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What units are used for pressure in the ideal gas law?

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A diagram showing a bottle with a balloon over its opening, first at room temperature and then heated. As the bottle is heated, the air inside expands, inflating the balloon to demonstrate the principle of thermal expansion.
gas

In the ideal gas law, the standard unit for pressure within the International System of Units (SI) is the pascal (Pa), which is defined as one newton per square metre (N/m²).12

While the pascal is the official SI unit, other units for measuring pressure are also frequently encountered in scientific literature and practical applications, including:

  • Atmosphere (atm): Defined as exactly 101,325 Pa.13
  • Bar: Equivalent to 100,000 Pa (or 0.1 MPa).4
  • Kilopascal (kPa): Equal to 1,000 pascals, often used because the pascal itself is a relatively small unit.3
  • Pounds per square inch (psi): A non-SI unit where one psi is approximately 6.895 kPa.13

When using the ideal gas law equation (PV=nRT), it is essential to ensure that the units for pressure, volume, and temperature are consistent with the universal gas constant (R) being utilized.12