Business envelope sizes are referenced by a number such as #9 or #10. The chart below indicates the most common sizes in use today:
| Size | Width x Length |
| #6 1/4 | 3 1/2″ x 6″ |
| #6 3/4 | 3 5/8″ x 6 1/2″ |
| #7 | 3 3/4″ x 6 3/4″ |
| #7 3/4 | 3 7/8″ x 7 1/2″ |
| #8 5/8 | 3 5/8″ x 8 5/8″ |
| #9 | 3 7/8″ x 8 7/8″ |
| #10 | 4 1/8″ x 9 1/2″ |
| #11 | 4 1/2″ x 10 3/8″ |
| #12 | 4 3/4″ x 11″ |
| #14 | 5″ x 11 1/2″ |