Pressure in the International System of Units
Pressure is defined as force per unit area, and in the International System of Units (SI), the coherent derived unit is the pascal (Pa), equal to one newton per square metre (N/m²). This reference explains the SI unit and its decimal multiples, how these units relate to everyday and technical pressure measurements, and where non‑SI units such as bar and atmosphere remain common. The aim is to clarify exact definitions and practical conversions so you can move between standards confidently.
Definition of the Pascal
The pascal is the SI unit of pressure, named after Blaise Pascal. One pascal is one newton applied uniformly over one square metre. Because the newton itself is kg·m/s², the pascal can also be expressed as kg/(m·s²). In many engineering and scientific contexts, pressures are large relative to one pascal, so the kilopascal (kPa, 1,000 Pa) and the megapascal (MPa, 1,000,000 Pa) are commonly used. The pascal is a coherent unit, meaning it is derived directly from the base SI units of mass, length, and time without additional numerical factors.
Pascal multiples and submultiples
- Kilopascal (kPa): 1,000 Pa, often used in meteorology and engineering.
- Megapascal (MPa): 1,000,000 Pa, typical for mechanical stress and material strength.
- Gigapascal (GPa): 1,000,000,000 Pa, used in geology and high‑pressure physics.
Non‑SI Units Still in Common Use
While the pascal is the coherent SI unit, other units are widely employed in practice. The bar, close to one average atmospheric pressure, is accepted for use with the SI in many countries. Standard atmospheres (atm) and millimetres of mercury (mmHg) or inches of mercury (inHg) remain common in weather reporting, aviation, and medicine. These are not SI units, but conversions to pascals are well defined and stable.
Common pressure units and exact conversions
| Unit | Name | Exact SI relation | Typical use cases |
|---|---|---|---|
| Pa | pascal | 1 Pa = 1 N/m² | Scientific and engineering standards |
| kPa | kilopascal | 1 kPa = 1,000 Pa | Everyday engineering and weather |
| MPa | megapascal | 1 MPa = 1,000,000 Pa | Material strength and mechanical systems |
| bar | bar | 1 bar = 100,000 Pa = 100 kPa | Automotive tyres, process engineering |
| atm | standard atmosphere | 1 atm = 101,325 Pa | Weather, aviation, chemistry |
| mmHg | millimetre of mercury | 1 mmHg ≈ 133.322 Pa | Blood pressure, barometric pressure |
| inHg | inch of mercury | 1 inHg ≈ 3,386.389 Pa | Aviation and weather in some regions |
Pascal in Everyday and Technical Contexts
Atmospheric pressure at sea level is close to 101,325 Pa, or roughly 101.3 kPa, which is often rounded to 1 bar for practical use in many industries. Tyre pressures are commonly quoted in bar or in pounds per square inch (psi) in some markets, though bar aligns cleanly with SI because it is exactly 100,000 Pa. In scientific work, reporting in pascals or kilopascals keeps units consistent with other SI measures such as energy, volume, and temperature, simplifying calculations and reducing conversion errors.
Using SI Pressure Units Correctly
To avoid mistakes, ensure unit consistency: when using the ideal gas law or other equations, match the pressure unit with the units of volume and temperature. For example, if using SI base units, express pressure in pascals, volume in cubic metres, and temperature in kelvins so that the gas constant has a single coherent value. When working with bar or atmospheres, convert to pascals before combining with other SI quantities, or use the appropriate form of the gas constant that matches your units.
Practical checklist
- Use pascals or kilopascals when full SI coherence is required.
- State the unit explicitly in any table, datasheet, or report.
- Convert non‑SI values to SI before combining them in calculations.
- Prefer symbols (Pa, kPa, bar, atm) to avoid ambiguity.
- Verify gas constants and conversion factors for the units you are using.
Historical Context and Adoption
The SI system has evolved since its mid‑20th century adoption, with the pascal being recommended for pressure since the early days of coherent SI. Over time, meteorologists, engineers, and scientists have standardized on the pascal as the official unit, while certain fields retain bar or atmospheres for convenience. Modern standards bodies such as the International Bureau of Weights and Measures (BIPM) and the International Union of Pure and Applied Physics (IUPAP) reaffirm the definitions and exact numerical values for these conversions, ensuring continuity over decades. Core standards are stable and not time‑sensitive.
Key Differences and When to Convert
Because the SI unit is small relative to everyday pressures, multiples such as kilopascal and megapascal keep numbers manageable, while bar and atmosphere remain convenient for approximate comparisons to ambient air pressure. Exact conversion factors are fixed and internationally agreed, so you can confidently switch between units as needed. For peer‑reviewed work or regulatory submissions, using SI units is typically expected, whereas bar and atmospheres may still appear in specifications, weather reports, and clinical measurements.
Summary and Best Practices
The SI unit of pressure is the pascal, with kilopascal, megapascal, and gigapascal as common multiples. Non‑SI units such as bar and atmosphere have precise, stable definitions in SI terms and remain useful in specific domains. Use the appropriate unit for your field, state it clearly, and convert consistently using exact defined factors. This approach ensures accuracy, clarity, and long‑term usefulness of your technical communication and calculations.