King size to queen size bed conversion is one of the easiest bed modifications you can make. Queen mattresses are the same length as standard kings, and bed rails are designed to adjust along the width. These instructions are for converting simple, standard bed rail frames. Solid wooden frames are difficult enough to convert that you're better off finding a replacement frame.
Examine the bed rails. Bed rail assembly uses nuts, often wing nuts, to attach the two halves. The nuts run through holes drilled in the short sides of the bed frames. In some cases, the rails have a series of holes. Check that the smallest possible setting is wider than 60 inches, the width of a queen mattress. Some rails are built to be compatible for queen and king mattresses, in which case you can stop at this step.
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Slide the rails together until the frame width is 60 inches. Mark both overlapping rails at two points where they overlap.
Put on your safety goggles. Drilling metal throws sparks and slivers of metal. If doing this in the bedroom, have a fire extinguisher handy. There is a very slight, but real, chance that flying sparks can ignite your drapes, bedclothes or carpet.
Drill through the rails at all the points you've marked, using a drill bit the same diameter as the bolts that come with the bed rail set.
Reassemble the bed rails in the 60-inch wide position. Bolt them in place through the holes you've just drilled.