How to Calculate the Amount of Nitrogen in a Fertilizer Bag Do not be misled by a large nitrogen number on the fertilizer bag (the first number in the N-P-K analysis). To find the amount of nitrogen in a bag of fertilizer, you must calculate the pounds of nitrogen per 1,000 sq. ft. To calculate the pounds of nitrogen in a bag of fertilizer, multiply the bag's weight by the percent nitrogen (this is the first number in the N-P-K designation on the front of the bag). This will tell you the pounds of nitrogen in the bag. Then divide the pounds of nitrogen by the area the bag states it will cover to get the pounds of nitrogen per 1,000 sq. ft. Note: Fertilizer bags usually come in 5,000 or 10,000 sq. ft. For example a 19 lb. bag of fertilizer with an analysis of 26-4-12 (N-P-K) covering 5,000 sq. ft.: 19 lbs. × (26 ⁄ 100) = 4.94 lbs Total nitrogen in the bag 4.94 lbs nitrogen in the bag ÷ 5,000 ft² bag = 0.98 lbs. of nitrogen ⁄ 1,000 ft² Learn more about university-recommended fertilization requirements.