Lesson Twelve: Pressure and PSI

Introductory-level water treatment students need to know only one simple fact pertaining to pressure and psi (pounds per square-inch). This fact, which is generally found on most mathematical constants sheets, is that 1.0 psi = 2.31 feet of water. This means that every 2.31 feet of water elevation in a tank exerts 1 psi of pressure onto the walls of any pipe or tube that the water in the tank drains downward into.
Many constants sheets also state that 1.0 foot of water = .433 psi. This constant is more easily understandable if it is reversed to state .433 psi = 1 foot of water. By reversing this constant, it matches the form of the 1.0 psi = 2.31 constant.

1.0 psi = 2.31 feet of water
.433 psi = 1 foot of water


The vertical elevation of water from one point to another point is called "head." Sometimes mathematical problems will use the term "head" instead of the term "water elevation." For introductory-level math purposes, the two terms can be considered as equivalent