3 Types of pressure and units from around the world

 13 October 2026

What is pressure?

By definition, pressure is the amount of force applied straight down (perpendicular) onto a surface per unit of area.

You can calculate it using this simple formula:

P = F A

where:	P	=	Pressure
	F	=	The total perpendicular force applied
	A	=	The surface area receiving that force

How is pressure physically created?

One easy way to visualise pressure is by thinking of it as the combined weight of all the molecules stacked on top of a surface. This idea works best for solids and liquids.

The illustration above shows a solid block resting on a flat surface.

Every single molecule inside that block has weight because gravity is pulling it downward. Since weight acts as a downward force, each molecule applies a tiny push against the surface.

When you sum up all these tiny forces, you get the overall pressure.

When applying this idea to gases, things get a little tricky because gas molecules aren’t neatly stacked – they float around freely. So, how do gases exert a force on a surface?

Gas molecules are in constant, rapid motion. As they move, they carry momentum and kinetic energy. They constantly crash into each other and bounce off any surface in their way.

Every time a molecule collides with a surface, it transfers a bit of momentum to it, creating a microscopic force perpendicular to that surface.

The sum of all these constant collisions generates measurable pressure!

What are the different types of pressure?

There are three primary types of pressure:

• Absolute pressure
• Gauge pressure
• Differential pressure

The key difference between them is the baseline or “zero point” chosen on the measurement scale. For absolute pressure, a perfect vacuum serves as the absolute zero point. For gauge pressure, standard atmospheric pressure is used as the reference point. For differential pressure, there is no fixed zero point because it simply compares two arbitrary pressures.

The following drawing shows the different types of pressure. The starting point of each arrow coincides with a chosen reference point. Note that absolute pressure and differential pressure are always positive, while relative (gauge) pressure can also be slightly negative. In the latter case, we also call it a partial vacuum. Theoretically, the maximum partial vacuum is -1,013 bar gauge, which corresponds to a perfect vacuum.

In principle, every pressure measurement is a comparison between the pressures at two different places.

• Absolute pressure compares the pressure at a specific location against a complete vacuum.
• Gauge (relative) pressure compares the pressure against normal sea-level atmospheric pressure (around 1012 mbar).
• Differential pressure measures the difference between any two random pressure points.

Pressure measuring instruments are designed to handle these specific types of pressure and are categorised accordingly.

Absolute pressure

Measuring anything requires a reliable starting point. For absolute pressure, the starting point is a perfect vacuum – the lowest pressure possible in the universe, where pressure is zero.

A perfect vacuum means every single particle has been removed from a sealed container. Without particles to collide or push against walls, pressure cannot exist.

As a result, absolute pressure values are always positive. Negative values don’t exist in absolute terms because you can’t go below zero pressure!

Gauge pressure (relative pressure)

Instead of comparing measurements to a perfect vacuum, gauge pressure compares the measured pressure to the standard atmospheric pressure at sea level, which is roughly 1013.25 mbar (14.696 psi).

At any given location, the difference between absolute pressure and gauge pressure is always about 1 bar (14.50 psi).

Gauge pressure can be positive or negative.

• Overpressure (Positive Gauge Pressure): When the measured pressure is higher than atmospheric pressure.

P_o = P_abs – P_atm

• Underpressure / Partial Vacuum (Negative Gauge Pressure): When the measured pressure drops below atmospheric pressure.

P_u = P_atm – P_abs

When using the term “partial vacuum”, we usually drop the minus sign. For instance, if a vacuum cleaner operates at an absolute pressure of 0.8 bar, we can describe it as having a 0.2 bar underpressure (or partial vacuum).

Differential pressure

Sometimes, engineers and scientists need to measure the difference between two arbitrary pressure points without relying on atmospheric pressure or a vacuum as a baseline. This is called differential pressure.

Technically, absolute and gauge pressures are also differences between two points, but differential pressure only tells you how much two points differ – it doesn’t tell you the total baseline pressure at either point.

For instance, knowing there is a 3 bar differential pressure between Point A and Point B tells you how different they are, but it doesn’t tell you the exact pressure level at either point or which one is higher on its own.

Are there any other types of pressure?

All pressure types we’ve discussed depend on baseline reference points or comparing two pressures.

However, you will often hear specific terms for certain kinds of pressure. Some common examples are:

• Vacuum pressure
• Atmospheric pressure
• Hydrostatic pressure
• Dynamic pressure

These are not separate categories from the main three; rather, they are specific names given to pressures in certain scenarios, and all of them can still be expressed as one of the three types of pressure: absolute, gauge, or differential.

So, there aren’t other fundamental types – just specific names!

Here is a closer look at these common terms.

