How To Measure Your Belt Size.

Your belt size is different from your pants size and is usually larger by several inches. The BEST way to measure your belt size is by using an old belt. This is done by laying the old belt on a flat surface and measuring from the inside of the roller bar on the buckle to the center of the most comfortable fitting hole in the belt. The pictures below show how this looks when I measure my own belt size which is currently 43".
Measuring from inside of roller buckle.

Measure to the most comfortable belt hole.

If you don't have a belt to measure your belt size with there are some rough guides but you should keep in mind that these aren't accurate for all people. My suggestion would be to measure your belt size with an old belt or at a store with a belt off the rack. That said here is a table guide that may be used to approximate your belt size:

Pants Size Belt Size
25 30-31
26 31-32
27 32-33
28 33-34
29 34-35
30 35-36
31 36-37
32 37-38
33 38-39
34 39-40
35 40-41
36 41-42
37 42-43
38 43-44
39 44-45
40 45-46
41 46-47
42 47-48
43 48-49
44 49-50
45 50-51
46 51-52
47 52-53
48 53-54
49 54-55
50 55-56