USPS Small Flat Rate Box
Dimension size:Length x Width x Height
Inches: 8.625 x 5.375 x 1.625
Millimeters: 219.075 x 136.525 x 41.275
Centimeters: 21.9075 x 13.6525 x 4.1275
USPS Medium Flat Rate Box
Dimension size:Length x Width x Height
Inches: 11 x 8.5 x 5.5
Millimeters: 279.4 x 215.9 x 139.7
Centimeters: 27.94 x 21.59 x 13.97
USPS Large Flat Rate Box
Dimension size:Length x Width x Height
Inches: 12 x 12 x 5.5
Millimeters: 304.8 x 304.8 x 139.7
Centimeters: 30.48 x 30.48 x 13.97