Pressure, Head and Density: The Working Physics
How pressure, liquid head and density relate, the units you will meet, and how to calculate a hydrostatic pressure with confidence.
11 min
Activities: 0 of 2
One equation behind most of this course
Pressure is force per unit area. In a liquid at rest, the pressure caused by the liquid above a point depends only on three things: the liquid's density, the acceleration due to gravity, and the vertical height of liquid above the point. This is the hydrostatic equation:
pressure = density × g × height
Here g is about 9.81 metres per second squared. With density in kilograms per cubic metre and height in metres, the answer is in pascals. The shape and width of the vessel do not matter. Only vertical height counts.
Engineers often use specific gravity (SG): the liquid's density divided by the density of water. Water has an SG of 1.0. A light crude oil might have an SG near 0.85. A useful shortcut is that one metre of water produces about 9.81 kilopascals (kPa), which is about 98.1 millibar (mbar).
Units you will meet
The pascal (Pa) is the SI (International System of Units) unit. Kilopascal (kPa) and megapascal (MPa) are common on datasheets.
Quick calculations
Fill in the missing values or words.
One metre of water produces about kPa. A liquid with a density 85% of water's has a specific gravity of . In the hydrostatic equation, only the height of liquid matters, not the shape of the vessel.
An open tank contains 4.0 m of liquid with SG 0.90. What is the approximate pressure at the bottom caused by the liquid?