What Envelope Size Needs Extra Postage?

By Staff WriterLast Updated Mar 29, 2020 12:20:49 PM ET

Any envelope larger than the maximum dimensions for a United States postal letter (6 1/8 inches high, 11 1/2 inches long and 1/4 inch thick) requires extra postage. An envelope that doesn't exceed these dimensions and weighs one ounce or less will require only a single First Class postage stamp.