How many ounces can a letter weigh before you need to add another stamp?

Standard-sized letters up to 1 ounce can be mailed with a single first-class stamp. Above that, additional postage is required. Starting in January 2014, the first ounce of mail requires 49 cents postage and additional ounces are 21 cents.

Unusually shaped envelopes, those that do not bend easily, have clasps instead of adhesive or are lumpy, are charged a nonmachinable surcharge of 21 cents, according to the postage calculator on the USPS website. Oversized envelopes, more than 11.5 inches x 6.125 inches or more than 0.25 inches thick, are also subject to higher postal rates regardless of the weight. The 2014 rate for large envelopes is 98 cents.