Postal Standards - The Graphic Design: Reference and Specification - page 118

Package
A three-dimensional mail piece contained in a box, thick envelope, or tube.

Type Height Width Thickness Weight

Postcard Min 3 ½ inches

(89 mm); Max 4 ¼
inches (108 mm)

Min 5 inches
(127mm); Max 6
inches (152 mm)

Min .007 inches
(.178 mm); Max
.016 inches inch
(.406 mm)

n/a

Letter Min 3 ½ inches

(89 mm); Max 6
1/8 inches (156
mm)

Min 5 inches
(127mm); Max
11 ½ inches (292
mm)

Min .007 inches
(.178 mm); Max
¼ inch (6 mm)

Max 3.3 oz (94 g)

Large envelope
or flat

Min 6 1/8 inches
(155 mm); Max 12
inches (305 mm)

Min 11 ½ inches
(295 mm); Max
15 ¾ inches
(400mm)

Min ¼ inch (6
mm); Max ¾ inch
(19 mm)

Max 13 oz (369 g)
for first class; Max
16 oz (454 g) for
standard class