IGCSE Physics / Density and pressure
Calculating pressure in a liquid revision notes
Edexcel International GCSE Physics (4PH1), chapter 18: Density and pressure, specification point 5.7. Free lesson.
Watch the video lesson · IGCSE Physics past papers
Liquid pressure difference depends on the liquid’s depth, density and gravitational field strength. You can also rearrange the equation to find depth, or add atmospheric pressure to find the total pressure.
How to calculate pressure difference
Pressure increases with depth because there is more liquid above pressing down. Use p = h × ρ × g. The answer is the pressure difference caused by the liquid, not the total pressure including the air. p is pressure difference in pascals (Pa). h is depth, or height difference, in metres (m). ρ is liquid density in kilograms per cubic metre (kg/m³). g is gravitational field strength in newtons per kilogram (N/kg). Use 10 N/kg in these calculations.
Convert units before substituting
The equation needs depth in metres and pressure in pascals. Convert any centimetres or kilopascals before you put values into the equation. To convert centimetres to metres, divide by 100. For example, 80 cm = 0.8 m. To convert kilopascals to pascals, multiply by 1000. For example, 50 kPa = 50 000 Pa.
Find depth by rearranging
If pressure is known but depth is not, first substitute the values into the original equation. Then divide by density × gravitational field strength to make h the subject.
Find total pressure
The liquid causes a pressure difference, but the air also presses on the surface. To find the total pressure at a depth, add atmospheric pressure to the pressure difference from the liquid. Total pressure = atmospheric pressure + liquid pressure difference. Use matching units when adding. Convert kilopascals to pascals first if needed, then convert the final answer if the question asks for kPa.