Trundle bed mattresses are commonly 75 inches by 39 inches. They vary in thickness from 8 inches to just a few inches thick. Trundle beds are designed to be rolled into the space beneath a traditional bed.
Trundle bed frames, which contain a set of casters to make rolling easier, are generally slightly smaller than a regular bed to enable the trundle to fit beneath the other bed frame. People with back issues may have trouble using a trundle bed, since it rests close to the floor. Trundle beds are designed to save space when not needed, while making it easier to provide extra sleeping space.