There are many ways on how to calculate blood pressure, but the most common method is through the use of a sphygmomanometer. The sphygmomanometer is wrapped around your upper arm 1-2 inches above the line of your elbow.
Make sure that the sphygmomanometer is not tightly or loosely wrapped around the arm for this may affect the accuracy of the blood pressure taken. First feel the radial pulse with your index and middle finger while pumping the handbulb. When you feel radial pulse stop, look at the sphygmomanometer and take note of the result.
Next, place the stethoscope in the brachial pulse and start pumping the handbulb while adding 30-40 mmHg to the first result you got. Slowly release the air in the cuff while looking at the sphygmomanometer and listening to the first and last sound. The first sound that you hear will be the systole and the last sound is the diastole.
A normal blood pressure will be 120/80, more than that will be considered as pre-hypertensive or hypertensive.