A king-size bed is the largest bed made in standard mattress sizes, except for the California king, which is 4 inches less in width but 4 inches longer. The standard measurement for a king-size bed is 76 inches wide by 80 inches long. Although there is standardisation now in bed frames and mattresses, you will still need to measure your bed frame to get an exact fit for a new mattress.
Measure the width and length of the area provided for the mattress. Work from the inside edge of the framework and take into account that the mattress may slide under the shelving of the headboard on frames with bookcases or shelves.
Find the depth of the mattress required by holding your tape measure at the top of the slats where the box spring would sit and measure to the top of the area provided for the mattress.
Subtract an inch from the length and width of the bed as well as from the depth, so that when sliding the mattress in place there is enough space for clearance.
Measure the doorways and hallways through which the bed or mattress will have to pass and confirm that there is enough clearance for the shortest end of either.