Use a shoe size conversion chart to convert U.S. sizes to Mexican. These charts can be found from retailers and converting services online, which generally list the chart on the product page. Because shoes made by different manufacturers may not fit the same, it is recommended to refer to the size chart specific to the retailer before converting.
The online chart available on Convert Your Shoe Size, for example, states that a U.S. men's size 9 is equivalent to a size 7.5 in Mexico. By selecting features such as Man or Woman, and adding in a shoe size from the U.S., U.K., Canada or other available country, the shoe size is converted to show the equivalent size in Mexico.