A duvet (the filler) and its own duvet cover often are created by different companies. There are no standard sizes, so obtaining the two to fit together depends upon careful measurements. Your duvet and cover may have to be wider and longer to fit a super-thick bed, or shorter and narrower if you sleep on a thinner mattress on a platform bed.
Measure to Be Certain
Measure from the bottom edge of the bed on one side of the bed, around the surface, across the top and down to the bottom edge of the bed on the opposing side. Add 2 inches to this measurement for the width of the filler, or duvet. Measure from the head of the bed to the foot and Also down to the bottom of the mattress. Add 1 inch to get the duvet length. Adjust these measurements to suit your own personal tastes; you may need your duvet longer down the sides, or even shorter. Purchase a duvet cover with exactly the exact same size specifications since the filler.