Pure water at sea level weighs 62.4 pounds per cubic foot. Thus, if the base of a cubic foot is divided into 144 one-inch squares, the weight of the column of water one foot high covering a square inch area would be 0.433 pounds. Or 0.433 pounds per square inch of pressure exists at the base of this cubic foot of water. Since, pressure is measured in pounds per square inch, the area covered at the base of a water column is not relative to the pressure. As a rule of thumb, it is said that a column of water one foot high creates a pressure of approximately 1/2 pound per square inch, or psi. Another term used for measuring pressure aside from psi is inches of mercury, or "Hg. This is because mercury manometers are often used to measure pressure differentials. However, mercury weighs 13.5 times the weight of water. Therefore, one "Hg is approximately 1/2 psi.