 |
| 50 | 500.0 | 250.0 | 200.0 | 144.9 | 108.7 | 83.3 |
| 75 | 750.0 | 375.0 | 300.0 | 217.4 | 163.0 | 125.0 |
| 100 | 1000.0 | 500.0 | 400.0 | 289.9 | 217.4 | 166.7 |
| 150 | 1500.0 | 750.0 | 600.0 | 434.8 | 326.1 | 250.0 |
| 200 | 2000.0 | 1000.0 | 800.0 | 579.7 | 434.8 | 333.3 |
How to use the fertiliser calculator
The tool has two modes. Nutrient requirement estimate gives rounded, illustrative N, P₂O₅ and K₂O figures (kg/ha) from crop, yield and simple soil index inputs, minus optional soil N and manure N credits. Treat this as a planning discussion aid, not a recommendation. NPK product rate turns nutrient targets and a bag analysis into kg/ha of product, bags and optional cost, using the nutrient you choose to "match" (nitrogen, phosphate or potash).
Fertiliser calculation formula
To supply a nutrient from a granular product, scale from percentage to kg/ha of product:
Product rate kg/ha = required nutrient kg/ha ÷ nutrient percentage × 100
Example: To apply 60 kg N/ha using a 20% nitrogen fertiliser: 60 ÷ 20 × 100 = 300 kg/ha of product.
How to convert nutrient requirement into product rate
Divide the nutrient you need in kg/ha by the matching percentage in the product (expressed as a fraction of 100). That gives kg/ha of product. If you need several nutrients, choose which one the application should "hit" first, then check surpluses or shortfalls on the others.
What do N, P and K mean?
On labels and in agronomy, N is nitrogen (growth and protein). P₂O₅ is phosphate expressed as phosphorus pentoxide, the usual convention for phosphate fertilisers in the UK and EU. K₂O is potash, expressed as potassium oxide. Your soil analysis and fertiliser bag list percentages of these forms.
How to read NPK fertiliser analysis
A label such as 20-10-10 means about 20% N, 10% P₂O₅ and 10% K₂O by weight of the product. Compound grades supply more than one nutrient; straight fertilisers often have two zeros (for example 46-0-0 urea).
Nutrient requirement vs product rate
Nutrient requirement is kg/ha of N, P₂O₅ or K₂O you want to cover. Product rate is kg/ha of the actual fertiliser you spread or spray (solids here). The same nutrient need needs less kg/ha if the product contains a higher percentage of that nutrient.
Fertiliser rate examples
- Nitrogen only: 60 kg N/ha from 20% N: 60 ÷ 20 × 100 = 300 kg/ha product.
- Compound NPK: Matching N with 20-10-10 at 60 kg N/ha implies 300 kg/ha product, which also delivers 30 kg P₂O₅/ha and 30 kg K₂O/ha.
- Field area: At 300 kg/ha over 10 ha, total product = 3,000 kg.
- Bags: 3,000 kg ÷ 600 kg bags ≈ 5 bags (order 5 full bags or adjust for part bags).
Frequently asked questions
How do I calculate fertiliser rate per hectare?
Divide the nutrient you want to supply (kg/ha) by the percentage of that nutrient in the product (as a decimal), or use: product kg/ha = nutrient kg/ha ÷ percentage × 100.
What does 20-10-10 fertiliser mean?
It usually means about 20% nitrogen, 10% phosphate as P₂O₅ and 10% potash as K₂O by weight of the product.
How much fertiliser do I need for one hectare?
Multiply the product rate in kg/ha by the number of hectares. The calculator does this when you enter area after working out kg/ha of product.
How do I convert nitrogen requirement into kg of fertiliser?
Use the N percentage on the bag. For example 60 kg N/ha from 34.5% N is 60 ÷ 34.5 × 100 kg/ha of product.
What is the difference between N, P₂O₅ and K₂O?
N is nitrogen. P₂O₅ is the standard way phosphate fertiliser content is declared. K₂O is the standard way potash content is declared on labels.
Can this calculator be used for lawns?
Yes for the arithmetic. Lawn programmes often use smaller kg/ha figures or grams per square metre; convert g/m² to kg/ha by multiplying by 10 if needed.
Is this calculator the same as a nutrient management plan?
No. It performs simple arithmetic from numbers you supply. A nutrient management plan covers legal limits, organic manures, timings, soil testing and record-keeping.
Why does fertiliser product rate differ from nutrient rate?
Product rate is the weight of fertiliser you apply. Nutrient rate is the weight of N, P₂O₅ or K₂O inside it, determined by the analysis percentage.
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