Pressure – Units of pressure

What is Pressure?

One of the important physical quantities that appears everywhere on Earth, continuously exerting force and influencing every object present on this planet, is pressure. The type of pressure acting on objects varies depending on the surrounding environment.

Here are some examples:

  • For every object existing on the Earth’s surface (land, sea, etc.), it experiences the pressure exerted by the atmosphere.
  • For every object existing underwater, it experiences the pressure exerted by the water.

There are many other environments as well, but these are the two closest and most common examples. The magnitude of pressure differs depending on the nature of the surrounding environment, and it is most noticeable and apparent when we are on land or when we dive underwater.

Pressure (symbol: P or p).

In today’s article, we will review some old knowledge and explore some new knowledge related to this physical quantity.

What is Pressure?
What is Pressure?

Definition of Pressure

Pressure is defined as the force acting perpendicular to the surface of any object per unit area.

Formula for Calculating Pressure

Based on the definition, pressure can be calculated using the following formula:

P = F/S


  • P: Pressure, measured in Pascal (Pa)
  • F: Force acting perpendicular to the surface, measured in Newtons (N)
  • S: Area of the surface subjected to the force F, measured in square meters (m2)
Formula for Calculating Pressure
Formula for Calculating Pressure

Unit of Pressure

With the quantity of pressure, various units of pressure measurement are used, such as psi, bar, Pa, mmH2O, mmHg, in.Hg, and more.

They can be divided into two different types: units of pressure measurement according to the International System of Units (SI), and units of pressure measurement not according to the SI.

Unit of Pressure
Unit of Pressure

Units of Pressure Measurement According to the SI

Based on the formula for calculating pressure and the conventions regarding the units of physical quantities in the International System of Units (SI):

The unit of pressure measurement according to the SI is:

  • Pa, read as Pascal.
  • Or N/m2, read as newtons per square meter.
  • Where: 1 Pa = 1 N/m2
Units of Pressure Measurement According to the SI
Units of Pressure Measurement According to the SI

The Pascal unit is the unit of pressure measurement in the SI system. It is named after the physicist and mathematician Blaise Pascal, who made significant contributions related to fluids and pressure. In recognition of his major contributions, his name was used to designate the unit of pressure measurement and Pascal’s law, which is still applied in many current mechanical devices.

Physicist Blaise Pascal
Physicist Blaise Pascal

Other Common Units of Pressure Measurement Not in the SI System

Unit of Pressure Measurement PSI

PSI stands for “Pound Per Square Inch,” which means the amount of pounds per square inch. It is the standard unit of pressure measurement in the United States.


Pound is a unit of weight commonly used in the traditional measurement systems of the United Kingdom, the United States, and some other countries (if you are a fan of boxing, you are probably familiar with this unit when the organizers rank fighters by weight in pounds).

We have 1 Pound = 0.4359237 kg.

Inch is a unit of length measurement and is also commonly used in the above-mentioned countries and some other nations. Consequently, square inch refers to the area measured in square inches.

1 inch = 2.54 cm.

From the above information, we have: 1 PSI = 6895 Pa.

Bar Pressure Unit

When it comes to common pressure units, it is impossible not to mention Bar.

This unit was introduced by a Norwegian physicist and meteorologist named Vilhelm Bjerknes (1862 – 1951), who developed the method of weather forecasting. His weather equations are still used today.

In 2004, the Bar pressure unit was recognized in European countries.

We have: 1 Bar = 100,000 Pa

Atmospheric Pressure Unit

One of the popular pressure units that is not part of the international measurement system is “atm.”

The English name for the atmospheric pressure unit is “standard atmosphere,” which represents the standard atmospheric pressure.

But why is it called that? And why do we need an additional pressure unit when we already have one?

There are two fundamental reasons based on the nature of atmospheric pressure and the need for calculations.

Firstly, atmospheric pressure changes with altitude. As altitude increases, the pressure decreases. Therefore, in many cases, it is difficult to determine a specific pressure value for design calculations or other purposes.

To address this, a reference point was established for convenience in calculations and conversions. The reference altitude is based on the average sea level, where the measured pressure is 1,013,325 Pa, and it is conventionally defined as 1 atm.

We have: 1 atm = 101,325 Pa.

In addition to the mentioned pressure units, there are many other units used to measure pressure, such as mmHg, at, torr, and more.

Common Pressure Types Used in Engineering Fields

Partial Pressure

According to Wikipedia, in a mixture containing multiple gases, each gas in the mixture has its own partial pressure, which is referred to as partial pressure. In this case, the partial pressure of any gas in the mixture is understood as the pressure of that gas in the entire initial space, after removing other gases.

To understand this type of pressure further, consider the following example:

Imagine a box that meets ideal conditions such as being airtight, capable of removing certain gases, or introducing gases.

