What Are USPS Flat-Rate Box Sizes?
As of 2015, the U.S. Postal Service offers two sizes of flat-rate boxes for Express one-to-two-day delivery. It also offers five sizes of flat-rate boxes for Priority one-to-three-day delivery.
The Express service flat-rate boxes measure 13 5/8 inches by 11 7/8 inches by 3 3/8 inches and 11 inches by 8 1/2 inches by 5 1/2 inches. The cost for shipping these boxes, as of January, 2015, is $44.95.
The Priority service large flat-rate boxes measure 23 11/16 inches by 11 3/4 inches by 3 inches and 12 inches by 12 inches by 5 1/2 inches. As of 2015, the shipping cost is $17.90, unless shipping to an APO, FPO or DPO address, in which case the rate is $15.90. The medium flat-rate boxes for Priority service are 13 5/8 inches by 11 7/8 inches by 3 3/8 inches and 11 inches by 8 1/2 inches by 5 1/2 inches. Shipping charges are $12.65. There is one small-size box offered for Priority service. It measures 8 5/8 inches by 5 3/8 inches by 1 5/8 inches and costs $5.95 to ship.
Note that all dimensions are inside dimensions. Outside dimensions are slightly larger. Up to 70 pounds can be shipped in these boxes without additional charges applying. Flat-rate offers the same rate to all 50 states, as of 2015. All flat-rate packaging may be ordered through the U.S. Postal Service or can be picked up at a local post office. Several flat-rate envelopes are available as well.