Physical Standards

Mail Piece Dimensions

Mailing costs and classifications depend on size and adherence to USPS dimensional standards:

Postcards: Must be 3½" x 5" to 4¼" x 6" and at least 0.007" thick for postcard rates. Larger or thicker cards incur letter-rate postage. Nonprofit bulk rates apply for cards up to 6⅛" x 11½".

Letters: Dimensions range from 5" x 3½" to 11½" x 6⅛" and up to ¼" thick. Over 3.5 ounces or outside these dimensions, mail is charged flat rates. Folding documents to letter size can reduce postage costs.

Flats: Exceed 11½" x 6⅛" or ¼" thick but not more than 15" x 12" or ¾" thick. Must be flexible, rectangular, and uniformly thick.

Design Tips:

  • Postcards require a blank 4½" x ½" area for barcoding.
  • Letter-size mail must be horizontally rectangular to avoid surcharges for nonmachinable pieces.
  • Use 60# paper or heavier for mailable folded brochures.

Proper folding and design reduce costs, ensure machinability, and prevent delays.

Standard Flat-mail size diagram