A standard king size bed, or Eastern King, is 76 inches wide by 80 inches long. A regular twin bed is 39 inches wide by 75 inches long, while an extra-long twin bed is 80 inches long. You can create a roomy king size bed inexpensively with two twin beds placed side by side. Hold the beds securely together with a ratchet strap or buy a bed-joining kit. Although the overall dimensions are slightly different, all standard king size bedding will fit the double-twin bed.
Remove the bedding from both twin beds, if necessary.
Push the twin beds together, side by side, with the top and bottom edges aligned
Wrap the ratchet strap around the outside of both mattresses and hook the ends together. Tighten the ratchet to hold the mattresses snugly in place.
Roll up a twin size quilt batting lengthwise, starting at one side. Press the roll halfway into the crack between the mattresses. Press down the top of the quilt batting roll to create a flattened padded strip over the full length of the crack.
Place the king size mattress pad on the joined twin mattresses.
Place king size sheets on the newly-created king size bed.
When using regular-length twin beds, place the box springs horizontally and the mattresses vertically to keep the beds from separating further. For added comfort, put a foam mattress topper under the mattress pad. Buy a Create-a-King, Extend-A-Bed or other mattress joining system instead of making your own with a ratchet strap and quilt batting, if desired.
Do not try to combine twin beds on frames with side rails.