Enter the size of your plot, the NPK grade printed on the fertilizer bag and the nutrient target you are aiming for in kilograms per hectare. You get the application rate, the total kilograms to buy, how many 25 kg and 50 kg bags that is, and how much of each nutrient you actually apply. Nothing is uploaded — the arithmetic runs in your browser.
Area must be above zero; grades are 0–100% and targets up to 10,000 kg/ha.
Area must be above zero; grades are 0–100% and targets up to 10,000 kg/ha.
Enter the area, the grade from the bag and at least one nutrient target.
How it works
A bag labelled 15-15-15 contains 15% nitrogen, 15% phosphorus as P₂O₅ and 15% potassium as K₂O; the rest is filler and carrier salts. To supply a target of a single nutrient you divide the target by that percentage: 90 kg/ha of nitrogen from a 15% fertilizer needs 90 ÷ 0.15 = 600 kg/ha of product.
When you enter more than one target, each nutrient implies its own rate and they rarely agree. The tool shows every individual rate and then recommends the largest one, so no target falls short — the nutrient behind it is the limiting one. The table then shows what the other nutrients receive at that rate, which is usually more than you asked for. If the surplus is uncomfortable, split the job: apply a compound fertilizer to the lowest requirement and top up the limiting nutrient with a straight fertilizer such as urea or KAN.
Areas can be entered in square metres, ari (1 ar = 100 m²) or hectares, and everything is converted to hectares internally. Bag counts are rounded up to whole bags, since that is how fertilizer is sold. Targets and grades are handled in the oxide form printed on bags (P₂O₅, K₂O), not in elemental phosphorus and potassium — mixing the two is the single most common way to over- or under-apply.
Practical examples
One hectare of maize with NPK 15-15-15
Targets of 90 kg N, 60 kg P₂O₅ and 60 kg K₂O per hectare imply 600, 400 and 400 kg/ha respectively. Nitrogen sets the rate at 600 kg/ha — 24 bags of 25 kg — and phosphorus and potassium each end up 30 kg/ha above target.
Urea on half a hectare
A 5,000 m² field and a 100 kg/ha nitrogen target from urea (46-0-0) needs 217.4 kg/ha, so 108.7 kg in total: three 50 kg bags with some left over.
A phosphorus-led grade
With NPK 8-16-24 and a target of 80 kg/ha P₂O₅, the rate is 500 kg/ha. That also delivers 40 kg of nitrogen and 120 kg of potassium per hectare, which suits potato and vine crops.
A 10-ar kitchen garden
10 ari is 1,000 m². Aiming for 30 kg/ha of nitrogen with 15-15-15 gives 200 kg/ha, which is only 20 kg for the whole garden — a single bag.
Frequently asked questions
Which rate should I actually use when the three targets disagree?
The recommended figure — the largest single rate — guarantees no nutrient is short. If the resulting surplus of the other two is too high, use a fertilizer whose ratio is closer to your targets, or apply the compound product to the lowest requirement and top up the rest with a straight fertilizer.
Are P and K entered as elemental or oxide values?
Oxides — P₂O₅ and K₂O, exactly as printed on the bag and as written in soil-analysis recommendations here. If your recommendation is in elemental P or K, convert first: P × 2.29 = P₂O₅ and K × 1.205 = K₂O.
How do I convert the other way?
Multiply P₂O₅ by 0.436 to get elemental phosphorus, and K₂O by 0.830 to get elemental potassium. So 60 kg/ha of P₂O₅ is about 26 kg/ha of P.
What is an ar and why is it an option?
One ar is 100 m², and ari are the everyday unit for gardens and small plots in Serbia — 10 ari is 1,000 m², and 100 ari make a hectare. Entering the unit you already think in avoids conversion mistakes.
Can I use it for a single fertilizer like urea or KAN?
Yes. Enter 46 for nitrogen and leave phosphorus and potassium at zero for urea, or 27 for KAN. Only enter a target for the nutrient the product actually contains.
Why is the bag count rounded up?
Because fertilizer is sold in whole bags. The total kilograms figure is exact; the bag count is what you carry home, so it always rounds up and the surplus stays in the shed.
Does the tool know how much nutrient my soil already holds?
No, and no calculator can. Targets should come from a soil analysis for your parcel, ideally with an agronomist reading the report. This tool only converts a target into a product quantity.
Should I apply everything at once?
Phosphorus and potassium are usually applied before sowing, while nitrogen is commonly split between a basal dose and one or two top dressings so less is lost to leaching. The rate is the same either way — the timing is an agronomic decision.
What does the nutrient share percentage tell me?
It is N + P₂O₅ + K₂O added together. A 15-15-15 product is 45% nutrients and 55% other material, which is why cheaper low-analysis fertilizers cost more per kilogram of actual nutrient than the price per bag suggests.
Can I compare two fertilizers for value?
Yes — run the same target with both grades. The one needing fewer kilograms per hectare carries more nutrient per bag; multiply by the bag price to see which is genuinely cheaper per hectare.
Are there limits on what I can enter?
Grades are 0–100% and their sum cannot exceed 100, since a bag cannot hold more nutrient than its own mass. Targets go up to 10,000 kg/ha, far above any realistic recommendation.
Is this agronomic advice?
No. It is arithmetic on the numbers you enter. Follow the product label, your soil analysis and local rules on nutrient application, especially near watercourses.
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