There is no set amount of fps lost per grain of arrow weight; instead, the number of fps lost will depend on the bow’s draw weight, the arrow length, the arrow spine, the type of fletching, and other factors. It is important to note that the total amount of fps lost will also depend on the type of bow used. A bow with a higher draw weight will require more energy to draw back and will result in a more significant amount of fps lost when reducing the weight of the arrow. Additionally, arrows with a stiffer spine and larger fletching will require more energy to propel them forward, resulting in more fps lost when reducing the arrow weight. Therefore, it is essential to consider all these factors when determining how much fps is lost when reducing the arrow weight.