Blood Pressure is measured using an instrument called the spygnomanometer. It can also be measured digitally. Blood pressure is always given as two numbers - systolic pressure (when the heart contracts) and diastolic pressure (when the heart relaxes). It is measured in mmHg (millimetres of mercury). The normal blood pressure range is 110/70 to 120/80 with systolic being the first number and diastolic being the second number.
It is very difficult to correctly define normal blood pressure as the blood pressure range can vary widely. A single blood pressure monitor reading is not reliable. Blood pressure monitor readings must be taken at different times of the day to determine average blood pressure over a period.
Read more: How to Measure Blood Pressure on http://www.medindia.net/animation/blood-pressure.asp#ixzz2ve0AOJdO