What is atmosphere?
An atmosphere (atm) is a pressure unit equal to mean atmospheric pressure at sea level, approximately 101,325 pascals. It is used as a reference in chemistry, diving tables, and altitude calculations.
Real-world uses
Atmospheres are used as a reference pressure in chemistry (standard conditions: 1 atm, 25°C), scuba diving (each 10 m of seawater adds about 1 atm), and high-pressure industrial applications like autoclaves and pressure vessels.
History
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0°C under standard gravity. Evangelista Torricelli first measured atmospheric pressure in 1644 using a mercury barometer, establishing this foundational concept.
Common mistakes
Confusing standard atmosphere (101,325 Pa) with technical atmosphere (1 kgf/cm² ≈ 98,066.5 Pa). Also, assuming atmospheric pressure is constant at 1 atm everywhere—it varies with altitude, weather, and temperature.
What is inch of mercury?
Inch of mercury (inHg) is a pressure unit representing the pressure exerted by a one-inch column of mercury. It is used in aviation altimetry, barometric pressure readings, and vacuum measurements in the United States.
Real-world uses
Inches of mercury are used in US aviation for altimeter settings (standard is 29.92 inHg), weather reporting on American TV broadcasts, and HVAC system vacuum measurements. American barometers are traditionally scaled in inHg.
History
Inches of mercury originate from the mercury barometer invented by Torricelli in 1644. The use of inches (rather than millimetres) reflects the Imperial measurement tradition. US aviation adopted inHg for altimeter settings, while most other countries use hPa.
Common mistakes
Confusing inches of mercury with millimetres of mercury—29.92 inHg = 760 mmHg = 1 atm. Also, assuming the conversion from inHg to psi is simple; 1 inHg ≈ 0.491 psi, not a round number.
When is this conversion used?
Converting between atmosphere and inch of mercury is common when working across metric and imperial systems, such as international trade, travel between countries with different measurement standards, or following instructions from a different region. Tyre pressure gauges, HVAC systems, and industrial equipment may use different pressure units depending on the manufacturer and region. Converting between psi, bar, and kPa is common in automotive and engineering work.
Worked examples
1 atmosphere = 29.9212524 inch of mercury
1 inch of mercury = 3,386.389 pascal
How to convert atmosphere to inch of mercury
To convert atmosphere to inch of mercury, multiply the value by 29.9212524.
To convert inch of mercury back to atmosphere, multiply by 0.03342106.
Measurement standards
The pascal (Pa) is the SI derived unit of pressure, defined as one newton per square metre. The World Meteorological Organization mandates the hectopascal (hPa) for atmospheric pressure reporting in aviation and weather services.
Did you know?
The pressure at the bottom of the Mariana Trench, nearly 11 km deep, exceeds 1,086 bar — over a thousand times atmospheric pressure. At this depth, water is compressed by about 5%, making it measurably denser than at the surface.
Quick reference: atmosphere to inch of mercury
| atmosphere | inch of mercury |
|---|---|
| 0.1 | 2.99212524 |
| 0.5 | 14.9606262 |
| 1 | 29.9212524 |
| 2 | 59.8425048 |
| 5 | 149.60626201 |
| 10 | 299.21252402 |
| 25 | 748.03131005 |
| 50 | 1,496.06262009 |
| 100 | 2,992.12524019 |
| 250 | 7,480.31310047 |
| 500 | 14,960.62620095 |
| 1,000 | 29,921.25240189 |
Common values
| atmosphere | inch of mercury | |
|---|---|---|
| Car tyre | 2.17123119 atmosphere | 64.96595636 inch of mercury |
| Standard atmosphere | 1 atmosphere | 29.9212524 inch of mercury |
| Blood pressure (systolic) | 0.15790772 atmosphere | 4.72479683 inch of mercury |
| Deep-sea submersible | 1,085.61559339 atmosphere | 32,482.97818118 inch of mercury |
| Bicycle tyre | 6.11892425 atmosphere | 183.08587702 inch of mercury |