An 8x10 rug will fit comfortably under a king size bed. In order to achieve this, you will need at least 24 inches of clearance from the edge of the rug to the side of the bed frame. It is also important to consider the orientation of your furniture when deciding on Rug placement. For example, if your headboard is against a wall, you may want to choose a runner or smaller area rug so that it does not impede traffic flow in your bedroom.