Standard board strength comes in: C Flute (Industry Standard). About 3/16” Thick B Flute (Commonly used for medium sized mailer boxes). About ⅛” thick. E Flute (Thinner material commonly used for smaller boxes under 6”). About 3/32” Thick Standard double-wall and Heavy-Duty double-wall is approx. 5/16” thick or more. Based on the size and style that you choose, we make an industry standard default selection for you, in order for your box to close evenly.