If you have a queen size mattress and a full size headboard, you can pair them together to make your bedroom appear as though it has a complete bed. In most cases you'll need a converter piece or a converter kit to allow the pieces to attach together although they are different widths. You can purchase these conversion pieces online or at specialty bed and mattress stores.
It Will Look Just Fine
Full size mattresses are 54 inches wide, and queen size mattresses are 60 inches wide, which means there's a difference of only 6 inches in their widths. If you place a full size headboard with a queen size mattress, the mattress will be slightly wider than the headboard but will extend beyond each end about 3 inches or maybe even less, depending upon the exact width of the headboard. This amount is small enough that it will look just fine and go unnoticed by most people, especially once the bed is made and fully dressed with a quilt or comforter and plenty of pillows.
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Headboard, Frame and Mattress Only
If you have a queen size mattress, a queen size metal bed frame and a full size headboard, you can use modification plates to connect them together. You attach these plates toward the bottom of the narrower headboard to extend the width at that point. To join the headboard and frame together, you simply attach the wider frame to the plates as well. The queen size mattress will fit perfectly on the queen size frame. You might also attach the headboard directly to the wall, and simply push the mattress and frame flush with the headboard.
Mattress and Complete Bed
With a queen size mattress and a complete bed -- meaning a headboard, foot board and rails -- purchase a full-to-queen converter kit. Before making the purchase, note whether your headboard and foot board are held onto the rails with bolts or hooks, and purchase a converter that corresponds. Follow the instructions included with the converter kit, which essentially has you connect new, longer rails to the headboard and foot board, place the mattress on top of the rails and secure the mattress onto the rails with retainer brackets.
Other Considerations
Once you've installed your full-to-queen converter kit, you won't need the full-size rails that came with your bed. Discard them only if you are certain that you will never again want to use your bed with a full size mattress. Put them away in a safe place if your full size bed is a valuable antique or if you might want to use it with a full size mattress someday.