What size postcard is cheapest to mail through USPS?

The cheapest postcard to mail through USPS is one that fits the First-Class Mail postcard rate, which is the lowest postage tier available. To qualify, the card has to stay inside a specific size and thickness window.

The card must also be rectangular, with an aspect ratio between 1.3 and 2.5, since odd shapes lose the postcard rate. Measure the trimmed, finished size, not your working file with bleed included.

A 4" x 6" postcard is the go-to because it sits comfortably inside those limits and qualifies for the postcard rate, around $0.56 per piece versus $0.73 at the letter rate. It also carries the lowest per-unit printing cost of the common postcard sizes, which is why it is the default for high-volume sends like appointment reminders, event promos, and loyalty offers where reach matters more than size. Go larger than 4.25" x 6" and the piece jumps to letter postage, so staying in range saves money on every single mailing. If you need more room, plan for letter-rate postage rather than shrinking your design.