AgriAuditor nutrient planning tool
Fertilizer purchase calculator
Convert approved N, P₂O₅ and K₂O targets into product quantities, bags and a budget using your supplier prices. Free to use; no account required.
Build your purchase plan
Enter approved nutrient targets to compare product quantities and purchase costs. Crop references come from broad demand classes, not a soil test; replace them before budgeting.
Preview of product quantities. Calculate to confirm valid inputs and create a purchase plan.
What to enter, and in which units
Nutrient rates are in kilograms per hectare and expressed in fertilizer-analysis units: N as elemental nitrogen, phosphorus as P₂O₅ and potassium as K₂O. If your soil report gives elemental P and K, convert them first — see the soil-test and unit conversion guide.
- Target crop from the crop list
- Loads a crop-profile planning reference. It is not a soil-test recommendation; replace the three targets with locally approved values.
- Cultivated area hectares
- Scales per-hectare rates into product quantities and 50 kg bag counts. 1 ha = 2.471 acres.
- Approved nutrient targets kg N, P₂O₅ and K₂O/ha
- Enter the area-basis targets supplied by a lab report, local recommendation or qualified adviser. No hidden pH or organic-matter multiplier is applied.
- Nitrogen credit kg N/ha
- Enter only an adviser-approved credit for soil mineral N, manure, legumes or another documented source. It is subtracted one-for-one from the N target.
Worked example: nutrient target to product weight
The arithmetic the calculator runs on every result: convert the soil-report unit to the label unit, then divide by the product analysis. The 45 kg elemental P/ha and 60 kg elemental K/ha figures are illustrative, not a crop recommendation.
| Step | Arithmetic | Metric | Imperial |
|---|---|---|---|
| Phosphorus to label units | 45 × 2.291 | 103.1 kg P₂O₅/ha | 92.0 lb/acre |
| Phosphate product weight | 103.1 ÷ 0.46 | 224.1 kg/ha of a 46% product | 199.9 lb/acre |
| Potassium to label units | 60 × 1.2046 | 72.3 kg K₂O/ha | 64.5 lb/acre |
| Potash product weight | 72.3 ÷ 0.6 | 120.5 kg/ha of a 60% product | 107.5 lb/acre |
Imperial figures use 1 kg/ha = 0.8922 lb/acre. Product mass always follows the same rule: nutrient required ÷ fertilizer analysis as a decimal. That arithmetic does not decide whether the nutrient rate belongs on a particular field.
Formulas and factors used
- Unit conversion: P₂O₅ = P × 2.291; K₂O = K × 1.2046.
- Nitrogen: entered N target minus the entered, adviser-approved N credit. Both values use kg N/ha; the result cannot fall below zero.
- Phosphate and potash: the entered P₂O₅ and K₂O targets pass through unchanged to the product-conversion step.
- Products: DAP (18% N, 46% P₂O₅) sized on phosphate first; the nitrogen it supplies is subtracted before urea (46% N) is sized; MOP (60% K₂O) covers potash.
Starting prices are illustrative: 0.70/kg urea, 0.85/kg DAP and 0.65/kg MOP. Choose a currency and replace them with supplier quotes. Exact-quantity cost uses the required product mass; purchase cost rounds each product up to whole bags. Neither includes tax, transport or application.
Limits of this estimate
The calculator can accept a locally approved recommendation, but it cannot interpret a raw soil test. It does not know the extraction method, CEC, soil depth, yield goal, manure analysis, environmental limits or field history that determine a real application.
It does not diagnose a nutrient deficiency, set a lime or sulfur rate, or approve an application. Use it to check arithmetic and compare scenarios, then verify the rate with a soil test, local water-quality rules and a qualified adviser.
For the label-unit background — what the three numbers mean and which soil-test inputs change the decision — read the soil-test and unit conversion guide.
Read the AgriAuditor methodology →Sources for the conversion factors
- Penn State Extension: How Much Phosphorus and Potassium Are Really in Your Fertilizer?Elemental P and K conversion factors.
- Penn State Extension: Soil TestingWhy soil tests guide fertilizer and lime decisions.
- University of Minnesota Extension: Interpreting Soil Tests for Fruit and Vegetable CropsReading fertilizer labels and calculating product amounts from nutrient needs.
- University of Minnesota Extension: Quick Guide to Fertilizing PlantsGuaranteed analysis, product selection, and over-application limits.
Source check: 10 August 2026. Editorial owner: AgriAuditor Research. These links are cited for guidance; AgriAuditor does not republish the linked extension text.
Questions about the NPK calculator
What does this NPK fertilizer calculator actually calculate?
It converts the N, P₂O₅ and K₂O targets you enter into product weights of urea (46% N), DAP (18% N, 46% P₂O₅) and MOP (60% K₂O), including the nitrogen supplied by DAP. It does not create a soil-test recommendation.
How do soil-test N, P and K values map onto the inputs?
Use a locally interpreted recommendation expressed in kg/ha. If it gives elemental P and K, convert them first — P₂O₅ = P × 2.291 and K₂O = K × 1.2046. A raw ppm result cannot be entered directly because converting concentration to an area basis also requires sampling depth, bulk density, extraction method and local calibration.
Are the results in kg/ha or lb/acre?
Nutrient rates are shown in kg/ha and product quantities in kilograms and bags of your chosen weight. To convert to imperial, multiply kg/ha by 0.8922 to get lb/acre. The worked example on this page shows both columns.
Why does DAP change how much urea I need?
DAP is 18% nitrogen as well as 46% phosphate, so the phosphate application already delivers nitrogen. The calculator subtracts that contribution before sizing the urea, which is why the urea figure falls when the phosphate requirement rises.
Can I use this instead of a laboratory soil test?
No. This is a planning and arithmetic-checking scenario, not a lab-calibrated prescription. It does not collect lab P, lab K, CEC, soil depth or a locally calibrated recommendation. Verify any rate with a soil test and a qualified local adviser before applying it.