Are the npk rated per bag wt or by 100lb?

NPK ratings on fertilizer bags are typically expressed as percentages by weight, indicating the relative amounts of nitrogen (N), phosphorus (P), and potassium (K) in the product. These percentages apply to the total weight of the bag, not specifically to a per 100-pound basis. For example, if a bag is labeled 10-20-10, it means that in a 50-pound bag, there are 5 pounds of nitrogen, 10 pounds of phosphorus, and 5 pounds of potassium.