Its about 34 feet. There are several smart answers concerning Standard Temperature/Pressure, sea level, relative/absolute pressure etc, but for considering an open head in plumbing needed to add an extra 1 bar to the already existing 1 bar of atmospheric pressure all around us, then the answer is that the head needs to be about 34 feet above the point of measurement (or shower or tap or whatever) unless you add a pump. Each 34 feet of water head adds another 1 bar.