The opening in my stool which is exactly like the one pictured is 18 inches.  The cushion I received is 16 inches round.  It doesn't look as nice as 18 inches would but not worth returning to receive another that could be the same size.  Didn't like that it arrived stuffed inside a box  without being covered with a plastic bag or something else to protect it.  Beautiful color that matches the cushion for the chair I ordered at the same time.