The formula
A bag labeled 26-5-10 contains 26% nitrogen, 5% phosphate and 10% potash by weight. To deliver a given amount of nitrogen, divide it by the nitrogen share written as a decimal:
Penn State Extension's example: (1.0 lb nitrogen per 1,000 sq ft) ÷ 0.26 = 3.8 lb of 26-5-10 per 1,000 sq ft. The same 3.8 lb also carries 0.19 lb of phosphate and 0.38 lb of potash. For the whole lawn, multiply by the area in thousands of square feet:
Illinois Extension's example uses a 15-0-10 product on 7,000 sq ft at 1 lb of nitrogen per 1,000 sq ft: 6.67 × 7 = 46.7 lb, which the extension rounds to about 47 pounds.
How much nitrogen per application
Illinois Extension suggests 1 pound of nitrogen per 1,000 square feet for each application. Both Illinois and Purdue say not to exceed 1 pound of nitrogen per 1,000 square feet at one time when the nitrogen is in a quick-release form. Purdue notes there can be exceptions for fertilizers that release nitrogen more gradually, such as organic and coated products, and Illinois recommends controlled-release sources for most lawn applications. Penn State's acre example uses 0.75 lb, and the tables below include 0.5 and 0.75 lb for lighter feedings.
Pounds of product per 1,000 sq ft
Find the first number on your bag in the left column:
| N on bag | 0.5 lb N | 0.75 lb N | 1 lb N |
|---|---|---|---|
| 10% | 5.0 lb | 7.5 lb | 10.0 lb |
| 15% | 3.3 lb | 5.0 lb | 6.7 lb |
| 16% | 3.1 lb | 4.7 lb | 6.3 lb |
| 20% | 2.5 lb | 3.8 lb | 5.0 lb |
| 24% | 2.1 lb | 3.1 lb | 4.2 lb |
| 26% | 1.9 lb | 2.9 lb | 3.8 lb |
| 28% | 1.8 lb | 2.7 lb | 3.6 lb |
| 30% | 1.7 lb | 2.5 lb | 3.3 lb |
| 32% | 1.6 lb | 2.3 lb | 3.1 lb |
The lower the nitrogen percentage, the more product you spread for the same feeding. A 10% product needs 3.0 times as much weight as a 30% product.
For the whole lawn
First measure the lawn. The square footage calculator adds up rectangles and other shapes, and the area calculator handles circles, triangles and other shapes such as a round bed you need to subtract. Then, with a 24% nitrogen product:
| Lawn | 0.5 lb N | 0.75 lb N | 1 lb N |
|---|---|---|---|
| 2,500 sq ft | 5.2 lb | 7.8 lb | 10.4 lb |
| 5,000 sq ft | 10.4 lb | 15.6 lb | 20.8 lb |
| 7,000 sq ft | 14.6 lb | 21.9 lb | 29.2 lb |
| 10,000 sq ft | 20.8 lb | 31.3 lb | 41.7 lb |
Subtract the house, driveway, patio and beds before you multiply. Spreading the full figure over a smaller real lawn raises the rate on every square foot.
How far a bag goes
A bag holds its weight times the nitrogen percentage in actual nitrogen. Purdue's example: a 40-pound bag of 28-5-10 has 0.28 × 40 = 11.2 pounds of nitrogen. At 1 lb per 1,000 sq ft, that covers 11,200 sq ft. Coverage at three rates:
| Bag | N in bag | Covers at 0.5 lb | Covers at 0.75 lb | Covers at 1 lb |
|---|---|---|---|---|
| 15 lb, 24% N | 3.6 lb | 7,200 sq ft | 4,800 sq ft | 3,600 sq ft |
| 40 lb, 28% N | 11.2 lb | 22,400 sq ft | 14,933 sq ft | 11,200 sq ft |
| 50 lb, 26% N | 13.0 lb | 26,000 sq ft | 17,333 sq ft | 13,000 sq ft |
| 50 lb, 15% N | 7.5 lb | 15,000 sq ft | 10,000 sq ft | 7,500 sq ft |
Penn State works the 50-pound bag of 26-5-10 through the rounded rate of 3.8 lb per 1,000 sq ft and gets 13,200 sq ft; the unrounded figure is 13,000 sq ft. For a 30,000 sq ft area it divides the area by the coverage: 2.3 bags.
One lawn, start to finish
An example: a lot with 7,400 sq ft of open ground around the house, minus a 300 sq ft patio and a 900 sq ft bed, leaves 6,200 sq ft of grass. The bag in the shed is 30 lb of a 20-0-5 product, and the plan is 0.75 lb of nitrogen per 1,000 sq ft.
