mmHg to Millibar Converter
Common Conversions
| mmHg | mbar |
|---|---|
| 0.01 | 0.01333 |
| 0.1 | 0.13332 |
| 0.25 | 0.33331 |
| 0.5 | 0.66661 |
| 1 | 1.33322 |
| 2 | 2.66645 |
| 5 | 6.66612 |
| 10 | 13.332 |
| 25 | 33.331 |
| 50 | 66.661 |
| 100 | 133.322 |
| 1000 | 1333.22 |
Why this conversion matters in chemistry
Boiling-point barometric correction is a worked example. A 760 mmHg standard atmosphere is 1013.25 mbar — the reference an atmospheric numerical-weather model uses, and equivalently the reference pressure a chemistry lab uses to correct distillation boiling points to standard atmospheric. The multiplier of 1.33322 mbar per mmHg reduces to 1 atm = 760 mmHg = 101325 Pa = 1013.25 mbar. It's the unit step at the boundary of mercury-manometer readings and the pressure-controller displays modern equipment uses.
Formula
Where the factor comes from
Here the mercury is real and the arithmetic follows the column. A millimeter of mercury is the pressure that a millimeter of the fluid exerts under its own weight: density times gravity times height, with the density fixed by convention at 13595.1 kg/m³ and gravity at the standard 9.80665 m/s². That product is 133.322387 Pa. The millibar has no fluid in it whatsoever — it is exactly 100 Pa, the bar's 10⁵ moved by the milli- prefix. Dividing one by the other gives 1.333224 mbar per mmHg. Read mmHg as the torr instead, defined straight from the atmosphere, and the same figure emerges as 1013.25/760 = 1.3332237, differing from the column value by about 1.4 parts in 10⁷.
Precision and significant figures
Four-thirds is the shortcut worth carrying: 1.333224 sits 0.0082 percent below 4/3, so multiplying a mercury reading by four and dividing by three is good to roughly one part in 12000 — finer than any manometer you will ever read. On 760 mmHg it overshoots by 0.08 mbar. Beyond that, six figures is generous and the two definitions of mmHg agree far past the point of interest. What actually limits the answer is the column: read to the nearest millimeter, a barometric measurement is uncertain by about 1.3 mbar, so the fourth figure in the converted value is already soft.
Worked Examples
Standard atmospheric pressure expressed in both unit systems.
A small pressure increment on a manometer scale.
About a low-vacuum gauge reading for solvent removal.
About a moderate vacuum on a distillation setup.
Common mistakes
Legacy solvent tables typed into modern controllers
Reduced-pressure boiling-point tables for common solvents are published in mmHg, while rotary-evaporator and pump controllers take their setpoint in millibars. Entering 100 from an mmHg table into an mbar field asks for 100 mbar rather than 133 mbar — a quarter deeper than intended, which is roughly the margin that separates a controlled distillation from a flask that bumps.
The factor assumes a 0 °C column
That 13595.1 kg/m³ density is mercury at 0 °C. A column standing in a 25 °C room is less dense and rises higher for the same pressure, by roughly 0.14 mmHg per kelvin near atmospheric — about 3.4 mmHg, or 4.6 mbar, at room temperature. Barometric work applies the temperature correction to the reading before the conversion, not after it.
Close magnitudes hide a mislabeled dataset
The two units differ by only a third, so a vapor-pressure curve digitized in mmHg and plotted against instrument data in millibars gives a smooth, plausible, entirely wrong comparison. A hundredfold unit error announces itself immediately; a 33 percent one just looks like a systematic offset and gets blamed on calibration. Label the axis with its unit, every time.