Fertilizer calculator: how much fertilizer do I need for my lawn?
25 lb of fertilizer
5,000 square feet at 1 lb of nitrogen per 1,000 sq ft from a 20% N product · 1 bag of 50 lb · for 100 × 50 ft of lawn
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
25 lb of fertilizer
5,000 square feet at 1 lb of nitrogen per 1,000 sq ft from a 20% N product · 1 bag of 50 lb
Source: Purdue Extension AY-22-W, Fertilizing Established Cool-season Lawns, the product-rate method and the nitrogen limits per application, read October 15, 2026. Data sheet (PDF)
A 5,000 square foot lawn fed 1 lb of nitrogen per 1,000 square feet takes 25 lb of a fertilizer with 20% nitrogen, such as a 20-0-5, because 1 ÷ 0.20 is 5 lb per 1,000 square feet.
A worked example
Purdue's own case, a 5,500 sq ft lawn (110 by 50 ft) fed 0.5 lb of nitrogen with an 18-0-10 product sold in 15 lb bags, needs 15.3 lb of fertilizer, 2 bags of 15 lb. 0.5 ÷ 0.18 is 2.78 lb of product per 1,000 square feet, and 5,500 ÷ 1,000 is 5.5, so 2.78 × 5.5 = 15.29 lb, the figure in the Purdue publication. Without rounding the 2.78 first it is 15.28, shown as 15.3.
How much nitrogen per 1,000 square feet should one application have?
Purdue Extension puts a single application normally between 0.5 and 1.0 lb of actual nitrogen per 1,000 square feet, and no more than 1.0 lb when the nitrogen is water-soluble, the quick-release kind. When more than half of the product's nitrogen is slow-release, up to 2.0 lb in one application is acceptable. Larger amounts are split into two or more applications. The calculator marks a rate over those limits next to the result.
How much nitrogen does a lawn need in a year?
For established cool-season lawns, Purdue's Table 2 gives a yearly total of 1 to 3 lb of nitrogen per 1,000 square feet for the lowest maintenance, 2 to 4 lb for moderate maintenance with or without regular irrigation, and 3 to 5 lb for the highest maintenance with regular irrigation. At 1 lb per application, 3 lb a year is three applications; enter each one above to get the pounds of product.
What else it is called
Lawn fertilizer is labeled by its N-P-K analysis, the three numbers on the bag. Bags also print a coverage in square feet, which is the maker's own rate; this calculator works from the nitrogen instead, so you can compare two products or feed at a different rate than the bag assumes.
Pounds per 1,000 square feet
| Nitrogen on the label | 0.50 lb N | 0.75 lb N | 1.00 lb N | 2.00 lb N |
|---|---|---|---|---|
| 10% N | 5.0 lb | 7.5 lb | 10.0 lb | 20.0 lb |
| 16% N | 3.1 lb | 4.7 lb | 6.3 lb | 12.5 lb |
| 20% N | 2.5 lb | 3.8 lb | 5.0 lb | 10.0 lb |
| 24% N | 2.1 lb | 3.1 lb | 4.2 lb | 8.3 lb |
| 29% N | 1.7 lb | 2.6 lb | 3.4 lb | 6.9 lb |
| 32% N | 1.6 lb | 2.3 lb | 3.1 lb | 6.3 lb |
| 46% N | 1.1 lb | 1.6 lb | 2.2 lb | 4.3 lb |
Pounds of product per 1,000 sq ft. Purdue AY-22-W: normally 0.5 to 1.0 lb of N per application, no more than 1.0 lb of water-soluble N, and 2.0 lb only when over half the N is slow-release.
Where the numbers come from
- Method and limits: Purdue Extension AY-22-W, Fertilizing Established Cool-season Lawns (Bigelow, Camberato and Patton). Pounds of product = (target lb of nitrogen per 1,000 sq ft ÷ the nitrogen fraction on the label) × (area ÷ 1,000), worked in the publication for 5,500 sq ft at 0.5 lb with an 18-0-10: 15.29 lb. The target is normally 0.5 to 1.0 lb of nitrogen per 1,000 sq ft; no single application should carry more than 1.0 lb of water-soluble nitrogen, or 2.0 lb when more than half the nitrogen is slow-release. Mature cool-season lawns take 1 to 5 lb of nitrogen per 1,000 sq ft a year. Read October 15, 2026. Purdue AY-22-W (PDF)
- The three numbers on a bag, such as 20-0-5, are the percent of nitrogen, phosphate and potash by weight; the calculator uses the first. Bags are the pounds divided by your bag weight, rounded up.
The rates and limits are Purdue's for cool-season lawns: Kentucky bluegrass, ryegrass and the fescues. For warm-season grasses such as bermuda, zoysia or St. Augustine the arithmetic is the same with the nitrogen rate you choose. Phosphorus and potassium are set by a soil test, not by this count.
By Javier Miralles · Last updated . How this site is made.
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