These are the U.S. Post Office requirements to keep in mind when designing an envelope:
- All mail pieces 1/4″ thick or less must be rectangular in shape, at least 3 1/4″ high and at least 5″ long.
- Any mail piece less than 4 1/4″ in height must be at least .007″ (7 pt.) thick.
- Any mail piece greater than 4 1/4″ in height or 6″ in length must be at least .009″ (9 pt.) thick.