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Pounds of lawn fertilizer and bags to buy from the N-P-K numbers on the bag and the pounds of nitrogen you want per 1,000 sq ft.
With these inputs:
| Result | Value |
|---|---|
| Fertilizer for 5,000 sq ft | 15.6 lb |
| Per 1,000 sq ft | 3.13 lb |
| Bags of 15 lb | 2 bags |
| Nitrogen delivered | 5 lb |
| Bag analysis | 32-0-4 |
| Phosphate (P₂O₅) per 1,000 sq ft | 0 lb |
| Potash (K₂O) per 1,000 sq ft | 0.13 lb |
| Formula | 1 ÷ 0.32 = 3.125 lb per 1,000 sq ft |
Rule from Iowa State University Extension: pounds of nitrogen wanted ÷ the bag's nitrogen percentage (as a decimal) = pounds of product per 1,000 sq ft. A soil test gives the rate your lawn actually needs; set your spreader from the bag's chart for this many pounds per 1,000 sq ft.
A fertilizer label gives percentages, not coverage, so the amount to spread depends on the first number. Iowa State University Extension's rule: pounds of product per 1,000 sq ft = pounds of nitrogen wanted ÷ the nitrogen percentage as a decimal. Enter the lawn, the N-P-K from the bag, the bag weight and the nitrogen rate, and the calculator returns the pounds to spread, the bags to buy, and the phosphate and potash each 1,000 sq ft receives.
Divide the pounds of nitrogen you want per 1,000 sq ft by the first number on the bag as a decimal. Iowa State Extension's example: 1 lb of nitrogen from a 32-0-4 fertilizer is 1 ÷ 0.32 = 3.125 lb of product per 1,000 sq ft. Multiply by your square feet ÷ 1,000 for the whole lawn.
At 1 lb of nitrogen per 1,000 sq ft, 3.125 lb × 5 = 15.6 lb of product — two 15 lb bags. At 0.5 lb N it is half that, 7.8 lb.
They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). Iowa State Extension: the first number is the percentage of the fertilizer's weight that is nitrogen, which is why the rate is worked from it.
Iowa State Extension's lookup table covers 0.25 to 1 lb of nitrogen per 1,000 sq ft per application. Follow your soil test and the bag directions, and don't spread more than the label allows.
For 1 lb of nitrogen, 1 ÷ 0.10 = 10 lb of 10-10-10 per 1,000 sq ft, which also delivers 1 lb of phosphate and 1 lb of potash. For 0.5 lb N it is 5 lb.
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