Why are there so many different shoe size systems? Confused about shoe sizes? Well, You're not alone. Unfortunately there are a number of different systems in use. The first problem is that the various international shoe sizes are based on different measuring systems.
UK shoe sizes are based on an imperial system with units of one-third of an inch.
US shoe sizes for ladies are also based on an imperial system with units of one-third of an inch, however they use a different starting point to the British scale. EU shoe sizes have a French metric origin with units based on two-thirds of a centimeter. To further confuse the issue, a distinction in sizing is frequently made between Men's, Women's, Adult and Junior shoe sizes within the same scale, generally being a full size out across the range. For some illogical reason:

• a US Men's 5 is equivalent in size to a US Women's 6
• a US Men's 5 is approximately the same size as a US Junior 4
• a UK Adult 5 is approximately a UK Junior 4 (but different from the equivalent US size!)
• a EU Adult 35 is approximately a EU Junior 34

To add a little more confusion, there are two scales used in the USA, the "standard" (known as "FIA") and "common" scales (caused by an error of the US Army during WW1). Although the "standard" scale is more correct, the "common" scale is more widely used. These are (go figure!) approximately ½ size different.
Also there exisits specialised systems such as Mondopoint for ski boots, skates and the like.
Finally, many manufacturers have adopted their own sizing systems, that although they appear to conform with one system or the other, may be completely different.

2008 American Eagle $5 Gold Proof One-tenth with Box