FOC = (CG – GC) / GC * 100
FOC = Front of Center, or the additional length of the area that is towards the front of the arrow’s GC.
GC = Geographic Center of the arrow. This is the physical halfway point from the point of your broadhead or field point to the tail of the nock.
CG = Center of Gravity of the arrow. This is the point at which the arrow balances. Heavier points will bring this towards the front of an arrow.
For example, we have an arrow that is 32.375 inches in total length. Our GC will be 16.19. When we balance the arrow with its 125gn broadhead and lighted nock, it balances at 18.38 inches instead of at the GC value of 16.19. If we plug these into our formula we get:
FOC = (18.38 – 16.19) / 16.19 * 100
FOC = 2.19 / 16.16 * 100
FOC = .14 * 100
FOC = 14%