Modern king size bed is the largest bed made in standard mattress sizes, with the exception of California king, which is 4 inches less in width but four inches longer. The standard measurement under a king size bed is 76 inches wide by 80 inches long.
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Measure the width and length of the area referred to the mattress. Work from the inner edge of the frame and take into account that the mattress can slide under shelves in the gable on frames with bookshelves or shelves. Finding the depth of the mattress is required by holding your tape measure on top of the lamellae where the spring box would sit and measure to the top of the area meant for the mattress. Subtract one inch from the length and width of the modern king size bed as well as from the depth so that when the mattress slides into place there is enough space for the finish. Measure doorways and corridors through which the bed or mattress will need to pass and confirm that there is enough room for the shortest end of either.
A modern king size bed mattress is 76 inches wide by 80 inches long. A California king mattress is longer by 4 inches (84 cm long), but narrower by 4 inches (72 inches wide).