According to the American Heart Association, a healthy blood pressure reading is equal to or less than 120 over 80. The top number refers to systolic pressure, which is the pressure against artery walls when the heart pumps. The bottom number, diastolic pressure, represents pressure between beats.
WebMD states that blood pressure readings ranging from 120 over 80 to 139 over 89 indicate prehypertension. Readings higher than 140 over 90 indicate high blood pressure. According to the American Heart Association, blood pressure readings greater than 180 over 110 are extremely dangerous and represent a hypertension crisis, which is a medical emergency.