Your lawn
N-P-K on the bag (%)
Fertilizer to spread
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- Rate of product–
- Bags to buy–
- Phosphate (P₂O₅) applied–
- Potash (K₂O) applied–
Estimate only. Set your spreader from the product label; spreader settings differ by brand and model.
Follow the product label and local rules. Some states and counties restrict phosphorus on lawns or ban fertilizing in summer or before heavy rain (for example, Florida county blackout periods). Keep fertilizer off driveways and sidewalks and away from water.
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How the fertilizer calculator works
Lawn rates are set in pounds of nitrogen per 1,000 sq ft. The first number on the bag is the percent of nitrogen, so pounds of product per 1,000 sq ft = pounds of nitrogen ÷ (N% ÷ 100). Multiply by your lawn area in thousands of square feet for the total.
Example from Purdue University Turf: 0.75 lb of nitrogen with a 16-8-8 product is 0.75 ÷ 0.16 = 4.69 lb per 1,000 sq ft, or about 37 lb for an 8,000 sq ft lawn. Penn State Extension works the same way: 1 lb of nitrogen from 26-5-10 is about 3.8 lb of product per 1,000 sq ft.
The phosphate and potash lines show what that same amount of product also puts down, using the second and third numbers.
How much nitrogen should I put on my lawn?
Many university extension guides suggest no more than about 1 lb of nitrogen per 1,000 sq ft in one application, and less (around 0.5 lb) for quick-release products or in hot weather. How many times a year depends on your grass type and region; your state's extension service publishes a calendar.
What do the three numbers on a fertilizer bag mean?
They are the percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 50 lb bag of 24-0-6 holds 12 lb of nitrogen, no phosphate and 3 lb of potash.
How do I work out how much fertilizer to put on my lawn?
Divide the pounds of nitrogen you want per 1,000 sq ft by the first number as a decimal. For 0.75 lb of nitrogen with a 16-8-8 product: 0.75 ÷ 0.16 = 4.69 lb of product per 1,000 sq ft.