SI Pressure Units: Pascal (Pa) and Its Prefixes
The SI unit for pressure is the pascal (Pa). SI pressure values often look “too small” in Pa, so engineers typically use KPA O MPa, while meteorology commonly uses hPa.
What exactly is 1 pascal?
From the SI derived-unit relationships, pascal is expressed in base units as:
- N = kg·m·s⁻² (newton)
- Pa = kg·m⁻¹·s⁻² (pascal)
From those two lines, you can directly derive the familiar engineering identity:
1 Pa = 1 N/m² (one newton of force acting on one square meter).
Why Pa is often replaced by kPa or MPa
1 Pa is small. That’s why SI prefixes matter:
Common SI multiples used in engineering
- 1 kPa = 10³ Pa
- 1 MPa = 10⁶ Pa
- 1 GPa = 10⁹ Pa
Meteorology special case: hPa
Meteorology commonly uses hectopascal:
- 1 hPa = 100 Pa
- 1 hPa = 1 mbar
Practical SI scaling examples
- Typical atmospheric pressure: ~101.325 kPa (sea-level reference)
- Pneumatics: 600 kPa is a common scale (≈ 6 bar)
- Hydraulics: 10–35 MPa is a common working range
Converting non-SI to SI (recommended for datasheets)
- bar → Pa: multiply by 100,000
- psi → Pa: multiply by 6,894.757
- mmHg → Pa: multiply by 133.3224
- atm → Pa: multiply by 101,325
Engineering clarity
For global buyers, a clean pattern is:
- Primary (SI): kPa / MPa
- Secondary (market): bar / psi in parentheses
- Always state reference if relevant: (abs) / (g) / (Δp)
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FAQ (SI Pressure Units)
Is Pa a base unit?
No, it’s an SI derived unit.
Why does weather use hPa instead of kPa?
Because atmospheric pressure sits near ~1000 hPa, and 1 hPa = 1 mbar historically.
Is 1 bar exactly 100 kPa?
Yes: 1 bar = 100 kPa = 10⁵ Pa.
Is mmHg an SI unit?
No. It’s still common in medical/manometer contexts and convertible to Pa.
What’s better for hydraulics: bar or MPa?
Both work, but MPa is SI-friendly and maps neatly to material/mechanical specs.
What’s the safest “universal” SI unit to publish?
Usually KPA (HVAC/IAQ) or MPa (industrial/hydraulics), depending on magnitude.
