What is a Millimeter of Mercury?

The millimeter of mercury equals approximately 133.322 pascals and is the universal medical unit for blood pressure measurements worldwide. Normal blood pressure is around 120/80 mmHg (systolic/diastolic). The unit's origin is the manometer: a column of mercury rises 1 mm for every 133 Pa of pressure difference. Mercury barometers historically measured atmospheric pressure (760 mmHg = 1 atm at sea level). Beyond medicine, mmHg appears in vacuum-system specifications, certain laboratory contexts, and altimeter settings (in inHg in the US, in mmHg or hPa elsewhere). The mmHg relates to the torr (1 mmHg ≈ 1.0000003 torr — essentially identical), the pascal (1 mmHg ≈ 133.3 Pa), the atmosphere (760 mmHg = 1 atm), and inches of mercury (1 inHg = 25.4 mmHg). The unit's persistence in medicine reflects historical inertia and the precision of mercury manometers.

Visual reference for how the millimeter of mercury relates to other pressure units. Each row links to the full converter for that pair.

• Blood pressure measurement globally
• Vacuum-system specifications
• Historical scientific texts

• ✓1 mmHg ≈ 133.32 Pa ≈ 1 Torr.
• ✓760 mmHg = 1 atm.
• ✓Blood-pressure cuffs display mmHg worldwide.

• ✗Confusing mmHg with mmH₂O (different unit, factor of 13.6).
• ✗Mixing mmHg with torr in precise vacuum physics — they differ by one part per million.
• ✗Writing blood pressure in kPa — use mmHg for medical continuity.