A fluid is a substance that can flow, so it is either a liquid or a gas. The particles in a fluid are constantly moving and colliding with any surface they touch. Each collision pushes on the surface, so the pressure in a fluid causes a force normal to the surface, which means at right angles to it.
Pressure depends on how much force is spread over how much area. It is calculated with: pressure = force normal to the surface ÷ area of that surface, or p = F ÷ A. Pressure is in pascals (Pa), force is in newtons (N) and area is in metres squared (m2). For example, a force of 200 N spread over 0.5 m2 gives a pressure of 400 Pa. The same force on a smaller area gives a larger pressure.
Higher tier only. The pressure due to a column of liquid is found with p = h ρ g, where h is the height of the column in metres, ρ is the density of the liquid in kg/m3 and g is the gravitational field strength in N/kg. In a calculation g is always given. Pressure at a point increases with the height of the column above it, because there is more liquid pressing down, and with the density of the liquid, because each metre of denser liquid has more mass. An object that is partly or fully submerged has a greater pressure on its bottom surface than on its top surface. This gives a resultant force upwards called the upthrust, which influences whether the object floats or sinks.