You need to select the correct duvet cover to match your duvet.
If you're not sure, just take a look at your duvet's size on its label or check your existing bedding. Here are the main bed sizes and their matching duvet cover sizes.
Bear in mind that some brands can vary by several inches, so we've just listed these figures as a general guide:
For a Twin/Twin XL duvet you'll need a twin duvet cover measuring approximately: 66" x 86-90"
A full/double duvet  takes a full duvet cover measuring approximately: 80" x 80"- 88" x 88"
A queen duvet takes a queen duvet cover measuring approximately: 90" x 88-90"
A king duvet  takes a king duvet cover measuring approximately: 102" x 90" - 104" x 92"
A Californian king duvet takes a Californian king duvet cover measuring approximately: 104" x 94"