Lawn Fertilizer Calculator
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.
Worked example: 100 × 50 ft lawn with 32-0-4
| Step | Working | Result |
|---|---|---|
| Area | 100 × 50 | 5,000 sq ft |
| Product per 1,000 sq ft (1 lb N) | 1 ÷ 0.32 | 3.125 lb |
| Whole lawn | 3.125 × 5 | 15.6 lb |
| Bags of 15 lb | 15.6 ÷ 15, rounded up | 2 bags |
| Potash delivered | 3.125 × 0.04 | 0.13 lb K₂O per 1,000 sq ft |
Pounds of product per 1,000 sq ft
| Bag (first number) | 0.5 lb N | 0.75 lb N | 1 lb N |
|---|---|---|---|
| 10% nitrogen | 5.0 lb | 7.5 lb | 10.0 lb |
| 16% nitrogen | 3.1 lb | 4.7 lb | 6.3 lb |
| 24% nitrogen | 2.1 lb | 3.1 lb | 4.2 lb |
| 29% nitrogen | 1.7 lb | 2.6 lb | 3.4 lb |
| 32% nitrogen | 1.6 lb | 2.3 lb | 3.1 lb |
| 46% nitrogen | 1.1 lb | 1.6 lb | 2.2 lb |
Every row is the same rule from Iowa State University Extension's guide to lawn fertilizer rates (last reviewed April 2026): divide the nitrogen you want by the nitrogen percentage as a decimal. Its own example is 1 lb of nitrogen from 32-0-4, which is 3.125 lb of product per 1,000 sq ft. The extension also points out that a soil test gives the rate your lawn actually needs — the bag's numbers only tell you how much of each nutrient is inside.
Reading the bag
The three numbers are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 15 lb bag of 32-0-4 holds 4.8 lb of nitrogen, no phosphate and 0.6 lb of potash; the rest is carrier. Set the spreader from the bag's chart for the pounds per 1,000 sq ft above, and go over the lawn in two directions at half rate for even coverage.
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Frequently asked questions
- How do I calculate how much fertilizer to put on my lawn?
- 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.
- How much 32-0-4 do I need for 5,000 square feet?
- 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.
- What do the three numbers on a fertilizer bag mean?
- 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.
- How much nitrogen should I put down at one time?
- 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.
- How much 10-10-10 do I need per 1,000 square feet?
- 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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