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 |