The amount of lawn fertilizer determines how well your grass thrives. Using too little or too much can lead to a sick looking lawn. Accurately calculating the amount of fertilizer needed will prevent this. Nitrogen fertilizers differ widely in their percentages of nitrogen from 1 percent to 46 percent. Use the type best for your area by talking to local experts for their recommendations for your lawn type and region.
Measure the length and width of your lawn in feet.
Multiply the length times the width to get the total square footage for your lawn. For instance, a 50 foot by 10 foot lawn has an area of 50 feet x 10 feet = 500 square feet.
Look at your purchased lawn fertilizer for the amount of nitrogen in it. Find the three numbers on the front of the bag indicating the percentages of nitrogen, phosphorus and potassium, respectively. Use the first number as the percentage of nitrogen.
Look up the percentage on a nitrogen percentage chart and see the corresponding amount of pounds needed to apply to each 1,000 square feet of your lawn. For instance, a fertilizer with a percentage of nitrogen of 35 percent requires 2.9 lbs per 1,000 square feet.
Divide the total square footage of your lawn by 1,000 square feet to get the relative size of your lawn. For the example, 500 square feet/1,000 square feet is ½.
Multiply the relative size of your lawn by the number of pounds per 1,000 square feet to determine the amount of lawn fertilizer to apply. For the example: 2.9 lb. per 1,000 square feet x ½ = 1.45 pounds of lawn fertilizer for your lawn.