Calculator Inputs
Choose your input basis, enter a fertilizer grade, and add an application amount. The calculator outputs both label and elemental values.
Example Data Table
This sample uses a common 10–10–10 label grade with a 5 kg application across 100 m². Use “Load example” to copy it into the form.
| Scenario | Grade basis | Grade | Amount | Area | N delivered | P delivered | K delivered |
|---|---|---|---|---|---|---|---|
| Balanced feeding | Label (N–P2O5–K2O) | 10–10–10 | 5 kg | 100 m² | 0.500 kg | 0.218 kg | 0.415 kg |
| Elemental view | Elemental (N–P–K) | 10.0–4.4–8.3 | 5 kg | 100 m² | 0.500 kg | 0.218 kg | 0.415 kg |
Formula Used
- P2O5 = P × 2.291
- P = P2O5 × 0.4364
- K2O = K × 1.2047
- K = K2O × 0.8301
How to Use This Calculator
- Select whether your numbers come from a label grade or elemental grade.
- Enter the N, P (or P2O5), and K (or K2O) percentages.
- Enter how much fertilizer you plan to apply, and choose the unit.
- Turn on per-area rates if you want feeding per surface area.
- Press Submit to display results above the form.
Understanding fertilizer grades and oxides
Most garden fertilizers show N–P2O5–K2O, not N–P–K. The oxide convention is standardized for labels. This calculator converts phosphate and potash into elemental P and K using fixed factors (P = P2O5 × 0.4364; K = K2O × 0.8301). The same bag grade can look lower in elemental form, although the product is identical.
How conversion changes nutrient expectations
If you read a 10–10–10 label as elemental, you overestimate phosphorus. Applying 5 kg of 10–10–10 delivers 0.50 kg N, 0.50 kg P2O5, and 0.50 kg K2O. Converted, that equals about 0.218 kg elemental P and 0.415 kg elemental K. Seeing both views helps you match soil test targets often reported in elemental terms.
Rates per area for beds, lawns, and containers
The per-area option translates total nutrients into practical dosing metrics. Enter your treated area to get g/m² for quick garden math, plus kg/ha and lb/acre for agronomic comparisons. This makes it easier to scale one plan across different plot sizes and avoid double feeding when switching products.
Comparing products by delivered nutrients
Two fertilizers can share the same N number but deliver different P and K once converted. Use the calculator to standardize on nutrient mass rather than bag weight. To keep nitrogen constant, adjust fertilizer amount until delivered N matches your baseline, then verify P and K remain within your target range. This supports steady growth while reducing excess runoff risk.
Professional notes for planning and recordkeeping
Record the grade basis, application mass, and area for repeatable results. Keep decimals modest (2–3 places) to reflect real-world spreading accuracy. Validate that each percentage stays between 0 and 100, and confirm units before exporting. For liquid concentrates, enter the product mass equivalent to dry weight shown on the label. Use the CSV download to maintain a feeding log across seasons, then refine timing and rates using plant response and soil testing.
FAQs
What is the difference between P and P2O5 on fertilizer labels?
P2O5 is a labeling convention for phosphorus content. Elemental P is smaller: P = P2O5 × 0.4364. The calculator shows both so you can compare products and soil targets consistently.
Why do many products list K as K2O instead of elemental K?
K2O is the traditional labeling basis for potash. Elemental K is smaller: K = K2O × 0.8301. Converting prevents dosing errors when a guide uses elemental values.
Can I use this for liquid fertilizers or concentrates?
Yes, if you know the mass of product applied. Enter the equivalent product weight for your dose. If you only have volume, convert to mass using the product density from the label or datasheet.
How are per-area results calculated?
The tool divides delivered nutrient mass by treated area, then outputs g/m² plus common farm units (kg/ha and lb/acre). This helps scale a plan from one bed to a whole lawn.
Does this replace soil testing or crop recommendations?
No. It converts label grades into comparable nutrient amounts. Use local guidance, plant needs, and soil tests to choose targets, then use the calculator to check what your chosen product actually supplies.
What if my product already lists elemental P and K?
Select the “Elemental grade (N–P–K)” basis and enter the values directly. The calculator will compute the equivalent label view (P2O5 and K2O) for comparisons with standard bag grades.