Calculate fertiliser from an N-P-K label

Convert a nitrogen recommendation and N-P-K label into product kilograms and whole bags.

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ToolFertiliser Calculator
Input
Output
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How it works

Area × nitrogen grams per square metre gives actual nitrogen required, which is divided by the first N-P-K number as a decimal to obtain product mass. Kilograms are then divided by the selected pack size and rounded upward.

Formula
product grams = area × target nitrogen g/m² ÷ (label N% ÷ 100)
  • University of Minnesota Extension defines the label numbers as nutrient percentages and uses required nutrient ÷ fractional concentration.
  • Soil-test rate, product analysis, and bag size are required rather than supplied as lawn defaults.

Worked example

A 100 square metre lawn
20-10-10 feed at a 3g nitrogen per square metre target
Input
											Area m2: 100
Npk: 20-10-10
Target nrate gper m2: 3
Bag size kg: 20
Output
												Product needed g: 1500
Product needed kg: 1.5
N percent used: 20
Bags needed: 1

When to use this

Soil-test recommendations become product kilograms, lawn feeds can be compared by nitrogen concentration, and purchase lists round the result to whole bags.

Edge cases

  • 20-10-10 means 20% nitrogen by product weight, not a 20 g/m² spreading rate.
  • A 0-10-10 analysis cannot meet a nitrogen target and is rejected instead of dividing by zero.
  • Meeting the nitrogen target with a balanced product may overapply phosphorus or potassium; their soil-test requirements are not checked here.

References