Atmospheres to mmHg (Torr) Converter
Common Conversions
| atm | mmHg |
|---|---|
| 0.01 | 7.6 |
| 0.05 | 38 |
| 0.1 | 76 |
| 0.25 | 190 |
| 0.5 | 380 |
| 0.75 | 570 |
| 1 | 760 |
| 1.5 | 1140 |
| 2 | 1520 |
| 3 | 2280 |
| 5 | 3800 |
| 10 | 7600 |
Why this conversion matters in chemistry
mmHg — sometimes called torr — is the unit you'll see most often in vapor-pressure data, especially in older references like the Antoine equation tables that still get used today. The trouble comes when the vapor pressure needs to drop into an ideal gas law that expects atm. A Raoult's-law partial pressure for methanol at 25°C might come out as 127 mmHg, which is 0.167 atm. Skip dividing by 760 and the answer is off by a factor of 760 — a mistake that survives all the way through a homework problem without tripping any obvious alarms.
Formula
Where the factor comes from
This pair has two defensible answers, and which one you want depends on how strictly you read the symbol. Treat mmHg as a synonym for the torr — which is what essentially every chemistry table and every manometer scale does — and 1 atm = 760 mmHg exactly, since the torr is defined as one seven-hundred-and-sixtieth of the standard atmosphere. Take the strict route instead, where the conventional millimeter of mercury is built from an assigned density of 13595.1 kg/m³ and standard gravity 9.80665 m/s², and 1 mmHg = 133.322387 Pa, which puts 1 atm at 759.99989 mmHg. The two disagree by about one part in seven million. That gap is a fossil: a 760 mm conventional mercury column actually delivers 101325.0144 Pa, and the round 101325 was adopted in its place.
Precision and significant figures
Use 760, and use it as an exact integer. The strict definition's 759.99989 differs in the seventh significant figure, and no mercury manometer ever built resolves that. Figures in your result should come from the reading, so a column read to the nearest millimeter supports three or four and no more. Precision leaks away in the physical measurement rather than the arithmetic: a column at room temperature rather than its 0 °C reference stands roughly 0.14 mmHg high for each kelvin of excess, and local gravity departs from the standard value by a couple of parts per thousand with latitude and elevation. Both corrections swamp the definitional difference.
Worked Examples
The anchor conversion. This ratio was the original definition of the standard atmosphere before the pascal came along.
Half an atmosphere. Typical working pressure for a vacuum distillation of a moderately volatile solvent.
Roughly the pressure an autoclave reaches during steam sterilization — enough to push water's boiling point above 120°C.
Water's vapor pressure at 25°C. Worth keeping in mind when interpreting bubble-point calculations or a room-temperature gas measurement.
Common mistakes
Antoine constants are unit-specific
Antoine coefficients are tabulated for one particular pressure unit, and the A term absorbs any change: going from mmHg to kPa shifts A down by 0.875. Convert the calculated pressure afterwards, or convert the constants, but never take mmHg-based coefficients and read the output as though it were kilopascals.
Applying the bar factor to atm
1 bar is 750.06 mmHg while 1 atm is 760, and the two get transposed constantly because both get called standard pressure. Using 750 where 760 belongs under-reports by 1.31 percent — the same 1.3 percent that separates the atmosphere from the bar, arriving by a route that is considerably harder to spot.
Open-arm manometers give differences
A U-tube open to the room reports the height difference between its arms, which is sample pressure minus ambient. Reading that difference as an absolute pressure omits the barometric term entirely. On a closed manometer with an evacuated reference arm the same reading really is absolute — and the two instruments look nearly identical on the bench.