Blood Pressure mmHg Explained

A normal blood pressure reading of 120/80 means

120 / 80 mmHg (systolic / diastolic)

What Blood Pressure Numbers Mean

Blood pressure is measured in millimeters of mercury (mmHg), a unit historically tied to mercury-column manometers, the original instruments used to measure it. A reading like '120/80' reports two values: the systolic pressure (120 mmHg, the pressure when the heart beats) and the diastolic pressure (80 mmHg, the pressure between beats). Both values are tiny compared to atmospheric pressure (760 mmHg), since blood pressure measures the pressure exerted by blood against artery walls, not ambient air pressure.

The Conversion Formula

To relate blood pressure in mmHg to the standard atmosphere, divide by 760:

To convert mmHg to Pascals, the SI pressure unit, multiply by 133.322:

Practical Examples

• 120 mmHg (normal systolic) ≈ 16,000 Pa, or about 0.158 atm
• 80 mmHg (normal diastolic) ≈ 10,666 Pa, or about 0.105 atm
• 140 mmHg (elevated systolic, hypertension threshold in many guidelines) ≈ 18,665 Pa
• 760 mmHg = 1 full atmosphere, about 6 times higher than typical systolic blood pressure

Why mmHg Remains the Medical Standard

Despite the Pascal being the official SI pressure unit, mmHg remains the universal standard in medicine for blood pressure measurement worldwide, because of its long historical use, the precision of mercury-based readings, and the familiarity of clinicians and patients with the resulting numbers.

Other Pressure Conversions

• • Convert psi to bar
• • Convert bar to psi
• • Convert psi to kPa
• • Convert psi to Pascal
• • Convert inHg to hPa
• • Tire Pressure: psi to bar

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