For European sizes, you can check a tag on a pair of your own shoes at home.  Here is a conversion chart to help.

Women's Shoe to Boot Size Chart
European USA Size
35 4.5
36 5-5.5
37 6
38 6.5-7
39 7-7.5
40 8
41 8.5
42 9-9.5
43 10

Men's Shoe to Boot Size Chart
European USA Size
40 7-7.5
41 8
42 8.5-9
43 9.5
44 10
45 10.5
46 11-11.5
47 12
48 12.5-13
49 14

 

Youth Shoe to Boot Size Chart
European USA Size
28 10
29 11
30 12
31 13
32 13.5
33 1.5
34 2
35 3
36 3.5
37 4.5