The exact diameter needed for your document varies based upon the thickness of the inner sheets of paper as well as the covers. Below is a chart that provides recommendations based on both the number of pages (using regular 20# bond) and the thickness of the stack being bound.
| Pages | Thickness | Comb Size |
|---|---|---|
| 12 | 1/8" | 3/16" |
| 25 | 3/16" | 1/4" |
| 40 | 1/4" | 5/16" |
| 55 | 5/16" | 3/8" |
| 70 | 3/8" | 7/16" |
| 80 | 7/16" | 1/2" |
| 105 | 1/2" | 9/16" |
| 125 | 9/16" | 5/8" |
| 150 | 5/8" | 3/4" |
| 175 | 3/4" | 7/8" |
| 200 | 7/8" | 1" |
| 225 | 1" | 1-1/8" |
| 265 | 1-1/8" | 1-1/4" |
| 320 | 1-1/4" | 1-1/2" |
| 375 | 1-1/2" | 1-3/4" |
| 425 | 1-3/4" | 2" |