There are several different standards for measuring the size of shoe and conversion from one type to another is often quite complicated.
The most important standards are English, US/Canadian and European. Actually, in US two different standards are used, differing by about 1/2 size, or 1/6 inch. Additionally, these standars have different methods with dealing with children/adults and men/women. Luckily, most shoe stores have conversion tables that you can borrow.
In the U.K., U.S. and Australia one shoe size equates to 1/4 inch. As the starting points differ, you will have to add a fixed amount of inches depening on what type of shoes you are dealing with. For women's shoes this is 8 3/8 inch in U.K., 8 1/8 inch in Australia and 7 3/4 inch in U.S. For men's shoes it is 8 1/4 inch in U.K. and Australia and 8 1/8 inch in U.S.
In Europe, 1 shoe size is equal to 1/5 inch, with always 2 extra inches added, independent of wether they are for children or women.
Shoe fabricants have somehow decided upon that women should not have feet larger that 10 1/2 inches while men should not exceed 11 1/2. If your feet exceed this, you might have to go to specialist stores, but conditions have greatly improved in the last decades.