Shoe Size
In Korea, millimeters (MM) are used to measure shoe size. If you are lucky, the label of your shoes will list the shoe size in US, UK, EUR and JPN. Since Japan (JPN) size is in centimeter (CM) and is more common to find internationally, you can easily convert to Korean size with JPN size reference (by adding zero or multiplying by 10, approximately). 

In case you do not know your US, UK, JPN or Korean shoe size, you can measure your bare foot length from toe to heel and add approx.10mm to get your Korean shoe size.

​Otherwise, you may want to refer to the chart as your guideline. However, it is advised to always try on footwear if possible. 
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Picture
www.koreancravings.com
Picture
www.koreancravings.com

Clothing Size
Beside the general way of using Small, Medium and Large, the Korean clothing sizing sometimes can be confusing, especially for women clothing. You could find the S, M, L standard is quite different with US standard of S, M, L. 

Sometimes you also find 'free' size, usually local product or a not established brands. You may asked the salesperson to try or if it's online they will provide the accompanying chart explaining the exact measurement of the item. 

Here are the guidelines but again, it is advised to try the clothes on before purchasing. Most online shopping in Korea will accept refund too. 

Picture
suzychung (blog.korea.net)
Picture
suzychung (blog.korea.net)
Children clothing size