Bar to mmHg Converter
Common Conversions
| bar | mmHg |
|---|---|
| 0.01 | 7.501 |
| 0.05 | 37.503 |
| 0.1 | 75.006 |
| 0.25 | 187.516 |
| 0.5 | 375.031 |
| 1 | 750.062 |
| 1.01325 | 760 |
| 1.5 | 1125.093 |
| 2 | 1500.124 |
| 5 | 3750.31 |
| 10 | 7500.62 |
Why this conversion matters in chemistry
Antoine-equation tables and other classic vapor-pressure references use mmHg. Modern industrial vapor-pressure curves and IUPAC-standard thermodynamic data use bar. The conversion is a multiply by 750.062 — a factor that looks ugly but drops directly out of 1 atm = 760 mmHg combined with 1 atm = 1.01325 bar. Water's vapor pressure at 100°C is 1.01325 bar, which reads as 760 mmHg; at 25°C, 0.0317 bar reads as 23.8 mmHg. Reading across bar-based and mmHg-based data is the main use case, and the conversion is exact by construction.
Formula
Where the factor comes from
Which number you get depends on what you take mmHg to mean, and both answers are defensible. Read it as the torr — which is what vapor-pressure tables and manometer scales intend — and the route runs bar to atmosphere to torr: 10⁵ ÷ 101325 × 760, or exactly 3040000/4053, giving 750.061683 mmHg per bar. Read it strictly and the conventional millimeter of mercury is built from an assigned mercury density of 13595.1 kg/m³ and standard gravity of 9.80665 m/s², putting 1 mmHg at 133.322387415 Pa and a bar at 750.061576 of them. The two disagree in the seventh significant figure, about one part in seven million. Neither involves a measurement; they are simply two different definitions, and the printed 750.062 satisfies both.
Precision and significant figures
750.062 is the practical form, and it already sits past the point where the mmHg-versus-torr ambiguity could bite: both definitions round to those same six figures and part company only in the seventh, a smaller gap than the residual left by the rounding itself. Round instead to a flat 750 and an atmosphere-scale reading lands about 0.06 mmHg light — invisible against a column you are reading to the millimeter. The reading constrains everything anyway. A mercury column read by eye yields three or four figures at best, and the column's temperature matters far more than the factor's digits, since mercury expands enough to shift a near-atmospheric reading by roughly a tenth of a millimeter per kelvin above its 0 °C reference.
Worked Examples
IUPAC standard pressure. Just below 1 atm, which is where the 10 mmHg gap between 1 bar and 1 atm originates.
Sea-level atmospheric pressure expressed in both units. The anchor that ties the two scales together.
A moderate reduced pressure, typical for rotovap distillation of mid-volatility solvents.
Roughly the elevated pressure inside an autoclave at 121°C during steam sterilization.
Common mistakes
Digital mmHg is a relabeled pascal reading
A digital gauge displaying mmHg is measuring with a diaphragm and dividing by 133.322, not balancing mercury. The temperature and local-gravity corrections that a real column needs do not apply to it, and applying them anyway introduces an error where none existed. Know which kind of instrument produced a number before deciding whether it wants correcting.
Water manometers also read in millimeters
Low-pressure work sometimes uses a water-filled manometer graduated in millimeters, and mm of water is not mm of mercury — the two differ by the density ratio, near 13.6. One bar is about 750 mmHg but roughly 10197 mmH₂O. Comparing a converted mmHg figure against a water column reading without that factor is wrong by more than an order of magnitude.
Inches of mercury on North American gauges
Many barometers and vacuum gauges are graduated in inches of mercury, where an inch is exactly 25.4 mmHg. One bar is 750.06 mmHg or 29.53 inHg, and both numbers get written as "Hg" on hurried notes. Reading a converted mmHg value against an inHg scale, or the reverse, misses by a factor of 25.4.