Mystic sells pre-cut mounts for every US postage stamp. That’s 407 different sizes of stamp mounts that make it extra easy for you to collect.
Stamp mounts are sold according to size in millimeters. The first number is the horizontal measurement, or width, across the mount. The second number is the vertical measurement, or height, of the mount. Example: A 45 x 30 mount measures 45 mm wide by 30 mm high.
Please Note: The mount size for self-adhesive stamps is generally 6-7mm larger in each direction because their backing makes them thicker.