Vacuum pressure

Strictly speaking, a vacuum is an empty space where absolute pressure is zero (0 bar absolute). This can only be achieved if all particles are removed from that space. In other words, the space is really empty. The perfect vacuum is only theoretically possible. It will never be technically possible to remove all the particles in a closed volume.

In real-world applications, we refer to any pressure lower than atmospheric pressure as a partial vacuum.

A “high vacuum” simply means the absolute pressure is extremely close to zero.

To create a vacuum, engineers use a vacuum pump to pull as many gas particles out of a container as possible. The efficiency of the pump dictates how deep the vacuum gets.

A widespread industrial example is the liquid ring vacuum pump. An off-center (eccentric) rotor spins inside a pump housing filled with a small amount of liquid. Centrifugal force pushes the liquid outward into a ring shape along the inner wall, forming a seal. Because the rotor is off-center, small pockets between the blades change in volume as they turn. As the pockets expand, they suck gas in; as they shrink, they squeeze the gas out. Pumps like this can reach pressures down to about 33 mbar absolute (0.4786 psi absolute).

Atmospheric pressure

Atmospheric pressure (or barometric pressure) is caused by the total weight of the air molecules in Earth’s atmosphere pushing down on the surface. Because air piles up from the ground up, atmospheric pressure is highest at sea level.

However, atmospheric pressure is constantly fluctuating. Changes in solar heating, night cooling, humidity, and weather systems (high- and low-pressure zones) cause air density to change constantly.

For the measurement of gauge pressure, this leads to a problem because this type of pressure is compared to that fluctuating atmospheric pressure.

Because actual air pressure changes continuously, engineers created a standardised benchmark called standard atmospheric pressure to keep measurements consistent. As a reference, the average atmospheric pressure at sea level has been chosen, which meets the following conditions:

Hydrostatic pressure

Hydrostatic pressure refers to the pressure exerted by a fluid at a given depth, caused purely by the weight of the liquid column above it.

It depends on three things: the liquid’s density, gravity, and the height of the liquid column.

Hydrostatic pressure is naturally a gauge pressure, calculated as:

P_hydro = ρ . g . h  (gauge pressure)

To calculate the total absolute pressure at that depth, you add the atmospheric pressure pushing on the top surface:

P_tot = P_atm + ρ . g . h  (absolute pressure)

As the atmospheric pressure is now accounted for in the equation, we are referring to a perfect vacuum, and so the total pressure becomes an absolute pressure.

Dynamic pressure

Dynamic pressure originates from Bernoulli’s equation for fluid dynamics. For smooth, steady flows of incompressible fluids, the sum of static pressure, kinetic energy, and potential energy stays constant along a streamline.

Dynamic pressure represents the kinetic energy component of a moving fluid (like water flowing through a pipe or air flowing over a wing).

It can be calculated with this formula:

q = 1 2 .ρ.v²

where:	q	=	Dynamic pressure
	ρ	=	Fluid density
	v	=	Flow speed

Pressure units across several continents

Different regions around the globe use different units to measure pressure.

• European Union: The International System of Units (SI) is mandatory under EU directive 80/181/EEC (EU Metric Directive). Pressure is primarily stated in Pascals (Pa) or bars, where 1 bar = 10⁵ Pa. Older units like mH₂O (meters of water) or mmHg (millimeters of mercury) have been phased out since 31 December 1977.
• United Kingdom: Historically, psi (pounds per square inch) was standard (14.5 psi  ≈  1 bar), but the UK has largely transitioned to using the bar as its primary unit.
• United States: psi remains the predominant unit for everyday and industrial pressure gauges.
• Asia: Units like MPa (megapascals) and kg/cm² (kilograms per square centimeter) are common.

Here is a quick conversion summary between key international pressure units:

Units	kPa	bar
1 kPa	1	0,01
1 MPa	1000	10
1 bar	100	1
1 mbar	0,1	0,001
1 atm	101,32500	1,01325
1 mH2O	9,80665	0,0980665
1 mmHg	0,133322368	0,00133322368
1 psi	6,89475729	0,0689475729
1 inH2O	0,249082	0,00249082
1 kg/cm²	98,0665	0,980665

How the unit of pressure refers to the type of pressure

Stating a pressure unit alone (like Pa, bar, or psi) can easily cause confusion if you don’t specify which type of pressure you are referring to.

Sometimes you can guess the type of pressure based on the context, but doubts can remain. Misinterpreting the baseline can lead to major engineering mistakes!

To prevent errors, it is good practice to clarify the type right after the unit – writing out bar absolute, psi gauge, or differential pressure.

You will often see letter suffixes added to units:

• a or A for Absolute (e.g., psia, bar(a))
• g or G for Gauge (e.g., barg, kPa(g))
• d or D for Differential (e.g., psiD

Note: While these shorthand suffixes are widely used in industry, official international standards organisations now deprecate them in favor of explicitly writing out the full context.

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