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If you have a king-size duvet cover but want to use it on a queen-size comforter, you may want to alter the duvet to fit the comforter, instead of using it as-is. Altering the duvet is as simple as sewing around the edges on three sides to leave it just slightly wider and longer than your comforter. You don't want to adjust the duvet to be the same dimensions as the comforter, because then it will be a very tight fit.
Measure the width and length of the queen-size comforter. Most queen comforters measure 86 inches by 94 inches, but there can be some variation depending on the comforter's style or the fill.
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Measure across the width of the king-size duvet. Subtract the width of the comforter from the width of the duvet. Divide that measurement in half, then subtract 1 inch from that measurement to find out how much you'll have to trim from each side. For example, if the king-size duvet is 104 inches wide and the comforter is 86 inches wide, subtract 86 from 104 to get 18. Divide 18 in half to get 9 inches, then subtract 1 to get 8 inches.
Turn the duvet wrong side out. Measure in the designated number of inches from each side of the duvet at the top and bottom and make a mark with a fabric marker. If your duvet has a zipper closer, pull the zipper closed halfway so you won't cut off the zipper pull when you alter the duvet.
Sew a straight row of stitches on each side of the duvet, from the mark on the top edge to the mark on the bottom edge.
Measure the length of the king-size duvet, then subtract the length of the queen size duvet, divide by two, then subtract 1 to find out how much you need to trim off of the length. For example, if your king-size duvet measures 98 inches and your comforter measures 94, you'll end up having to trim off 1 inch.
Measure down the amount you need to trim from the length and make a mark on each side, near the top of the duvet. You don't want to trim from the bottom, as that is where the opening and closures are.
Sew a straight line of stitches across the duvet from mark to mark.
Slide the comforter inside the duvet to make sure it fits. Trim the sides and top edge of the duvet to 1/2 inch away from the rows of stitching. Finish the raw edges inside the duvet with a zig-zag stitch.
Turn the duvet right side out, slide the comforter in and toss it on your bed.
In some cases, a king-size duvet is the same length as a queen-size, so you may not need to alter the length.