A sterile, plastic plate containing a series of holes of varying diameters intended to be used to measure the diameter of an inflated sizing balloon of a cardiac catheter to assist in determining the size of an atrial septal defect. It is typically used during transcatheter closure of the defect by positioning the balloon within the defect, recording the amount of fluid required to fill the balloon to the size of the defect, then refilling the balloon outside the body and passing it through the various holes of the plate to determine the diameter. This is a single-use device.