Confirm the dimensions of the comforter. If you’re buying a new comforter, the dimensions will appear on the packaging or in the product description if you’re ordering from a catalog or an online seller. If you’re covering an existing comforter, measure the length and width to the nearest inch.
Check the dimensions of the duvet cover. Like comforters, manufacturers generally list the dimensions on the packaging or package description. You could measure the duvet yourself if necessary.
Compare the dimensions. In general, the comforter should be within two inches -- bigger or smaller -- than the duvet cover in both length and width. Any larger or smaller and the duvet will not fit properly.