Safe blood pressure readings are below 120 for systolic and below 80 for diastolic blood pressure, states Mayo Clinic. These readings are measured in units of millimeters of mercury, or mmHg.
A blood pressure reading comprises the systolic blood pressure, which corresponds to the top number, and the diastolic blood pressure, which refers to the bottom number. For example, a blood pressure reading of 120/80, which is the upper limit for normal blood pressure levels, has 120 as systolic blood pressure and 80 as diastolic blood pressure.
A systolic reading between 120 and 139 and a diastolic blood pressure that ranges from 80 to 89 indicates the presence of pre-hypertension. Stage 1 hypertension may occur if the systolic blood pressure increases anywhere between 140 and 159 and the diastolic reading is between 90 and 99, notes Mayo Clinic.