Mid century king bed is the largest bed made in standard mattress sizes, except the king of California, which is 4 inches less in width, but 4 inches longer. The standard size of a king size bed is 76 inches wide by 80 inches long. Although there is standardization now in mattresses and box springs, you need to measure your bed frame to get an exact fit for a new mattress.
Measure the width and length of the area provided by the mattress. Work from the inside edge of the frame and keep in mind that the mattress can slide under the shelf of the mid century king bed headboard in frames with shelves or bookcases. Look for the required mattress depth by keeping the tape measure on top of the slats where the box spring would sit and measure at the top of the mattress area.
Subtract an inch from the length and width of the mid century king bed, as well as from the depth, so that by sliding the mattress in place there is enough room for separation. Measure the doors and corridors through which the bed or mattress will have to approve and confirm that there is not enough free space for the short end of any of them.