UPS flat-rate boxes range in dimensions from 18 inches by 13 inches by 3 inches for the largest box to 13 inches by 11 inches by 2 inches for the smallest box. The medium-sized box is 16 inches by 11 inches by 3 inches.
The weight limit is 30 pounds for the large box. The dimensions on the boxes are exterior dimensions and are rounded to the nearest inch. Small and medium boxes are accepted at UPS drop box locations while the large box must be taken to a UPS staffed location. The cost of UPS flat-rate boxes is free.