High blood pressure in an adult is indicated by a systolic reading — the top number of a blood pressure reading — of 140 or more. A diastolic level, indicated by the bottom number, of 90 or greater suggests hypertension, according to the National Heart, Lung and Blood Institute.
A systolic level of less than 120 and a diastolic level under 80 indicates normal adult blood pressure, the institute states. Prehypertension is indicated by top readings between 120 and 139, as well as bottom readings from 80 to 89. Prehypertension suggests that, unless an individual makes health changes, hypertension is very likely to develop.