Get at least one person to help you. Because of the weight, size and structure of a king-size mattress, you should not try to move it alone.
Grip the king-size mattress on each end, with you on one side and someone else on the other.
Turn the king-size mattress onto its side to move it through doorways. The mattress should fit through a standard interior or exterior doorway width-wise. Most doorways are about 80 inches tall, and width-wise, a king mattress is 76 inches.
Support the king mattress underneath as you move it. Avoid bending it to fit through spaces. This will damage some mattresses.
Transport the mattress by lying it flat when possible.