- Product per 1,000 sq ft: 0.75 ÷ 0.20 = 3.75 lb.
- For the lawn: 3.75 × 6.2 = 23.25 lb.
- Against the bag: 23.25 lb out of 30 lb is 78% of the bag, leaving 6.75 lb for the next feeding.
- Check the nitrogen: 23.25 × 0.20 = 4.65 lb of nitrogen on 6,200 sq ft, which is 0.75 lb per 1,000 sq ft.
How much per year
The yearly total depends on the grass, the sun and how much care the lawn gets:
- Illinois Extension: about 3 pounds of nitrogen per 1,000 sq ft per growing season for most full-sun lawns, split into two or three applications, and about half that for shade lawns. The number of applications follows lawn density, from one for a thin lawn to four for a dense one, where the early-summer feeding is only for irrigated lawns.
- Purdue Extension: 1 to 3 pounds a year for low-maintenance yards and 3.5 to 4 pounds for well-maintained lawns. Cool-season grasses grow in spring and fall, which makes those the best times to fertilize.
At 3 lb a season split into 3 feedings, each application is 1.0 lb of nitrogen per 1,000 sq ft. For a 5,000 sq ft lawn and a 24% product, that is 20.8 lb per feeding and 62.5 lb for the season.
Timing depends on where you live. For central Illinois, the extension gives early to mid-May in spring, August 15 to September 15 in late summer, and November for a late-season feeding about a week after the last mowing. The same page gives separate windows for northern and southern Illinois; your own state's extension service has the dates for your area.
Phosphorus, potassium and clippings
The second and third numbers matter less for an established lawn. Purdue says most established lawns have adequate phosphorus and that a soil test is the only reliable way to know phosphorus and potassium levels; phosphorus mainly helps when seeding. Penn State suggests soil tests every two or three years and notes that a soil test does not give reliable information about nitrogen needs, which is why nitrogen is set by the rates above. Purdue also points out that bagging clippings removes nitrogen the grass took from the soil, a loss that is smaller when clippings are mulched back into the lawn.
Watering is the other half of lawn care; see how much to water your lawn.
Frequently asked questions
How much nitrogen should I put on my lawn per application?
Illinois Extension suggests 1 pound of actual nitrogen per 1,000 square feet for each application. Illinois and Purdue Extension both say not to apply more than 1 pound of quick-release nitrogen per 1,000 square feet at one time; Purdue notes exceptions for products that release nitrogen more gradually.
How do I convert pounds of nitrogen to pounds of fertilizer?
Divide the pounds of nitrogen you want by the nitrogen percentage on the bag, written as a decimal. For a 26-5-10 product, Penn State Extension shows 1.0 ÷ 0.26 = 3.8 pounds of product per 1,000 square feet. Multiply by your lawn area in thousands of square feet for the total.
How much nitrogen does a lawn need per year?
Illinois Extension suggests about 3 pounds of nitrogen per 1,000 square feet per growing season for most full-sun lawns, split into two or three applications, and about half that in shade. Purdue gives 1 to 3 pounds a year for low-maintenance lawns and 3.5 to 4 pounds for well-maintained lawns.
What do the three numbers on a fertilizer bag mean?
They are the percentages of nitrogen, phosphate and potash, in that order. Penn State Extension's examples read a 26-5-10 bag as 26 percent nitrogen, 5 percent phosphate and 10 percent potash, so a 50-pound bag holds 13 pounds of nitrogen.
Does my lawn need phosphorus?
Often not. Purdue says most established lawns have adequate phosphorus and that a soil test is the only reliable way to know phosphorus and potassium levels, while phosphorus helps when seeding. Illinois Extension notes that high-phosphorus starter fertilizers are meant for newly seeded lawns and fresh sod.
Sources
- Penn State Extension, Calculations used to determine the amount of fertilizer needed to treat turf (1.0 ÷ 0.26 = 3.8 lb of 26-5-10; phosphate and potash; a 50-lb bag covers 13,200 sq ft; bags for an area; pounds per acre)
- University of Illinois Extension, Fertilizing your lawn (1 lb N per 1,000 sq ft per application; about 3 lb per season in sun, half in shade; timing windows; 47 lb of 15-0-10 for 7,000 sq ft; quick-release limit; soil test for phosphorus)
- Purdue Extension, A simple procedure for fertilizing cool-season lawns (no more than 1 lb N per 1,000 sq ft per application; 1 to 3 and 3.5 to 4 lb a year; 40-lb bag of 28-5-10 holds 11.2 lb N; phosphorus and soil tests; clippings)
- Penn State Extension, Turfgrass fertilization: a basic guide for professional turfgrass managers (soil tests every two or three years; not reliable for nitrogen; multiple applications over the season)