Pressure

What Is Pressure?

Pressure tells us how much force is pushing on a certain area. It answers the question, how hard is something pressing on a surface.

Imagine standing on soft ground. If you stand on two feet, you do not sink much. If you stand on one foot, you press harder into the ground. Your weight is the same, but the area is smaller, so the pressure is larger.

In simple words, pressure is how strongly a push is spread over a surface.

Definition

In science, pressure is defined using this idea:

Pressure = Force ÷ Area

• Force is the push or pull, usually measured in newtons (N).
• Area is the surface the force acts on, usually in square meters (m²).

The main SI unit (International System of Units) for pressure is the pascal, written as Pa.

1 pascal (1 Pa) means 1 newton of force spread over 1 square meter of area.

So, if you increase the force, pressure goes up. If you spread the same force over a bigger area, pressure goes down.

History / Origin

People have felt pressure forever, like air pushing on our bodies or water pressing on us when we dive. But the clear scientific idea of pressure grew slowly.

• Ancient times people noticed air and water could push on things, but they did not have a clear formula.
• 17th century scientists began to study air pressure. They did experiments with pumps and tubes filled with liquid.
• Blaise Pascal a French scientist, carefully studied how pressure changes with height in the air and in liquids. The unit pascal (Pa) is named after him.
• Robert Boyle studied how the pressure of a gas changes when you squeeze it into a smaller space. This led to Boyles law.

Over time, scientists agreed to use the pascal as the standard pressure unit in the SI system. Older units like atmosphere and millimetres of mercury are still used, but the pascal is now the main scientific unit.

Symbol & Abbreviation

There are symbols for the quantity pressure and for the units used to measure it.

• Quantity symbol for pressure usually p or P.

Main SI unit

• pascal written as Pa.

Common multiples of pascal

• kilopascal 1 kPa = 1 000 Pa.
• megapascal 1 MPa = 1 000 000 Pa.

Other common pressure units and abbreviations

• bar written as bar.
• atmosphere written as atm.
• pound per square inch written as psi.
• millimetre of mercury written as mmHg.
• torr written as Torr.

In equations you might see something like p = 101.3 kPa. This means the pressure is 101.3 kilopascals.

Current Use Around the World

Pressure is used in many parts of daily life, science, and industry all over the world.

• Weather and climate weather reports use air pressure to help predict storms and wind. Meteorologists often use hPa (hectopascal) or mb (millibar, very close to hPa).
• Medicine blood pressure is checked every day in hospitals and clinics. It is usually measured in mmHg (millimetres of mercury).
• Engineering and construction engineers use pressure to design pipes, tanks, boilers, and bridges. They often use Pa, kPa, or MPa.
• Cars and bikes tyre pressure is very important for safety and fuel use. In many countries it is measured in psi or bar.
• Diving and submarines divers and submarine operators watch pressure to stay safe under water. They think about how pressure increases as you go deeper.
• Industry factories use pressure in gas tanks, air tools, spray cans, and more. They pick units like bar, psi, or kPa depending on local rules.

Different regions may prefer different units. For example, Europe often uses bar and kPa for tyres, while the United States uses psi. Science papers usually use Pa or kPa.

Example Conversions

Because there are many pressure units, it is useful to know how to change from one unit to another.

Here are some key equalities:

• 1 atm = 101 325 Pa ≈ 101.325 kPa.
• 1 bar = 100 000 Pa = 100 kPa.
• 1 atm ≈ 1.01325 bar.
• 1 atm ≈ 14.696 psi.
• 1 atm ≈ 760 mmHg ≈ 760 Torr.

Simple conversion examples

Example 1 Convert 2 atm to kPa.

1. Use 1 atm ≈ 101.325 kPa.
2. 2 atm = 2 × 101.325 kPa = 202.65 kPa.

So 2 atm ≈ 203 kPa if we round to the nearest whole number.

Example 2 Convert 30 psi to bar.

1. Use 1 bar ≈ 14.5038 psi.
2. 30 psi ÷ 14.5038 ≈ 2.07 bar.

So 30 psi is a little above 2 bar.

Example 3 Convert 101.3 kPa to atm.

1. Use 1 atm ≈ 101.325 kPa.
2. 101.3 kPa ÷ 101.325 kPa ≈ 0.9998 atm.

So 101.3 kPa is almost exactly 1 atm. This is close to average air pressure at sea level.

Quick reference table

Related Units

Pressure connects to many related units. Here are the most common ones and how they relate.

• Pascal (Pa) the main SI unit of pressure, 1 Pa = 1 N per m².
• Kilopascal (kPa) 1 kPa = 1 000 Pa, often used for weather and tyre pressure.
• Megapascal (MPa) 1 MPa = 1 000 000 Pa, used for very high pressures in engineering.
• Bar 1 bar = 100 000 Pa, close to air pressure at sea level, often used in industry and diving.
• Atmosphere (atm) 1 atm = 101 325 Pa, based on average air pressure at sea level.
• Pound per square inch (psi) pressure from one pound of force on one square inch, common in the United States for tyres and tools.
• Millimetre of mercury (mmHg) based on how high a column of mercury rises under pressure, used in blood pressure and vacuum systems.
• Torr almost the same as 1 mmHg, often used in vacuum science.

All these units measure the same physical thing pressure. They just use different sizes or are based on different ideas, like air at sea level or a column of liquid.

FAQs

What is the basic unit of pressure in the SI system

The basic SI unit is the pascal (Pa). It means one newton of force acting on one square meter of area.

Is pressure the same as force

No. Force is just the push or pull. Pressure tells you how that force is spread over an area. The same force can give high pressure on a small area or low pressure on a large area.

Why do sharp objects cut better

A knife has a very small cutting edge area. When you push with your hand, the force is focused on that tiny area, so the pressure is very high. High pressure lets the knife cut more easily.

What is atmospheric pressure

Atmospheric pressure is the pressure of the air around Earth pressing on everything. At sea level it is about 1 atm, which is 101 325 Pa or about 101.3 kPa.

Why do my ears pop in a plane or in the mountains

As you go higher, outside air pressure becomes lower. Your ears have air trapped inside. When outside pressure changes, your ears adjust by letting air move in or out. You feel this as a pop.

How does pressure change under water

Water is heavy, so the deeper you go, the more water is above you, and the higher the pressure. This is why divers must be careful as they go deeper and when they come back up.

Why do we still use many different pressure units

Different fields and countries grew up with their own units. For example, doctors are used to mmHg for blood pressure, and car owners in the United States are used to psi for tyres. Science tries to use Pa, but everyday life still uses older units.

How can I quickly estimate pressure conversions

• 1 atm ≈ 100 kPa ≈ 1 bar.
• 1 bar ≈ 15 psi.
• 750 to 760 mmHg ≈ 1 atm.

These rounded values are not perfect, but they are good for quick mental estimates.

Can pressure be zero

In theory, zero pressure would mean no particles pushing on anything at all, a perfect vacuum. In real life we can get very low pressures, but never a perfect zero.

What is gauge pressure and absolute pressure

Absolute pressure is measured from a perfect vacuum, starting at zero. Gauge pressure is measured compared to the local air pressure. Many pressure meters, like tyre gauges, show gauge pressure. For example, a tyre at 220 kPa gauge pressure is at about 220 kPa above the outside air pressure.