Blood pressure charts are read by matching the numbers in an individual's blood pressure reading to the appropriate column -- systolic or diastolic -- on the chart, according to the American Heart Association. That match is compared to the blood pressure category, which ranges from normal to high blood pressure.
The systolic number is the top of a blood pressure result and measures arterial pressure during heartbeats, states the AHA. The lower number is the diastolic reading. This reports arterial pressure as the heart rests between beats. Normal blood pressure requires both a systolic reading under 120 and a diastolic measurement less than 80.
For hypertension to become a concern, only the systolic or the diastolic reading must be above normal, the AHA reveals. However, just one elevated result is not used to diagnose high blood pressure. Doctors consider a number of readings.
If, over time, the upper number is 120 to 139 or the lower number is 80 to 89, the patient has prehypertension, explains the AHA. This indicates that, without improving diet and exercise habits, high blood pressure is a real risk, Mayo Clinic declares. Systolic readings 140 and above, or diastolic readings 90 or greater suggest stage 1 or stage 2 high blood pressure. The larger the number, the higher the stage. Some charts also have a crisis level, with systolic results above 180 or diastolic results over 110.
People with blood pressure readings that fall into stage 1 or stage 2 are typically asked to make healthy lifestyle adjustments to lower their scores, discloses Mayo Clinic. If these changes are not successful, one or more medications are usually needed.