According to the Better Sleep Council, twin size mattress are approximately 38 inches wide by 75 inches long. The Bedding Experts, however, sells twin mattresses which measure 39 inches wide by 75 inches long. An exact measurement does not necessarily hold true across manufacturers.
With no industry standard, twin mattresses may come in a variety of lengths and width. The Bedding Experts' Twin-XL size measures five inches longer than their standard twin, but other retailers market similar-sized items as a standard twins. In order to know the exact measurements of any mattress, the exact model and brand need to be known, or it must be measured in person.