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Pressure due to a column of liquid is p = hρg, where h is depth, ρ is density and g is gravitational field strength. Free GCSE Physics notes and 13 FAQs.
Pressure in Liquid Columns is part of Pressure and pressure differences in fluids (physics only) in GCSE AQA Higher Physics.
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Pressure due to a column of liquid is p = hρg, where h is depth, ρ is density and g is gravitational field strength. Free GCSE Physics notes and 13 FAQs.
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Use this lesson to stay inside Pressure and pressure differences in fluids (physics only) while connecting the detail back to the wider Forces topic.
Read the core explanations from Pressure in Liquid Columns before testing yourself from memory.
Picture a submarine diving deeper into the ocean. At 10 metres, the crew feels fine. At 100 metres, the hull creaks. At 1000 metres, without proper engineering, the submarine would be crushed like a tin can.
This isn't mysterious — it's physics. The deeper you go in any liquid, the more pressure builds up above you. Understanding this pressure is crucial for everything from designing submarines to explaining why your ears pop when diving in a swimming pool.
Think of liquid pressure like a stack of books. Each book represents a layer of liquid. The book at the bottom feels the weight of every book above it. The more books you add (greater depth), the more weight presses down.
Here's the key insight: pressure depends on two things. First, how deep you go (height of the liquid column). Second, what liquid you're in (its density). Mercury creates much more pressure than water at the same depth because it's 13.6 times denser.
We need a way to calculate exactly how much pressure builds up. The equation that governs liquid pressure is surprisingly simple, but incredibly powerful.
Notice how this equation makes physical sense. Double the depth ($h$), and pressure doubles. Use a liquid twice as dense ($\rho$), and pressure doubles again. Gravity ($g$) is the driving force — without it, there would be no pressure at all.
Let's put the equation to work with some real-world calculations. These examples show you exactly how to approach pressure problems in your exams.
Worked example: Calculating pressure in a swimming pool
A swimmer dives to the bottom of a 3.0 m deep swimming pool. Calculate the pressure due to the water. (Density of water = 1000 kg/m³, g = 9.8 N/kg) [3 marks]
Answer: 29,400 Pa
Worked example: Comparing pressures in different liquids
Calculate the pressure 2.0 m below the surface of mercury. Compare this to the pressure 2.0 m below the surface of water. (Density of mercury = 13,600 kg/m³, density of water = 1000 kg/m³, g = 9.8 N/kg) [4 marks]
Answer: Mercury: 266,560 Pa, Water: 19,600 Pa. Mercury pressure is 13.6 times greater due to its higher density.
In many real situations, we need to find the pressure difference between two depths rather than the absolute pressure. This is particularly important in engineering applications.
Worked example: Calculating pressure difference
A submarine moves from 50 m depth to 200 m depth in seawater. Calculate the pressure difference. (Density of seawater = 1025 kg/m³, g = 9.8 N/kg) [3 marks]
Answer: 1506750 Pa (or 1.51 MPa)
This pressure difference explains why submarine hulls must be incredibly strong. The additional 1.5 million pascals of pressure could easily crush an unpressurised vessel.
Pressure calculations are common in GCSE physics papers. Here's how to maximise your marks and avoid the traps that catch other students.
Remember that examiners often test your understanding by asking you to compare different scenarios. Practice explaining why mercury barometers are shorter than water barometers, or why deep-sea creatures need special adaptations.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The density of mercury is 13,600 kg/m³, compared with 1000 kg/m³ for water. Because it is 13.6 times denser, mercury creates 13.6 times more pressure at the same depth.
The density of seawater is 1025 kg/m³, a little more than fresh water at 1000 kg/m³. It is used in p = hρg to find pressure in the sea, such as the pressure on a submarine.
The equation for pressure due to a column of liquid is p = hρg, where h is the depth in m, ρ is the density in kg/m³ and g is gravitational field strength in N/kg. A swimmer 3.0 m deep in water feels 3.0 × 1000 × 9.8 = 29,400 Pa.
Pressure increases with depth because the deeper the point, the greater the weight of liquid pressing down on it. Like a stack of books where the bottom book feels the weight of every book on top, double the depth and the pressure doubles.
Find the depth difference, then use Δp = Δh × ρ × g. A submarine moving from 50 m to 200 m in seawater (1025 kg/m³) has a pressure difference of 150 × 1025 × 9.8 = 1506750 Pa, about 1.51 MPa.
Liquid pressure depends on density, so a denser liquid gives more pressure at the same depth. At 2.0 m, mercury (13,600 kg/m³) gives 266,560 Pa while water gives 19,600 Pa: 13.6 times greater because mercury is 13.6 times denser.
Submarines need strong hulls because liquid pressure builds up the deeper they dive. Moving from 50 m to 200 m depth adds about 1.5 million pascals of pressure, which could easily crush an unpressurised vessel.
The pressure due to the water at the bottom of a 3.0 m deep swimming pool is 29,400 Pa, or 29.4 kPa. Using p = hρg with ρ = 1000 kg/m³ and g = 9.8 N/kg, p = 3.0 × 1000 × 9.8 = 29,400 Pa.
At 2.0 m depth, mercury gives a pressure of 266,560 Pa and water gives 19,600 Pa, so mercury's pressure is 13.6 times greater. The ratio equals the density ratio, because mercury is 13.6 times denser than water.
In p = hρg, h is the height of the liquid column, or depth, in metres. ρ is the density of the liquid in kg/m³ and g is gravitational field strength in N/kg, giving pressure in pascals.
Ears pop when diving in a swimming pool because the pressure builds up the deeper the diver goes. Liquid pressure increases with depth because of the weight of liquid on top.
Liquid pressure increases linearly with depth, unlike gas pressure, so doubling the depth doubles the pressure. Using a liquid twice as dense also doubles the pressure, because p = hρg.
Gravity is the driving force behind liquid pressure: without it there would be no pressure from a liquid column at all. The pressure is caused by the weight of liquid on top of a point, which is why g is in p = hρg.