Under Armour UA Phenom Kids Cushioned Crew Socks 3 Pack Size Youth 13.5k - 4.5 Fits kids size 13.5k (see below) through 4.5 (see below) Size Youth 13.5k is a children's shoe size, where the "k" stands for "kids". In general, youth shoes range in size from 1k to 13.5k. Here is how the sizing typically works: 1k to 13.5k: These sizes are for smaller, younger children. The "13.5k" is the largest size in this range. A Youth 4.5 shoe size indicates the 4.5 size in the youth (or big kid) sizing system, corresponding to a foot length of approximately 9 to 9.25 inches (about 23-23.5 cm). This size is generally for children who have outgrown toddler sizes and are approaching adult shoe sizes, typically fitting those aged 8 and above.