NPK Ratio Calculator
Reads a fertiliser grade: the ratio, what is in the bag and the elemental figures.
NPK ratio
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Total nutrients — %
In the bag
Elemental phosphorus — % P
Elemental potassium — % K
Reading this —
The formula
ratio = each grade number ÷ the smallest non-zero number P = P₂O₅ × 0.4364 K = K₂O × 0.8301
Grades are stated as the oxides by convention, and the factors come from their atomic weights.
What to watch out for
Two fertilisers with the same ratio can differ in strength: 10-5-5 and 20-10-10 are both 2 : 1 : 1, but it takes twice as much of the first.
Frequently Asked Questions
What does 20-10-10 mean?
20% nitrogen, 10% P₂O₅ and 10% K₂O by weight, a ratio of 2 : 1 : 1.
How much elemental phosphorus is in 10% P₂O₅?
4.36%.