mmHg to Bar Converter
Common Conversions
| mmHg | bar |
|---|---|
| 1 | 0.001333 |
| 10 | 0.01333 |
| 50 | 0.06666 |
| 100 | 0.13332 |
| 200 | 0.26664 |
| 400 | 0.53329 |
| 500 | 0.66661 |
| 600 | 0.79993 |
| 700 | 0.93326 |
| 750.062 | 1 |
| 760 | 1.01325 |
| 1000 | 1.33322 |
Why this conversion matters in chemistry
The bar is the pressure unit IUPAC picked when it wanted standard states in round SI-compatible numbers — 1 bar is exactly 100 kPa, which is a hair below atmospheric. Mercury-column units survive because manometers and vapor-pressure tables predate the switch. Water's vapor pressure at 25 °C is 23.8 mmHg, or about 0.0317 bar; the reduced pressure pulled by a rotary evaporator might be 100 mmHg, or 0.133 bar. Multiplying by 0.001333224 is the bridge between the two worlds — the instrument reading and the standard-state value a thermodynamic data compilation will report.
Formula
Where the factor comes from
No definition here mentions the other; the pascal brokers the entire relation. The bar is exactly 10⁵ Pa by decree. The millimeter of mercury reaches the pascal by a different kind of statement — as the torr, exactly 1/760 of the 101325 Pa standard atmosphere, giving 133.32236842… Pa; or as the conventional column, mercury assigned a density of 13595.1 kg/m³ under standard gravity 9.80665 m/s², giving 133.322387 Pa. Either route puts the quotient at 1.333224 × 10⁻³ bar per mmHg. The landmark falls where it falls: 1 bar is 750.0617 mmHg, deliberately un-round, because the bar was sized against the atmosphere rather than against mercury. IUPAC's move of the thermodynamic standard state from 1 atm to 1 bar is what keeps this pair in circulation.
Precision and significant figures
The two readings of mmHg agree to seven significant figures — 1.3332237 × 10⁻³ against 1.3332239 × 10⁻³ — so the choice between them is academic here and the printed 0.001333224 is honest. Five figures, 1.3332 × 10⁻³, runs low by 1.8 parts in 10⁵, which covers any manometer ever built. The binding constraint is the instrument that produced the mmHg number in the first place. Bench-scale pressures expressed in bar also arrive as awkward small decimals — water's vapor pressure at 25 °C is 0.0317 bar — and adding decimals to compensate does not recover information the gauge never held.
Worked Examples
Standard atmospheric pressure, the sea-level baseline for anything that isn't under vacuum.
Exactly 1 bar — the IUPAC standard pressure, useful for pinning down the right pressure to use in a thermodynamic calculation.
The vapor pressure of water at 25 °C — a number that comes up any time a gas is collected over water.
A reduced pressure in the range a vacuum filter or a gentle rotavap run would hold.
Common mistakes
Bar and millibar one keystroke apart
100 mmHg is 0.1333 bar and 133.3 mbar, and both numbers turn up on the same page whenever European instrumentation and thermodynamic tables appear in one calculation. A thousandfold slip between them drops a lab vacuum figure into the range of a pressurized reactor, or the reverse. A single decimal point carries the whole distinction.
Equilibrium constants carry a pressure basis
A Kp or a Henry's law coefficient is numerically tied to whichever pressure unit its compilation used. Converting an mmHg measurement to bar and then feeding it into a constant tabulated per atmosphere mismatches the basis by 1.3 percent in each pressure term. Find out what the constant is denominated in before converting anything into it.
Bar-scale pressures no manometer could reach
Turning a 5 bar reactor pressure into 3750 mmHg is arithmetically fine and physically absurd — that describes a 3.75 m mercury column. The figure came from a Bourdon gauge or a transducer whose accuracy class sets the significant figures, and quoting it in millimeters of mercury implies a mercury measurement that nobody made.