The rapid growth of the feet and the width of the foot should be taken into account. In the official ISO specification,
for example, toe clearance is given special consideration when calculating
children's shoe sizes. While shoe sizes for
adults, regardless of the actual foot length, always include two shoe sizes as space for toe clearance. For children,
the exact foot length is taken into account for the shoe size. According to ISO, 8 percent of the foot length is included
as a surcharge in the formula for the children's shoe size.