PSI to mmHg Converter
Common Conversions
| psi | mmHg |
|---|---|
| 0.1 | 5.171 |
| 0.5 | 25.857 |
| 1 | 51.715 |
| 2 | 103.43 |
| 5 | 258.575 |
| 10 | 517.149 |
| 14.696 | 760 |
| 20 | 1034.299 |
| 30 | 1551.448 |
| 50 | 2585.747 |
| 100 | 5171.493 |
Why this conversion matters in chemistry
Pulmonary function test calibration manifolds operate on psi at the gauge but export mmHg in the patient report. A 2.0 psi calibration test pressure becomes 103.4 mmHg on the spirometer's calibration certificate, alongside the ambient barometric reading. A factor of 51.7149 mmHg per psi is just the standard-atmosphere identity (1 atm = 14.696 psi = 760 mmHg). Mostly bookkeeping at the boundary of pneumatic-equipment psi readings and mmHg clinical reporting.
Formula
Where the factor comes from
Standard gravity sits inside both definitions, and canceling it is the whole trick. A pound-force is the avoirdupois pound multiplied by exactly 9.80665 m/s². The conventional millimeter of mercury is an assigned density of 13595.1 kg/m³ multiplied by that identical acceleration and by one millimeter of height. Form the ratio and the 9.80665 disappears, leaving a bare comparison of areal masses: 0.45359237 kg over 6.4516 × 10⁻⁴ m² is 703.0696 kg/m², set against 13.5951 kg/m² for the millimeter of mercury, and the quotient is 51.714925 mmHg per psi. No mercury is weighed and no gravimeter is consulted anywhere in that chain. The density is assigned by convention rather than measured, so the factor comes out exact despite every ingredient sounding thoroughly physical.
Precision and significant figures
The quoted 51.7149 rewards a careful read, because the conventional millimeter of mercury and the torr separate at the seventh digit — 51.714925 against 51.714933 — and four figures cover both without anyone having to decide which was meant. Everything else in play is larger. Mercury's density falls about 0.018 percent per kelvin, so a real column at 22 °C stands some three millimeters taller near atmospheric than the 0 °C reference the unit assumes, until the temperature correction is applied. That gap is orders of magnitude wider than the definitional one, which is why the mmHg-versus-torr argument never actually reaches a bench.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
The conversion anchor — useful for any quick mental check.
About a low-vacuum gauge reading.
About a moderate compressed-gas regulator output.
Common mistakes
A gauge zero error is magnified in mmHg
One psi is 51.7 mmHg, so a Bourdon gauge sitting a tenth of a psi off zero — invisible on the dial, well inside its tolerance — becomes a 5.2 mmHg offset in the converted value. On a manometer that is a glaring, plainly readable discrepancy. Check and record the psi instrument's zero before converting, because the unit change turns a shrug into an argument.
Partial pressures need the absolute value
A regulator showing 100 psi is reading gauge, so the absolute pressure is 114.7 psia, or 5932 mmHg. Apply a 20 percent oxygen mole fraction to that and the partial pressure is 1186 mmHg. Multiply the mole fraction by the converted gauge figure of 5171 mmHg instead and you get 1034 mmHg, low by nearly 13 percent, with nothing in the arithmetic to flag it.
Converted values above 760 outrun the manometer
A mercury column is a bounded instrument: a U-tube with a one-meter arm cannot resolve much beyond 760 mmHg of differential. Convert a 30 psi regulator setting and you get 1551 mmHg, a perfectly correct number that no mercury manometer in the room can verify. Above roughly one atmosphere the mmHg label is bookkeeping only, and a dial or transducer has to be the reference.