Ugg Classic tall and short run a whole size bigger. You wear them without socks and they stretch.
If you are buying a Classic Tall or a Classic Short UGG boot from Deckers UGG Australis you must go down a full size. You wear them without socks and they stretch.
If you are buying a Sundance or Ultra then you only go down half a size as they tend to run true to size. However these also may stretch and I find that it is better to go down a full size in these also.
Each Ugg Boots is different. I once had 10 size 8 Ultra Talls and each one was different. Some were almost a size 7 and some were almost a size 9. So there is really no way to be 100% sure of the size. It is always safe to go down a size due to the fact that they stretch.
I am a size 8 1/2 and the Sundance Chesntut tall I had in a 8 was too big and rubbed. I got a size 7 and they fit perfectly.
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