I am usually a size 11.5 bought this boot in a size 12 (Clarks does not make 11.5's).  By no fault of Clarks or amazon, the 12 was massive on me so I returned it (without incident, thank you amazon.com).  I'm going to try a different style because the part around the ankle was WAY too loose on my scrawny ankles, which may be less of an issue with the proper size, but even if I placed my heel at the back where it belongs, the ankle width was crazy baggy.