The mmHg to atm and Torr to atm conversions both change a mercury-style pressure into standard atmospheres and back, and the mmHg page links to the Torr page as a closely related unit. The distinction is definitional. Torr is exactly one 760th of a standard atmosphere, so the Torr page divides by 760. Physical mmHg comes from the height of a mercury column and depends on mercury temperature, so the mmHg page uses the conventional 0 °C factor of 133.3224 Pa per mmHg, which makes one atmosphere about 759.99982 mmHg. In ordinary use the two units are treated as numerically equal, but they are not identical by definition.
What each calculator does
The mmHg to atm conversion works in both directions. In mmHg to atm mode it applies atm = mmHg × 133.3224 / 101325; in atm to mmHg mode it applies mmHg = atm × 101325 / 133.3224. It displays atmosphere results to six decimal places and mmHg results to three. Its worked example converts 610 mmHg: 610 × 133.3224 / 101325 = 0.8026317690599555, displayed as 0.802632 atm, and the round trip back through 759.9998199852389 returns exactly 610 mmHg. The page stresses that 760 mmHg is approximately — not exactly — one atmosphere, and that a blood-pressure value of 120 mmHg is a pressure difference, not an absolute pressure.
The Torr to atm conversion is vacuum-aware. It converts Torr to atm by dividing by 760, atm to Torr by multiplying by 760, and derives supporting rows for mmHg, pascals (133.32236842105263 Pa per Torr), and psi (0.0193367747 psi per Torr). Its worked example converts a vacuum chamber at 120 Torr: 120 / 760 = 0.1578947368, displayed as 0.157895 atm, with rows of 120 mmHg, 15,998.68 Pa, and 2.320413 psi. Entering 0.25 atm the other way returns 190 Torr. A perfect vacuum is 0 Torr, and standard atmosphere is 760 Torr.
Side-by-side
| mmHg to atm conversion | Torr to atm conversion | |
|---|---|---|
| Definition basis | Conventional millimeter of mercury at 0 °C | Exactly one 760th of a standard atmosphere |
| Primary factor | 133.3224 Pa per mmHg; 1 atm ≈ 759.99982 mmHg | 760 Torr per atm; Pa and psi rows derived from Torr |
| Atmosphere link | 760 mmHg converts to 1.000000 atm (approximately 1) | 760 Torr is exactly 1 atm |
| Supporting rows | Entered pressure and the conventional factor | Equivalent mmHg, pascals, and psi |
| Display | Atmospheres to six decimals, mmHg to three | Example output at six decimals, with Torr, mmHg, Pa, psi rows |
| Typical context | Barometers, medical readings, lab tables | Vacuum work, gas-law problems, manometers |
| Worked example | 610 mmHg = 0.802632 atm | 120 Torr = 0.157895 atm (also 120 mmHg, 15,998.68 Pa) |
When to use which
Use the Torr to atm conversion when your pressure is described in Torr, as is common in vacuum practice and gas-law problems. The Torr scale maps naturally to vacuum ranges: a rough vacuum might be hundreds of Torr, a stronger laboratory vacuum a few Torr or less, and atmospheric pressure is 760 Torr, which is easier to read than a small fraction of an atmosphere.
Use the mmHg to atm conversion when your source uses millimeters of mercury — a mercury barometer, a chemistry table, a weather article, or a medical context such as blood pressure. The page’s conventional-factor design and its explicit warning that mmHg is not a length measurement and that blood-pressure values are not absolute pressures make it the right stop for mercury-column numbers.
Limits and disclaimer
Neither calculator corrects for a mercury temperature other than the stated reference or for local gravity, and both are educational unit conversions rather than medical or meteorological interpretation. The near-equality between mmHg and Torr is not an identity: the mmHg factor is conventional and limited to the table precision, while Torr is exact. Keep the reference label with every value — absolute, gauge, vacuum, or below atmospheric — because converting the unit does not change what the measurement means, and a unit conversion cannot repair the uncertainty of the instrument that produced the reading.