In a pool, water pressure comes from the water's weight. The pressure is a function of the water depth, not the pool shape. If you start out with the density of water, you can easily calculate the pressure at a given depth in a pool. The final result is the following formula: to get the pressure in pounds per square inch (PSI) a certain number of feet down, just multiply .4333 by the depth in feet.
Assume the density of pure water: .03611 pounds per cubic inch of water.
Decide on the depth of the water at the point you need to know the pressure. (We'll use two feet for this example.)
Multiply .03611 by 12 to convert the density of one inch into mass for a column height of one foot. .03611 times 12, which equals .4333 pounds per foot of height per square inch of surface area pressed on. Note that this .4333 number is still a density.
Multiply the density 0.4333 by the depth in feet, since your density is per square-inch feet. At a depth of two feet, the pressure is 0.4333 times two, which is 0.8666 PSI.
Include atmospheric pressure by adding 14.70 PSI. So the total pressure two feet down from air and water combined is 0.87 plus 14.70, which equals 15.57 PSI.
- Recall that your body is subjected to a pressure of 14.7 PSI by the atmosphere all the time. To double this pressure on your body, you have to dive to a depth of 34 feet.
- If you're a physics whiz, you may have expected mention of the gravitational acceleration constant to convert the density from mass to gravitational force and are wondering where it is. The reason is wasn't used is that, while the metric unit kilogram measures mass, the pound measures force. In other words, the gravitational acceleration is already built into the definition of the pound. The 'slug' is instead the Imperial system's mass-measuring analogue of the kilogram, not the pound.