Torr to mmHg Converter
Common Conversions
| torr | mmHg |
|---|---|
| 1 | 1 |
| 10 | 10 |
| 25 | 25 |
| 50 | 50 |
| 100 | 100 |
| 200 | 200 |
| 400 | 400 |
| 500 | 500 |
| 600 | 600 |
| 700 | 700 |
| 760 | 760 |
| 1000 | 1000 |
Why this conversion matters in chemistry
Vacuum-science to clinical-physics translation crosses this identity. A diffusion-pump manifold sitting at 5 × 10⁻⁶ torr reads as 5 × 10⁻⁶ mmHg on a clinical-style gauge — identical for any practical purpose. The match was locked in by the IUPAC 1964 redefinition that fixed 1 torr at exactly 1/760 of a standard atmosphere, which the mmHg matches at standard mercury-column conditions. The conversion is the everyday type cast at the boundary between physical-chemistry vacuum work (torr) and clinical or physiology equipment (mmHg).
Formula
Where the factor comes from
These two reach a factor of almost, but not quite, one by routes that never reference each other. The torr is exactly 101325/760 Pa, a pure subdivision of the standard atmosphere with no fluid anywhere in it. The conventional millimeter of mercury is built the other way round, from a mercury density fixed by convention at 13595.1 kg/m³, standard gravity of exactly 9.80665 m/s², and a height of one millimeter — giving exactly 133.322387415 Pa. Dividing one exact value by the other yields 24125000000/24125003437, or 0.99999985753…, so a torr falls short of a millimeter of mercury by 0.142 parts per million. Both numbers are exact by convention; they simply come from different conventions, and their agreement to seven figures was designed rather than stumbled upon.
Precision and significant figures
That 0.14 ppm gap will never be the limiting term in anything you measure. Noticing it would require a pressure known to better than one part in ten million; primary standards work near that level, and ordinary laboratory gauges are three to five orders of magnitude coarser. The practical rule is to carry the number across unchanged, digits and all, and add nothing to it. The second half of that matters more than the first. Because the conversion costs nothing, there is a pull toward presenting the result more precisely than the source justified. A barometer that reported 754 torr reports 754 mmHg — not 754.00, and not 753.9999.
Worked Examples
Standard atmospheric pressure — the anchor.
About the vapor pressure of water at 20 °C.
Base unit equivalence — the conversion in its purest form.
About atmospheric pressure at the Everest summit.
Common mistakes
The identity does not extend to inches
Inches of mercury share the mercury convention but not the scale: one inHg is 25.4 mmHg, and therefore 25.4 torr to the same seven figures. Having internalized that torr and mmHg are interchangeable, it becomes easy to treat every mercury-denominated unit as interchangeable. A barometric reading of 29.9 inHg is about 760 torr, not 29.9 torr.
A real mercury column needs corrections
The conventional millimeter of mercury assumes one specific density and standard gravity. An actual manometer holds actual mercury at room temperature, less dense than the convention by close to 0.2 percent for every 10 K above its reference point, standing in whatever local gravity the building happens to have. Those corrections run into tenths of a percent — thousands of times larger than the definitional gap this page is about.
mmHg gauges that count down from ambient
Some instruments labeled in millimeters of mercury report vacuum as depth below ambient rather than as absolute pressure. A dial showing 700 on that kind of scale means roughly 60 torr absolute, not 700 torr — better than a tenfold difference hiding behind an identical-looking number. Establish whether zero on the face means hard vacuum or means the room before trusting a reading in either unit.