The Number That Looks Wrong but Isn't
Every American learns "120 over 80" and never questions the unit. Convert it to psi and the reading becomes 2.32 over 1.55 — numbers that look like a tire that needs air, not a healthy heart. The reason the same pressure reads 120 in one unit and 2.32 in the other is geometry: psi spreads the force over a square inch of artery wall, while mmHg measures the height of a dense mercury column that concentrates the same pressure into a thin tube. Mercury is 13.6 times denser than water, and a psi's worth of force lifts a 51.7 mm column of it. The scale difference is a physical property, not a medical convention — which is exactly why the conversion factor is exact.
psi = mmHg × 0.019336777
mmHg = 133.322387415 Pa · psi = 6,894.757293168 Pa
Exact — the ratio of two definitions, from NIST SP 811.
Mercury Readings on the Gauge Scale
| mmHg | psi | Context |
|---|---|---|
| 80 mmHg | 1.55 psi | Blood pressure diastolic — the 80 of 120/80. |
| 120 mmHg | 2.32 psi | Blood pressure systolic — the 120 of 120/80. |
| 140 mmHg | 2.71 psi | Hypertension threshold (WHO/ESC). |
| 258.6 mmHg | 5 psi | Industrial rough-vacuum operating point. |
| 760 mmHg | 14.696 psi | One standard atmosphere — where both scales anchor. |
The hypertension row shows why this conversion matters clinically: 140 mmHg — the threshold at which treatment begins — is 2.71 psi. A blood pressure transducer measures in pascals internally and converts to the unit its market demands; the conversion firmware inside an automated cuff does this exact multiplication thousands of times a day. When a cuff calibrated for mmHg meets a display configured for psi, a 2 psi rounding error is a 15 mmHg mistake on the chart — the kind of unit slip that turns a normal reading into an alarming one.
Engineering Context
The forward direction is psi to mmHg, the vacuum-gauge page. This pair completes the cross-connection between the two biggest scales in the pressure hub: the mercury family (mmHg to kPa, kPa to mmHg, mmHg to Pa, Pa to mmHg) and the American gauge family (psi to kPa, kPa to psi, psi to atm, atm to psi). The torr is the mmHg's scientific twin — see torr to Pa. All of it lives in the pressure hub.
More: psi to mmHg · mmHg to kPa · psi to kPa · Pressure Hub
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Frequently Asked Questions
How do I convert mmHg to psi?
Multiply by 0.0193368: psi = mmHg × 0.019337. A blood pressure of 120 mmHg is 120 × 0.019337 = 2.32 psi. The factor is exact — the reciprocal of 51.7149252, the ratio of the defined psi and mmHg.
What is normal blood pressure in psi?
The healthy reading of 120/80 mmHg is 2.32/1.55 psi. It looks startlingly small because psi spreads force over a full square inch while the mercury column concentrates it into a thin tube — but it is the same pressure, and it is why mmHg exists: to give physiological pressures convenient two-digit numbers.
Why does medicine use mmHg instead of psi?
Because the first blood pressure cuff, Riva-Rocci's 1896 sphygmomanometer, read directly off a mercury column — millimeters of mercury was the instrument's own scale, and clinical practice never left it. A mercury column at 0 °C is defined as 133.322387415 Pa, so the conversion to psi (6,894.757 Pa) is exact: 120 mmHg = 2.32 psi.
More: psi to mmhg · mmhg to kpa · Pressure Hub