Two Definitions, One Pressure
Psi is force per area: one pound of force spread over one square inch, with the pound pinned by the 1959 agreement (0.45359237 kg exactly). mmHg is a column: the pressure at the base of a 1 mm tall column of mercury, pinned by mercury's density at 0 °C and standard gravity. The two definitions have nothing in common mechanically — force-area versus fluid-column — yet both describe the same physical pressure, which is why the conversion factor is a ratio of the two definitions and therefore exact. Divide 6,894.757293168 Pa (one psi) by 133.322387415 Pa (one mmHg) and you get 51.7149252…, the number this page applies.
mmHg = psi × 51.714925203869214
psi = 6,894.757293168 Pa · mmHg = 133.322387415 Pa
Exact — the ratio of two definitions, from NIST SP 811 and the 1959 agreement.
Vacuum and Medical Numbers, Both Scales
| psi | mmHg | Context |
|---|---|---|
| 0.147 psi | 7.5 mmHg | A strong shop vacuum pump (1 kPa in disguise). |
| 1 psi | 51.7 mmHg | Mild vacuum — roughly 2.04 inHg. |
| 5 psi | 258.6 mmHg | Typical industrial rough-vacuum operating point. |
| 14.696 psi | 760 mmHg | One standard atmosphere — the anchor of both scales. |
| 2.32 psi | 120 mmHg | Blood pressure systolic — the 120 of 120/80. |
The last row is the one that surprises people: a normal blood pressure of 120 mmHg is only 2.32 psi. The heart pushes blood with less than two and a half pounds per square inch — which is exactly why mmHg exists as a unit. Mercury is 13.6 times denser than water and 51.7 times denser than a psi's worth of force-area pressure, so the same small physiological pressures produce convenient two- and three-digit readings on a mercury column. A medical monitor reading 120/80 and a pump gauge reading 2.3 psi are describing the same pressure; only the measuring tradition differs.
Engineering Context
The reverse direction is mmHg to psi. This pair completes the mercury side of the pressure hub, joining mmHg to kPa, kPa to mmHg, mmHg to Pa and Pa to mmHg; the psi side links through psi to kPa, kPa to psi, and the atmosphere anchors psi to atm, atm to psi. The mercury-column ancestor of this unit is the torr — see torr to Pa and Pa to torr. Everything lives in the pressure hub.
More: mmHg to psi · psi to kPa · mmHg to kPa · Pressure Hub
Related Unit Converters
Frequently Asked Questions
How do I convert psi to mmHg?
Multiply by 51.714925: mmHg = psi × 51.7149. A 5 psi vacuum is 5 × 51.7149 = 258.6 mmHg. The factor is exact — it comes from dividing the defined psi (6,894.757293168 Pa) by the defined mmHg (133.322387415 Pa).
How many mmHg is one atmosphere?
760 mmHg — the same pressure as 14.696 psi and 101.325 kPa. That is the mercury column Torricelli measured in 1643: about 760 mm tall at sea level. So psi and mmHg meet at the atmosphere: 14.696 psi × 51.7149 = 760 mmHg.
Why do vacuum systems use both psi and mmHg?
Because the two scales were born in different trades. American pumps and shop gauges are marked in psi (or inches of mercury — inHg, where 1 psi ≈ 2.036 inHg), while laboratories, anesthesia machines, and vacuum spec sheets still quote mmHg from the mercury-column tradition. Both measure the same pressure; the conversion factor 51.7149 is exact and this page is the bridge.
More: mmhg to psi · psi to kpa · Pressure Hub