Blood pressure is measured in units of millimeters of mercury (mmHg) . Blood pressure readings are always given in pairs, with the upper (systolic) value and the lower (diastolic) value. The systolic blood pressure is the pressure when the heart beats, while the diastolic blood pressure is the pressure on the blood vessels when the heart muscle relaxes. The readings are usually given as two numbers, for example, 120/80 mmHg, where 120 is the systolic pressure and 80 is the diastolic pressure. Blood pressure can be measured using a digital blood pressure monitor for automated readings or an instrument called a sphygmomanometer for manual readings. The gauge on the cuff of the sphygmomanometer uses millimeters of mercury (mmHg) to measure the pressure in the blood vessels. It is important to measure blood pressure on several different days and while resting to get reliable readings. When taking blood pressure for the first time, it makes sense to measure the blood pressure in both arms, because its sometimes high on only one side. The values that are higher are always the ones used for assessing blood pressure. After that, it is enough to measure the blood pressure only in the arm that produced the higher reading.