To determine whether or not a twin mattress fits through a door, compare the measurements of the doorway plus any immediate clearance necessary with the length and width of the mattress. A typical twin size mattress measures 39 by 74 inches with an average thickness 10 inches.
The average interior doorway of a house is 32 inches, which is too small for a mattress to fit through a doorway long-ways. If turned on its side, the mattress should only require 10 to 15 inches of space to fit through the doorway. If there are no immediate blockages when the door is opened and the house provides enough clearance to its destination, a typical twin size mattress should have no issues going through a door. Most mattresses can bend slightly which makes squeezing through tighter doorways and hallways possible, as well.