To determine the dimensions of a bunk bed mattress, measure the dimensions of the bed area where the mattress is placed in the bunk bed. Most bunk beds require standard-sized twin mattresses.
A twin mattress that is 39 inches in width and 75 inches in length fits snugly inside the frame of a typical bunk bed that features same-sized beds. Measure the length of the bed area of the bunk bed to determine if it requires a twin XL mattress that is 80 inches in length.
Sometimes a bunk bed features a full bed or a futon at the bottom. In such cases, buy one full-sized mattress or opt for a futon mattress for the bottom bunk area.