Fertilizer Calculator
Convert nutrient rates into product quantities for any fertiliser analysis.
Please note: Planning figures based on published averages. Soil type, climate, variety, season and local practice all shift these results substantially. Base real decisions on a soil test, local extension guidance and the product label — particularly for fertiliser, lime and any crop protection product, where the label is a legal document.
What the Fertilizer Calculator does
Fertiliser is sold by its nutrient analysis, so converting a nutrient target into a product rate means dividing by the percentage of that nutrient. Where several targets are set, the one demanding the highest product rate governs — and everything else arrives in whatever quantity that rate happens to deliver.
Formula
Product rate = Nutrient target ÷ Nutrient % × 100Total product = Rate × AreaNutrient supplied = Rate × Nutrient % ÷ 100
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Area | number | Yes | Accepts more than 0. |
| Area unit | one of 4 options | Yes | — |
| Nitrogen (N) target | kg/ha | Yes | Accepts more than 0. |
| Phosphate (P₂O₅) target | kg/ha | Optional | Accepts 0 or more. |
| Potash (K₂O) target | kg/ha | Optional | Accepts 0 or more. |
| Fertiliser product | one of 11 options | Yes | — |
| Custom N % | % | In some modes | Shown Fertiliser product is Custom analysis. |
| Custom P₂O₅ % | % | In some modes | Shown Fertiliser product is Custom analysis. |
| Custom K₂O % | % | In some modes | Shown Fertiliser product is Custom analysis. |
| Product price per tonne | selected currency | Optional | Accepts 0 or more. |
| Currency | one of 10 options | Optional | — |
How to use it
- Choose Area unit and Fertiliser product.
- Enter Area and Nitrogen (N) target.
- Fill in the remaining inputs the form shows for your choice.
- Optionally add Phosphate (P₂O₅) target, Potash (K₂O) target and Product price per tonne.
- Select Calculate.
Worked example
Applying 120 kg of nitrogen per hectare as urea (46-0-0) across 5 hectares.
- Area
- 5 ha
- N target
- 120 kg/ha
- Product
- Urea (46-0-0)
120 ÷ 46 × 100 = 260.9 kg/ha of urea. Over 5 hectares that is 1,304 kg — about 1.3 tonnes.
Reading the result
- "Still short of target" is the honest counterpart to "rate driven by" — whichever nutrient target is highest gets satisfied exactly, and the others arrive only incidentally at whatever ratio the product carries, so a shortfall shown here is a mathematical consequence, not a bug.
- "Cost per kg of nitrogen" (once price is entered) is the figure for comparing products or suppliers fairly, since a raw price per tonne does not account for differing nutrient concentrations.
Assumptions and limitations
- The calculator assumes a single blanket rate across the whole area — it has no concept of variable-rate application by zone, even though soil variability often calls for different rates in different parts of a field.
- Nutrient targets are treated as the total to hit in one pass — this does not model split applications, such as some nitrogen at planting and more top-dressed later.
Common mistakes
- Reading "nitrogen supplied" as sufficient without checking "still short of target" for phosphate or potash when those were also entered — a single-product application can look successful on nitrogen alone while leaving other nutrients well under target.
- Assuming a US-labelled product's stated analysis lines up with the metric N-P-K percentages behind a preset — some regionally sold products are blended and labelled differently.
Frequently asked questions
What do the three numbers on a fertiliser bag mean?
Percentage of nitrogen, phosphate (P₂O₅) and potash (K₂O) by weight. Urea at 46-0-0 is 46% nitrogen and nothing else.
Why can one product not meet all three targets?
Because the ratio is fixed. Unless your soil needs nutrients in exactly the product’s proportions, one runs out first and the rest must come from a second product or a blend.