Donaldson R F Fertiliser NPK Calculator
Work out how much fertiliser to apply for a target nutrient rate, using the N-P-K grade printed on the bag.
Fertiliser NPK grade (the numbers on the bag)
How it is worked out
The numbers on a fertiliser bag (for example 20-10-10) are the percentage, by weight, of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Nutrient needed = area × target rate. Fertiliser needed = nutrient needed ÷ (that nutrient's percentage ÷ 100). Because the product has a fixed ratio, applying enough to hit your target rate of one nutrient also applies a fixed amount of the other two — shown below so you can see if, say, you're over-applying phosphate to hit a nitrogen target.
Source: standard fertiliser application arithmetic (nutrient needed ÷ grade percentage), the method used in university/extension gardening guides, e.g. Iowa State University Extension: How to Calculate Fertilizer Rates for the Garden.
Planning plant spacing too? Try the Plant Spacing Calculator.