Initially, the box is opened under normal conditions, and immediately air flows into the box. Then, the box is sealed, and gases like nitrogen, carbon dioxide, argon, etc., are removed, leaving only oxygen inside, with the remaining amount equivalent to the initial amount in the air. Now, the pressure exerted by oxygen inside the sealed box is the partial pressure of oxygen, and the other gases follow the same principle (note that we consider the example under the same temperature conditions).

To determine the partial pressure of a specific gas “X” in a mixture, we use the following formula:

Px = (nx/nhh) × phh or Px = (Vx/Vhh) × Phh

In English, the translated text with HTML code intact would be:


  • Px: Partial pressure of gas x.
  • nx: Number of moles of substance x.
  • nhh: Number of moles of the mixture.
  • Phh: Pressure of the mixture.
  • Vx: Volume of gas x.
  • Vhh: Volume of the gas mixture.

Excess Pressure (Relative Pressure)

The relative pressure at a position is the pressure exerted by neighboring particles on that point.

For example, when measuring the pressure of a compressed gas cylinder, water pipes, etc., the pressure displayed on the pressure gauge is the relative pressure of that substance in the system.

Absolute Pressure

As you know, on the Earth’s surface, atmospheric pressure always exists (as mentioned at the beginning of the article), so when considering the pressure of a system, flow, device, etc., we need to add the atmospheric pressure to the relative pressure at that time and position. (Standard for imperfect vacuum environments)

P = Pd + P0

  • P: Absolute pressure
  • Pd: Excess pressure
  • P0: Atmospheric pressure.

Permeation Pressure

According to Wikipedia, Permeation Pressure is the minimum pressure required to apply to a solution to prevent the flow of the pure solvent through a semipermeable membrane towards the solvent-containing solution.

Static Pressure

Static pressure is fundamentally understood as the pressure that is uniform in all directions, corresponding to the pressure caused by a liquid when the liquid is not in motion.


P = p0 + ρgh


  • P0: Atmospheric pressure
  • ρ: Density of the fluid
  • g: Acceleration due to gravity
  • h: Height at the point where pressure is measured relative to the liquid surface.

Conversion between different units of pressure

Surely, during your studies and work, you have encountered various units of pressure.

Even pressure measuring devices are designed with different pressure ranges, as each country typically uses different units of pressure.

Considering the practical conditions in Vietnam, due to the limited development of science and technology, a wide variety of machinery and equipment must be imported, not only from one or a few countries but from many different countries.

Therefore, there are also many applied units of pressure in our country. In addition to the common units mentioned at the beginning of the article, we also have units that are powers of 10 of the basic units.

For example: Pa, kPa, MPa,…

Below are some formulas for converting units of pressure, or you can also refer to the pressure conversion table for more comprehensive information.

  • 1 bar = 100 kPa
  • 1 bar = 1.02 kg/cm2
  • 1 bar = 0.99 atm
  • 1 bar = 14.5 PSI
  • 1 bar = 750 Torr.
Conversion between different units of pressure
Conversion between different units of pressure

The role of pressure in daily life

Pressure is always present around us, but because we are so accustomed to its presence, we often do not pay attention to its existence.

To better understand the importance of pressure, let’s explore some fields related to pressure.

In medicine:

  • Blood pressure, which refers to the force of blood acting on blood vessels (commonly known as blood pressure), is a crucial indicator. If it is too high or too low, it can affect human life, and doctors also rely on blood pressure to diagnose diseases.
  • If you have ever seen cardiopulmonary resuscitation (CPR) methods, you know that they are also closely related to pressure (this article does not delve into medical aspects, so we won’t discuss it in detail).

In mechanical engineering:

  • The operation of machines using internal combustion engines is also related to the variation of pressure in the combustion chamber.
  • Pressure plays a role in various devices such as vacuum pumps and many other types of machinery.

In meteorology:

  • Pressure is a very important parameter in weather phenomena. Differences in atmospheric pressure can cause various weather conditions, and significant differences can lead to extreme weather phenomena.

These are just a few basic examples of the role of pressure in some important fields that directly impact human beings. Naturally, the role of this physical quantity covers almost all aspects of life.

Note: The above translation assumes that the HTML code is not intended to be displayed as code, but as formatting or layout instructions.

Pressure Measuring Devices

Understanding the importance of pressure in engineering, physicists and inventors have long been concerned with the pressure that devices must withstand during operation. In order to monitor pressure in various scenarios and serve research and calculation purposes, the pressure gauge was invented.

According to Wikipedia, the first pressure gauge was created by Gasparo Berti (1600-1643), an Italian mathematician, astronomer, and physicist. It was a mercury pressure gauge constructed between 1640 and 1643.

Over time, the pressure gauge has been improved and developed with various shapes and sizes, gradually becoming more refined. Now we have devices such as pressure gauges, U-tube manometers, blood pressure monitors, and more.

Modern pressure measuring devices have been enhanced in terms of accuracy and the ability to work under vibration, wider temperature ranges, and higher working pressure ranges. They integrate multiple pressure measuring units into a single product.

Although they may have different shapes, features, and application areas, they all share a common basic principle.

Pressure Gauge
Pressure Gauge
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