How to Calculate Underwater Pressure

Underwater pressure affects divers.

While we surface dwellers live at the bottom of an "ocean" of air, a vertical column of air is thinner than water and weighs less than a vertical column water. The weight of several hundred or several thousand feet of water pressing down is much greater than the weight of miles of air. Calculating the underwater pressure that results from those differences can mean the difference between life and death for divers, researchers and others whose work takes them beneath the greater weight of the "water column."

Things You'll Need

• Calculator

Instructions

1. Define the depth for which you wish to calculate the pressure. Determine whether the water is salt water or is it fresh water.

2. Multiply the depth in feet by 0.432 to calculate the underwater pressure in fresh water, in pounds per square inch (psi). If the depth of fresh water is 100 feet, then: 0.432 x 100 = 43.2 pounds per square inch.

3. Multiply the depth in feet by 0.445 to calculate the underwater pressure in salt water, in pounds per square inch. If the depth of fresh water is 100 feet, then: 0.445 x 100 = 44.5 pounds per square inch.

Tips & Warnings

• A less accurate, but "close" approximation can be made by remembering that underwater pressure changes by 1 atmosphere (14.7 pounds per square inch) for each 33 feet of depth below the surface in seawater. At 100 feet below the surface, that equals 44.5 pounds per square inch: (100 / 33) x 14.7 = 44.545 psi.

• If you plan to go diving, take a properly certified diving course, conducted by a certified instructor.

References

• Thinkquest.org: Diving: Pressure and Buoyancy

Resources

• PADI: Professional Association of Diving Instructors