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 | Coil Size |
|---|---|---|
| 30 | 3/16" | 6mm (1/4") |
| 42 | 1/4" | 7mm (9/32") |
| 54 | 9/32" | 8mm (5/16") |
| 65 | 5/16" | 9mm (11/32") |
| 75 | 11/32" | 10mm (3/8") |
| 85 | 3/8" | 11mm (7/16") |
| 92 | 7/16" | 12mm (15/32") |
| 105 | 15/32" | 13mm (1/2") |
| 115 | 1/2" | 14mm (9/16") |
| 125 | 9/16" | 15mm (19/32") |
| 135 | 19/32" | 16mm (5/8") |
| 142 | 5/8" | 17mm (21/32") |
| 155 | 21/32" | 18mm (11/16") |
| 160 | 11/16" | 19mm (23/32") |
| 172 | 23/32" | 20mm (3/4") |
| 190 | 3/4" | 22mm (7/8") |
| 200 | 7/8" | 23mm (15/16") |
| 220 | 15/16" | 25mm (1") |
| 250 | 1" | 28mm (1-1/8") |
| 270 | 1-1/8" | 30mm (1-3/16") |
| 280 | 1-3/16" | 32mm (1-1/4") |
| 290 | 1-1/4" | 33mm (1-5/16") |
| 310 | 1-5/16" | 35mm (1-3/8") |
| 330 | 1-3/8" | 38mm (1-1/2") |
| 350 | 1-1/2" | 40mm (1-9/16") |
| 390 | 1-9/16" | 45mm (1-3/4") |
| 440 | 1-3/4" | 50mm (2") |