Fertilizer Calculator
Calculate fertilizer quantity for a selected nutrient target, including accompanying nutrients and bag count.
On this page
CalculatorHow to useFormula & methodWorked exampleRead the label before doing the arithmeticOne product can supply several nutrientsFrequently asked questionsRelated calculatorsSourcesHow to use this calculator
- Choose a measurement system and enter the treatment area.
- Copy the three nutrient percentages from the fertilizer label. Select the nutrient that determines the application.
- Enter the recommended nutrient rate and bag weight, then check all nutrients supplied by that product quantity.
Formula and method
For the assumptions behind these results, read the explanation below. You can also explore Grass Seed and Compost.
A worked example
Supplying 5 g of nitrogen per m² across 100 m² requires 500 g N. A 20% N product supplies that in 2.5 kg of fertilizer, also adding 0.125 kg phosphate and 0.25 kg potash.
Read the label before doing the arithmetic
Fertilizer analysis is a set of mass percentages. A product containing 20% nitrogen has 0.2 units of nitrogen per unit of fertilizer mass. Dividing the required nitrogen mass by 0.2 therefore gives the product mass. The nutrient target is the amount of nutrient itself, not the amount of complete fertilizer.
On conventional N-P-K labels, the second and third numbers report phosphate as P₂O₅ and potash as K₂O equivalents. They are not elemental phosphorus and potassium percentages. This calculator keeps that label basis in both the inputs and the supplied-nutrient results. Do not enter an elemental recommendation without converting it to the matching basis.
One product can supply several nutrients
A single fertilizer quantity fixes the amount of every labeled nutrient applied. Meeting a nitrogen target may oversupply phosphate, so inspect the accompanying outputs before selecting a product. The calculator chooses the quantity from one selected target; it does not claim that this quantity balances all the soil’s needs.
Use a soil test and local crop guidance to choose an application rate. The example is arithmetic rather than a prescription for every lawn. Timing, product release rate, weather, and water management influence how plants use nutrients. The rounded bag count is an ordering aid: spread the calculated product mass, not automatically every bag purchased.
Frequently asked questions
Why is a zero-percent target rejected?
No finite quantity of a product can supply a nutrient it does not contain. Choose another product or a different target.
What are the rate units?
Use the unit selector beside the target rate to match your recommendation. Compatible rates such as grams per square meter and pounds per 1,000 square feet are converted automatically; bag weight has its own unit selector.
Sources and further reading
- Montana State University Extension — Fertilizer Rate Calculations ↗
Demonstrates dividing a nutrient target by the product’s nutrient fraction and explains N–P₂O₅–K₂O labeling. The application target must come from appropriate soil and crop guidance.
- National Institute of Standards and Technology — SI conversion factors ↗
Reference for physical unit conversions. The calculator converts measurements before applying its formula.
Our explanations are written for BiologyCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.