It’s rare for a duvet to be so big that it’s truly inconvenient, such as edges that touch the floor. To avoid buying a duvet that is too big, it’s best to calculate the right duvet size by measuring your mattress’s thickness. Take that number and add it to your mattress’s length, then multiply your mattress’s depth by two and add the number to your mattress’s width. That should give you a rough idea of the length and width you should shop for.