A valvuloplasty catheter has a dog-bone shaped balloon with semi-compliant smaller diameter waist and non-compliant larger diameter bulbous end regions. The balloon centers across the valve with the waist adjacent to the annulus. One bulbous region serves to hyperextend the valve leaflets and the other assists in stabilizing the balloon position to reduce migration. The semi-compliant waist increases in diameter as fluid enters the balloon until it comes into contact with the valve annulus. The pressure within the balloon per unit of volume delivery has a greater slope after contact with the annulus than before resulting in a change in slope for the pressure versus volume curve. The diameter of the balloon and annulus are determined at this inflection point when the balloon contacts the annulus.