Pressure is force per unit area. When you stand on snow with regular shoes, you sink. Put on snowshoes, and the same weight is distributed over a larger area, reducing the pressure and keeping you on top. This simple concept governs everything from hydraulic systems to atmospheric science.
In a fluid at rest, pressure increases linearly with depth. The formula is P equals P0 plus rho g h, where P0 is the surface pressure, rho is the fluid density, g is gravitational acceleration, and h is depth. Every 10 meters of water adds about one atmosphere of pressure. At 100 meters depth, a diver experiences about 11 atmospheres. Our Pressure Calculator computes these values.
This has practical consequences for diving. The human body is mostly water and incompressible, so the pressure itself is not the problem. The problem is the air in your lungs and other cavities, which compress at depth. This is why scuba divers breathe compressed air at ambient pressure. Ascending too fast causes the dissolved gases in your blood to form bubbles, causing decompression sickness, also known as the bends.
Atmospheric pressure decreases with altitude. At sea level, it is about 101,325 pascals. At the top of Mount Everest, about 33,000 pascals, roughly one third of sea level. This is why climbing without supplemental oxygen is so difficult. Your lungs get less oxygen per breath because there are fewer air molecules in each cubic meter of air at altitude.
Pascal’s principle states that pressure applied to a confined fluid is transmitted equally in all directions. This is the basis of hydraulic systems. A small force applied to a small piston creates pressure that acts on a large piston, multiplying the force. Hydraulic car lifts, brakes, and excavators all rely on this principle. A 100-newton force on a 10-square-centimeter piston connected to a 1,000-square-centimeter piston produces a 10,000-newton force.
Bernoulli’s principle connects pressure to fluid velocity. Faster-moving fluid has lower pressure. This explains lift on airplane wings, the operation of spray bottles, and why two sheets of paper blow together when you blow between them. It is an approximation that assumes no viscosity and no turbulence, but it works surprisingly well in many situations.