Usually two readings are done, two minutes apart, and then averaged.
Most doctors or clinics use a sphygmomanometer, where a column of mercury is linked to a cuff around your arm. The height of the mercury column, in millimetres, indicates your blood pressure.
A soft cuff is wrapped around your upper arm. Air is pumped into the cuff. As the pressure rises gently, your arm is squeezed until the blood flow through the artery is interrupted. A stethoscope is placed on the inner side of the elbow crease, just below the cuff. This is right above the main artery in your arm.
The cuff is then deflated slowly to allow blood flow again. The return of blood flow can be heard as a pulsating beat. The reading on the pressure gauge when this first sound is heard, is your systolic blood pressure.
As the cuff is deflated further, more beats are heard for a few seconds and then it disappears. The point at which the sounds disappear is recorded as the diastolic blood pressure. Optimal BP is 120/80mmHg.
(Dr Kathleen Coetzee, MBChB)