When reading blood pressure numbers, the top number, known as the diastolic number, measures the pressure in millimeters of mercury inside the arteries when the heart muscle contracts, and the bottom, or diastolic, number measures the pressure within the arteries between heartbeats, according to the American Heart Association. A normal blood pressure reading is less than 120 millimeters of mercury for the top number and less than 80 millimeters of mercury for the bottom number.
There are four states of hypertension, and doctors determine a patient's state based on consistent readings that fall within certain levels, notes the American Heart Association. When the upper number is between 120 and 139 or the lower number is between 80 and 89, the person has prehypertension, which is a precursor to high blood pressure.
If the top number is between 140 and 159 or the lower number is between 90 and 99, the person has Stage 1 hypertension, notes the American Heart Association. A consistent reading of 160 on the top or 100 or more on the bottom indicates Stage 2 hypertension. A systolic blood pressure reading of 180 or more or a diastolic reading of 100 or more indicates a hypertensive crisis, and the person should seek immediate medical attention to avoid a stroke or heart attack.