How this calculator works
The first number on a fertilizer bag is the percentage of nitrogen by weight. A 28-0-3 bag is 28% nitrogen, so each pound of product has 0.28 lb of N. Lawn recommendations are given as pounds of nitrogen per 1,000 square feet, so we work out the nitrogen you need and divide by the percentage.
Worked example: a 5,000 ft² lawn at 1 lb of N per 1,000 ft² needs 5 lb of nitrogen. With a 28% N fertilizer that is 5 ÷ 0.28 = 17.9 lb of product, which is two 15 lb bags.
Tips before you buy
- Calibrate your spreader. The settings on a bag are for that brand, and a poorly adjusted spreader leads to stripes and burns.
- Water the fertilizer in lightly after applying, and never apply before heavy rain, which washes it off.
- Match the timing to the grass: cool-season grasses (fescue, bluegrass) are fed in fall and spring, and warm-season grasses (bermuda, zoysia) in late spring and summer.
- A soil test tells you whether you need phosphorus or potassium at all. Many lawns only need nitrogen, and some states restrict phosphorus.
Tools & materials for this project
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Frequently asked questions
How much fertilizer do I need per 1,000 square feet?
To apply 1 lb of nitrogen, divide 1 by the nitrogen fraction on the bag. For a 28% N fertilizer that is 3.6 lb per 1,000 ft².
What do the three numbers on a fertilizer bag mean?
Nitrogen (N), phosphorus (P₂O₅) and potassium (K₂O), as percentages by weight. A 28-0-3 bag is 28% nitrogen, 0% phosphorus and 3% potassium.
How much nitrogen does a lawn need per year?
Roughly 2 to 4 lb of N per 1,000 ft², depending on the grass, climate and how green you want it, split over several applications.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.