What Is the Lawn Fertilizer Calculator?
This tool tells you exactly how many pounds of bagged fertilizer to spread on your lawn to hit a target nitrogen rate. Lawn-care guidance is almost always given as pounds of actual nitrogen per 1,000 square feet, but the bag at the store lists a nitrogen percentage. This calculator bridges the two so you do not over- or under-apply.
How to Use It
Enter your lawn area (square feet or square meters), the target nitrogen rate you want (a typical maintenance dose is 0.5 to 1 lb N per 1,000 sq ft), and the first number of the N-P-K analysis on the bag (the nitrogen percentage). The calculator returns the pounds of product to apply, plus the actual nitrogen that represents.
The Formula Explained
First, the actual nitrogen needed is the area scaled to thousands of square feet times the rate:
$$N_{lb} = \frac{A}{1000} \times R$$Here \(A\) = lawn area in square feet and \(R\) = target lb of nitrogen per 1,000 sq ft. Because fertilizer is only partly nitrogen, divide by the nitrogen fraction to get the bag weight:
$$\text{Product} = \frac{N_{lb}}{N/100}$$where \(N\) = nitrogen percent on the label.
Worked Example
Suppose you have a 5,000 sq ft lawn, want 1 lb N per 1,000 sq ft, and bought a 20-0-10 fertilizer (20% N):
$$N_{lb} = \frac{5000}{1000} \times 1 = 5\,\text{lb}$$ $$\text{Product} = \frac{5}{20/100} = 25\,\text{lb}$$You would spread 25 pounds of product to deliver 5 pounds of actual nitrogen.
FAQ
Which number on the bag is nitrogen? The first number of the three-part N-P-K analysis (e.g. the 20 in 20-0-10).
What rate should I use? Cool-season lawns often target 0.5-1 lb N per 1,000 sq ft per application; never exceed about 1 lb of quick-release nitrogen at once to avoid burning.
Does this work for slow-release fertilizers? Yes—the math is the same. Just enter the labeled nitrogen percentage.
