The #10 envelope, which measures 4 1/8 by 9 1/2 inches and weighs 0.0208 ounces, is the most commonly used, or “standard” envelope.
The face of a normal envelope is closed, and there is no address window. In both corporate and personal mails, this is the most often used envelope. The amount of postage required can be influenced by the weight of the contents and envelope. Mail in a standard-size envelope weighing less than 13 ounces can be dropped off at a USPS drop box with the appropriate postage; however, mail weighing more than that must be shipped from an authorised shipper.