A USPS First-Class Mail letter can be no more than one-quarter inch thick. Measure the finished mailpiece with its contents inside. An envelope that exceeds that thickness does not qualify as letter-size mail simply because it looks like an ordinary envelope or carries extra stamps. [1]
Thickness is only one letter requirement
USPS’s retail standards set a maximum letter length of 11½ inches and maximum height of 6⅛ inches. The usual minimum dimensions are 5 inches long, 3½ inches high, and 0.007 inch thick, with additional thickness requirements for larger letter dimensions. First-Class letter-size pieces generally have a 3.5-ounce weight limit before flat-size pricing applies. [1]
Measure after sealing the envelope. Include anything creating a raised area, such as a folded stack, clasp, or insert. A package can satisfy the quarter-inch limit and still fail a different requirement. Weight, shape, flexibility, and preparation should be checked separately.
Nonmachinable does not mean unlimited thickness
USPS identifies several nonmachinable letter characteristics, including certain aspect ratios, clasps, excessive rigidity, and uneven thickness caused by enclosed objects. Paying a nonmachinable surcharge does not change the maximum dimensions for letter-size mail. [1]
For example, a stiff invitation within the letter dimensions presents a different classification question from an envelope that is half an inch thick. Both need review, but the same solution should not be assumed. Avoid testing eligibility by forcing the contents through a narrow opening or compressing a bulge temporarily.
When an envelope may be a flat
Flat-size mail generally exceeds at least one letter maximum while staying within the flat maximums: 15 inches long, 12 inches high, and three-quarters inch thick. It must also meet flexibility and uniform-thickness standards. An envelope containing a box does not become a flat merely because its outside dimensions fit. [1][2]
Check the completed piece before buying postage
- Pack and seal it exactly as it will be mailed.
- Measure length, height, and the thickest area.
- Weigh it and check whether inserts shift or create a rigid lump.
- Ask USPS to classify an uncertain piece before mailing it.
Choose any evidence-of-mailing service separately from the size classification. A Certificate of Mailing documents mailing under its own rules, while Certified Mail signature procedures answer a different delivery question. Neither substitutes for correct packaging and postage.
Primary sources
Sources checked: October 31, 2026.