As of 2014, an 8-by-5-inch envelope requires one first-class postage stamp if the envelope and its contents weigh less than 1 ounce and the letter is less than 1/4 inch thick. Letters weighing more than 1 ounce require extra postage.
Each additional ounce increases the amount of postage required, as does thickness greater than 1/4 inch. Eight by 5 inches is within the maximum dimensions for a U.S. postal letter. The maximum is 6 1/8 inches high, 11 1/2 inches long and 1/4 inch thick. Any envelope larger than this is classified as a “large envelope” or “flat.” These require two first-class postage stamps for the first ounce, with more postage for each additional ounce. Large envelopes thicker than 3/4 inch require package-rate postage.