Pressure of a gas is the force that gas particles exert per unit area on the walls of their container. It comes from billions of fast‑moving molecules colliding with surfaces and pushing on them.

Common definition (school level)

  • In simple terms:
    Pressure === force ÷ area.
    For gases, this force is due to particle collisions with the container walls.

Main units used to measure gas pressure

The SI unit (standard scientific unit) of pressure is:

  • Pascal (Pa)
    • 1 Pa = 1 newton per square metre (1 N/m²)

Other very common units you’ll see in chemistry and physics:

  • Kilopascal (kPa)
    • 1 kPa = 1,000 Pa
  • Atmosphere (atm)
    • 1 atm ≈ 101,325 Pa
  • Bar
    • 1 bar = 100,000 Pa
  • Millimetre of mercury (mmHg)
  • Torr (very close to mmHg; in practice often taken as equal)
  • Pounds per square inch (psi), used a lot for tyres and engineering

Quick HTML table of units

html

<table>
  <thead>
    <tr>
      <th>Unit</th>
      <th>Symbol</th>
      <th>Approximate relation to 1 atm</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Pascal</td>
      <td>Pa</td>
      <td>1 atm ≈ 101325 Pa</td>
    </tr>
    <tr>
      <td>Kilopascal</td>
      <td>kPa</td>
      <td>1 atm ≈ 101.3 kPa</td>
    </tr>
    <tr>
      <td>Atmosphere</td>
      <td>atm</td>
      <td>1 atm = 1 atm (reference)</td>
    </tr>
    <tr>
      <td>Bar</td>
      <td>bar</td>
      <td>1 atm ≈ 1.013 bar</td>
    </tr>
    <tr>
      <td>Millimetre of mercury</td>
      <td>mmHg</td>
      <td>1 atm ≈ 760 mmHg</td>
    </tr>
    <tr>
      <td>Torr</td>
      <td>torr</td>
      <td>1 atm ≈ 760 torr</td>
    </tr>
    <tr>
      <td>Pounds per square inch</td>
      <td>psi</td>
      <td>1 atm ≈ 14.7 psi</td>
    </tr>
  </tbody>
</table>

Story-style picture: Imagine a sealed box full of super‑bouncy balls flying around in all directions. Every time they hit the walls, they push on them. The “push per area” of all those hits together is the gas pressure.

TL;DR:
Gas pressure = force per unit area due to gas particle collisions, usually measured in pascal (Pa), but also in kPa, atm, bar, mmHg, torr, and psi.