mmHg to Atmospheres Converter
Common Conversions
| mmHg | atm |
|---|---|
| 1 | 0.001316 |
| 10 | 0.01316 |
| 50 | 0.06579 |
| 100 | 0.1316 |
| 200 | 0.2632 |
| 380 | 0.5 |
| 500 | 0.6579 |
| 760 | 1 |
| 1000 | 1.3158 |
| 1520 | 2 |
| 2280 | 3 |
| 3800 | 5 |
Why this conversion matters in chemistry
The Dumas method for measuring a volatile liquid's molar mass is a classic example of why this conversion matters. You record the barometer reading in mmHg along with the flask volume and temperature, then use PV = nRT to back out moles. But R = 0.08206 L·atm/(mol·K) expects atm, not mmHg, so a barometric pressure of 748 mmHg has to get divided by 760 to become 0.9842 atm before it drops into the equation. Forgetting the division gives you an answer off by a factor of 760 — the kind of error that survives all the way through the calculation because the rest of the arithmetic looks fine.
Formula
Where the factor comes from
The direction runs backwards through history. The standard atmosphere was originally the pressure of a 760 mm mercury column at 0 °C under standard gravity, so dividing millimeters of mercury by 760 was once the definition rather than a conversion. International agreement later cut that tie and fixed the atmosphere at exactly 101325 Pa, a value chosen to sit as close as possible to the column it replaced. The torr was then defined as exactly 1/760 of that atmosphere, which restores the clean division: 133.32236842… Pa per torr divided by 101325 Pa per atm is exactly 1/760. Read mmHg strictly instead — as the conventional column, mercury assigned 13595.1 kg/m³ — and 760 mmHg comes to 1.00000014 atm. Nothing at a bench sees that.
Precision and significant figures
With 760 exact, the arithmetic contributes no uncertainty and the reading contributes all of it. A mercury column read to the nearest millimeter near atmospheric is uncertain by about one part in 760, or 0.13 percent, and that sets the floor no matter how many decimals the result gets written to. Watch the leading zeros on the way down. 23.8 mmHg becomes 0.0313 atm: three figures in, three figures out, because zeros ahead of the first nonzero digit are placeholders rather than measurements. Writing 0.03132 quietly promotes a three-figure reading to four, and 1/760 = 0.001315789… repeating will happily supply the extra digit.
Worked Examples
The defining equivalence. 760 mmHg was the original definition of the standard atmosphere before the pascal took over.
Half an atmosphere. Typical working vacuum for distillation of a moderately volatile solvent.
Water's vapor pressure at 25°C. The correction you apply when collecting a gas over water and needing the actual partial pressure of the dry gas.
Roughly the absolute pressure inside a steam autoclave at 121°C — enough to push water's boiling point above the sterilization threshold.
Common mistakes
Centimeter scales read as millimeters
Plenty of U-tube manometers and older references are graduated in centimeters of mercury, and some vacuum work is quoted in cmHg outright. Dividing a cmHg figure by 760 understates the pressure tenfold, and the result is a small decimal that looks entirely at home in a vacuum calculation. Check the scale legend before you touch the arithmetic.
Mixed units inside a Dalton subtraction
Partial pressures only add and subtract when every term shares a unit. Converting a measured total to atmospheres while leaving a tabulated water vapor pressure in mmHg produces a subtraction between two incompatible numbers, and because the vapor term is small the answer still looks plausible. Convert every term first, or none of them, but never just one.
L·atm is energy, but it is not joules
The atmosphere is not a coherent SI unit, so pressure–volume work computed as PΔV in liter-atmospheres arrives in a unit that has to be carried one step further: 1 L·atm is 101.325 J. Reporting the raw liter-atmosphere figure as though it were joules understates the work by a factor of 101, which is large enough to invert an energy balance.