There are several shoe width charts online. Many of them include separate charts for men's and women's sizes and include both North American and European sizes for easy reference.
You can measure the length and width of your foot by tracing your foot on a piece of paper and measuring the longest and widest points of your foot. Subtract 1/5 of an inch, or 5 mm, to account for the space between your foot and the pencil. Refer to the charts in any of the links provided to find your shoe size. For U.K. sizes, subtract 2.5 from the corresponding U.S. size. For Latin American sizes, subtract 1 from the corresponding European size.