Fluid Pressure
Pressure at depth in a fluid.
Formula
Variables
How it works
In a fluid at rest, the deeper you go, the more fluid sits above you, so the pressure increases with depth. The formula P = ρgh comes from the weight of the column of fluid above a point: denser fluids, stronger gravity, or greater depth all create more pressure. It gives the extra pressure due to the fluid itself, not any pressure already present at the surface.
When to use it
Use this when finding pressure below the surface of a liquid, such as in water tanks, lakes, dams, or submerged objects. It is also useful for comparing how pressure changes when the fluid, gravity, or depth changes.
Worked example, step by step
A diver is 12 m below the surface of seawater. If the water density is 1025 kg/m^3 and g = 9.8 m/s^2, find the pressure due to the water. P = ρgh P = 1025 × 9.8 × 12 P = 120540 Pa P ≈ 1.21 × 10^5 Pa So the pressure from the seawater at 12 m depth is about 121 kPa.
Common mistakes to avoid
- using the wrong units for depth or density
- forgetting that this formula gives fluid pressure from the liquid column only
- mixing up pressure with force and not multiplying by area when force is needed
Did You Know?
Water pressure rises about one atmosphere every 10 metres of depth, crushing unprotected submarines in the deep sea.
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