Flexibility Test for Flats (Large Envelopes)
SOURCE: USPS Domestic Mail Manual, Section 101: Physical Standards for Letters, Flats, and Parcels
NOTE: Flats are also called ‘Large Envelopes.’ Parcels are also called ‘Packages.’
Dimensions and Weight
Letters and Flats are distinguished from Packages by their combined flexibility and uniform thickness. Letters and flats are distinguished from one another by their dimensions:
• Letters can be a maximum of 11-1/2″ (292mm) long x 6-1/8″ (155mm) high, with a uniform thickness of up to 1/4″ (6mm).
• Flats (Large Envelopes) are more than 11-1/2″ (292mm) long—OR— more than 6-1/8″ (155mm) high. They can be a maximum of 15″ (381mm) long and 12″ (305mm) high, with a uniform thickness of up to 3/4″ (19mm).
• The maximum weight for Letters is 3.5 oz (100g); the maximum weight for Flats is 13 oz (368g).
In addition to boxes, rigid envelopes are considered packages; flexible, lumpy envelopes are too, in some cases.
NOTE: Flat, flexible envelopes marked DO NOT BEND may also be considered Packages.