The cannon bone is the most mature bone in terms of length at birth in the horse. At six months of age, the horse is 93 percent of its mature length from the knee to the ground, and this distance represents approximately 25 percent of the horse’s mature size. So you can calculate the horse’s mature size by measuring the length of leg from the knee to the ground divided by 93 x 100 x 4 = mature size. Additionally, the horse is 61, 84, 92 and 95 percent of its mature size at birth, 6, 12 and 18 months of age, respectively. There is a great deal of variation in horses and their growth rate and proportions, so these estimates are not exact predictors of mature height.