The American Heart Association states that normal blood pressure readings are less than 120 systolic and 80 diastolic. Readings of 120 to 139 over 80 to 89 are considered prehypertension, while readings greater than 140 over 99 are considered high. Readings greater than 180 over 110 indicate a crisis.
It should be noted, however, that ideal blood pressure readings for teenagers and children are often quite lower, according to Mayo Clinic. Additionally, an occasional high reading does not necessarily indicate hypertension as many illnesses and health conditions temporarily elevate blood pressure. For this reason, multiple high readings over a period of time are necessary to diagnose high blood pressure, says WebMD.