Mmhg to Psi Converter
Common Conversions
| mmHg | psi |
|---|---|
| 5.171 | 0.1 |
| 25.857 | 0.5 |
| 51.715 | 1 |
| 103.43 | 2 |
| 258.575 | 5 |
| 517.149 | 10 |
| 760 | 14.696 |
| 1034.299 | 20 |
| 1551.448 | 30 |
| 2585.747 | 50 |
| 5171.493 | 100 |
Why this conversion matters in chemistry
Medical-gas regulator testing is a typical place to need it. A 760 mmHg ventilator-drive pressure on the patient-side gauge is 14.70 psi on the cylinder-side regulator — the bridge value a biomed technician references when checking regulator droop against NFPA 99 medical-gas specifications. The constant of 1/51.7149 psi per mmHg is the standard-atmosphere identity (1 atm = 760 mmHg = 14.6959 psi). In practice you reach for it when SI mercury-column readings need to land in the US psi form pneumatic equipment uses.
Formula
Where the factor comes from
These two units share no ancestor. One is the height of a liquid, the other a force divided by an area, and the only thing joining them is that both have been redefined as fixed counts of pascals. So the route runs through the pascal twice. On the psi side the avoirdupois pound is exactly 0.45359237 kg and the inch exactly 0.0254 m, which with standard gravity puts 1 psi at 6894.757293 Pa. On the mercury side 1 mmHg is 133.322387415 Pa. Divide one by the other: 0.0193367775 psi per mmHg, or 51.7149 mmHg per psi. Every ingredient is exactly defined, yet the quotient never terminates — whatever you write down is a rounding of an exact ratio.
Precision and significant figures
Neither instrument in this exchange earns the six figures sitting in 51.7149. A mercury column read to half a millimeter already caps you near four figures at atmospheric, and a Bourdon dial is blunter: most are specified as a percentage of full scale rather than of reading, so a 30 psi face rated at two percent is uncertain by 0.6 psi — thirty-one millimeters of mercury — wherever the needle happens to sit. The mismatch runs one way. The manometer resolves finer than the gauge it is being compared against, so a converted value is almost always limited by the psi instrument rather than the mercury one.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
Exactly one psi in mmHg — the conversion anchor in reverse.
Half a psi — about a low-vacuum gauge reading.
30 psi — about a typical compressed-gas regulator output.
Common mistakes
Inches of mercury are not millimeters
US vacuum gauges are commonly graduated in inHg, and one inHg is exactly 25.4 mmHg — 0.4912 psi rather than 0.01934. Read a 25 inHg dial as though it were mmHg and you get 0.48 psi where 12.3 psi was right, the whole factor of 25.4 astray. Check the face for the length unit sitting in front of the Hg before converting.
Compound dials report vacuum as negative
A manometer reports absolute pressure; a compound dial reports the difference from the room. The same 100 mmHg absolute reads 1.93 psia on one scale and roughly −12.8 psig on the other. Convert the mercury figure and set it against the dial without applying the 14.696 offset, and two perfectly consistent instruments appear to disagree by the full 14.7 psi of an atmosphere.
Tolerances written in the other unit
A pressure-decay criterion of one psi over ten minutes is 51.7 mmHg — coarse enough that a mercury manometer shows the leak long before the dial twitches. Written the other way, a five mmHg tolerance is under a tenth of a psi, and no ordinary dial resolves it. Match the unit of an acceptance limit to the instrument that has to verify it.