General Mail Standards

All mail pieces (except keys and identification devices mailed under the applicable standards) that are 1/4 inch thick or less must be rectangular, at least 3-1/2 inches high and at least 5 inches long. All mailable matter must be at least 0.007 inch thick, weigh no more than 70 pounds and be no more than 108 inches in length and girth combined.

Letter-Size Mail Dimensions

• At least 5 inches long, 3-1/2 inches high and 0.007 inch thick.
• Not more than 11-1/2 inches long, 6-1/8 inches high or 0.25 inch thick.

Postcard Dimensions

• At least 3-1/2 inches high, 5 inches long and 0.007 inch thick.
• Not more than 4-1/4 inches high, 6 inches long or 0.16 inch thick.

Flat-Size Mail Dimensions

• More than 11-1/2 inches long, 6-1/8 inches high or 0.25 inch thick.
• May be wrapped, unwrapped, sleeved or enveloped.
• Must be rectangular, uniform in thickness and flexible.
• Not more than 15 inches long, 12 inches high or 0.75 inch thick.
  •  Standard Mail (A) Enhanced Carrier Route rate pieces may not be more than 14 inches long, 11-3/4 inches high or 0.75 inch thick.

*Must exceed at least one of these letter-size maximums.