How much postage do you need to mail a 5.25 x 6.5 inch envelope in the US?

To mail a 5.25 x 6.5 inch envelope in the US, you typically need a minimum of one First-Class Forever stamp, which covers up to 1 ounce. If the envelope weighs more than 1 ounce, additional postage will be required. It's advisable to check with the USPS or use their online postage calculator for the most accurate postage based on weight and dimensions.