Designs for newel posts are governed by the International Residential Code (IRC), which says handrail heights must be between 34 and 38 inches. The handrail height determines what height your newel post will be.
Local Codes Prevail
The most important factor is to contact the local building department in your city to determine what the current requirements are for handrail height. Many contractors cite examples of following the IRC, only to have their new construction fail inspection because it didn't meet local codes.
The staircase is often the first thing a building inspector will check in a home. It is important to have experience in constructing stairways when determining newel height. There are many factors that need to be taken into account, including measurements which factor in tread and carpeting, when determining newel height. Failing to meet an inspection can mean costly corrections.