King size bed frame dimensions – Although you can feel the sinking in a mattress king at the top, the sinking is actually a fault that runs the full thickness of the mattress. Modern mattresses are not compressed a fact that would only affect the upper half of the mattress. Rather, your king mattress loses tensile strength in tense areas, and sinks from the bottom to the top. Replacing the mattress can cost hundreds of dollars. Fortunately, propping up support from below is a relatively cheap and easy project.
Remove the mattress from the bed frame. Measure the space between the support rails. This should be very close to the standard size for a king size bed: 60 inches by 84 inches. Although you will feel the sagging of the top of the mattress, reinforcement at the bottom will provide support you will feel all the way through. Cut the sheets of plywood to match the dimensions. Unless you have unusual access to plywood, two sheets are needed to get the job done. Cut them to match the width of the mattress and half the length.
Sand one side of each plywood cutting board. You do not have to do it perfectly smooth, just soft enough that it will not snag the bottom of your mattress. Adjust the boards instead of the rails of your bed frame, and set your mattress in place on top. Fallen mattresses, unless the inner core itself is broken, are tilted all the way to the bottom. A rigid board below helps reduce the degree to which it can be compromised.