Aspect Ratio For Letters and Cards
To avoid postal surcharges, the aspect ratio of your mail piece should conform to the following rule:
Length divided by height = 1.3 to 2.5
Example: a #10 envelope (9.5x4.125) is 9.5 divided by 4.125 = 2.30.
A square envelope would have an aspect ratio of 1.0 and would not be eligible for automation discounts.