How to use the lawn fertilizer calculator
- Enter Lawn size (sq ft), the grass you will actually fertilize. Multiply length by width for each section and subtract the house, beds and driveway.
- Type Nitrogen % on the bag (first N-P-K number). On a 20-5-10 bag, that is 20.
- Set Target nitrogen (lb per 1,000 sq ft). One pound is the usual amount for a single application. Use less if your bag label says so.
- Enter the Bag size (lb) and, if you want the cost, the Price per bag ($, optional).
- Read how many bags to buy, how many pounds to spread and how much is left over.
How the amount is worked out
Penn State Extension gives the method in Calculations Used to Determine the Amount of Fertilizer Needed to Treat Turf: divide the pounds of nitrogen you want per 1,000 sq ft by the percent nitrogen on the bag (as a decimal), then scale to your lawn. This tool multiplies the lawn size in thousands of square feet by your target to get pounds of actual nitrogen, divides by the nitrogen fraction to get pounds of fertilizer, and divides by the bag size to count bags, rounded up. Kansas State University calls 1 pound of actual nitrogen per 1,000 square feet the most common application. Single-application limits vary by state. Florida's urban turf fertilizer rule (UF/IFAS EP353) caps quick-release nitrogen at 0.7 lb per 1,000 sq ft and allows up to 2 lb in spring or summer only for fertilizer that is at least 65% controlled-release.
Worked example with the default inputs
A 5,000 sq ft lawn, a 20% nitrogen bag, a target of 1 lb per 1,000 sq ft and a 40 lb bag. Nitrogen needed is 5 x 1 = 5.00 lb. Fertilizer to spread is 5 / 0.20 = 25.0 lb, which is 5.00 lb per 1,000 sq ft. That is 0.63 of a bag, so the tool says Buy 1 bag (40 lb each), with 15.0 lb left over. One bag covers 40 x 0.20 / 1 x 1,000 = 8,000 sq ft.
Limits and edge cases
- Only nitrogen sets the amount here. Phosphorus and potassium should follow a soil test from your local extension office.
- Targets above 1 lb of nitrogen per 1,000 sq ft show a warning, and the field stops at 2.
- Weed-and-feed products have their own label rates, so use this tool only to check the bag count.
- The tool does not set your spreader. Use the setting printed on the bag, and split the dose over two passes in different directions to avoid streaks.
- Not sure of your lawn size? Use the rectangle area calculator or the area converter. Planting beds around the lawn? Try the plant spacing calculator.
Frequently asked questions
How much fertilizer do I need for 5,000 square feet?
With a 20% nitrogen fertilizer at 1 lb of nitrogen per 1,000 sq ft, you need 25 lb of fertilizer. That fits in one 40 lb bag with 15 lb left over.
How many pounds of nitrogen per 1,000 square feet should I apply?
One pound per application is the most common amount. Rules vary by state: Florida limits quick-release nitrogen to 0.7 lb per 1,000 sq ft at a time and allows up to 2 lb only for fertilizer that is at least 65% controlled-release. Always follow your bag label.
How do I figure pounds of fertilizer from the N-P-K numbers?
Divide the nitrogen you want by the first number as a decimal. For a 16-4-8 bag at 1 lb of nitrogen, 1 / 0.16 = 6.25 lb of fertilizer per 1,000 sq ft.
How many square feet does a 40 lb bag cover?
It depends on the nitrogen percent. At 1 lb of nitrogen per 1,000 sq ft, a 40 lb bag of 20% nitrogen covers 8,000 sq ft and a 10% bag covers 4,000 sq ft.
Sources: Penn State Extension, Landschoot (2025); Kansas State University, Lawn Fertilizing Guide MF2916; UF/IFAS Extension, Urban Turf Fertilizer Rule for Home Lawn Fertilization (EP353).