Flexibility Test

Large Envelopes (Flats)

Minimum Flexibility
Large Envelopes (Flats) must be flexible.

Flexibility Test—All Large Envelopes (Flats)
Place the piece with the length parallel to the edge of a flat surface and extend the piece halfway off the surface.
Press down on the piece at a point 1 inch from the outer edge, in the center of the piece’s length, exerting steady pressure.
The piece is not flexible if it cannot bend at least 1 inch vertically without being damaged.
The piece is flexible if it can bend at least 1 inch vertically without being damaged and it does not contain a rigid insert.

Flexibility Test — Large Envelopes (Flats) 10 Inches or Longer
Place the piece with the length perpendicular to the edge of a flat surface and extend the piece 5 inches off the surface.
Press down on the piece at a point 1 inch from the outer edge, in the center of the piece’s width, exerting steady pressure.
Turn the piece around and repeat steps 1 and 2. The piece is flexible if both ends can bend at least 2 inches vertically without being damaged.

Flexibility Test — Large Envelopes (Flats) Less Than 10 Inches Long
Place the piece with the length perpendicular to the edge of a flat surface and extend the piece one-half of its length off the surface.
Press down on the piece at a point 1 inch from the outer edge, in the center of the piece’s width, exerting steady pressure.
Turn the piece around and repeat steps 1 and 2. The piece is flexible if both ends can bend at least 1 inch vertically without being damaged.

SOURCE: USPS Domestic Mail Manual, Section 101: Physical Standards for Letters, Flats, and Parcels