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.
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.
When is this conversion used?
Converting between inch of mercury and atmosphere 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 inch of mercury = 0.03342106 atmosphere
1 atmosphere = 101,325 pascal
How to convert inch of mercury to atmosphere
To convert inch of mercury to atmosphere, multiply the value by 0.03342106.
To convert atmosphere back to inch of mercury, multiply by 29.9212524.
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: inch of mercury to atmosphere
| inch of mercury | atmosphere |
|---|---|
| 0.1 | 0.00334211 |
| 0.5 | 0.01671053 |
| 1 | 0.03342106 |
| 2 | 0.06684212 |
| 5 | 0.1671053 |
| 10 | 0.33421061 |
| 25 | 0.83552652 |
| 50 | 1.67105305 |
| 100 | 3.34210609 |
| 250 | 8.35526524 |
| 500 | 16.71053047 |
| 1,000 | 33.42106094 |
Common values
| inch of mercury | atmosphere | |
|---|---|---|
| Car tyre | 64.96595636 inch of mercury | 2.17123119 atmosphere |
| Standard atmosphere | 29.9212524 inch of mercury | 1 atmosphere |
| Blood pressure (systolic) | 4.72479683 inch of mercury | 0.15790772 atmosphere |
| Deep-sea submersible | 32,482.97818118 inch of mercury | 1,085.61559339 atmosphere |
| Bicycle tyre | 183.08587702 inch of mercury | 6.11892425 atmosphere |
Available Pressure units
More inch of mercury conversions
- Convert inch of mercury to pascal
- Convert inch of mercury to kilopascal
- Convert inch of mercury to bar
- Convert inch of mercury to pound per square inch
- Convert inch of mercury to atmosphere
- Convert inch of mercury to millibar
- Convert inch of mercury to hectopascal
- Convert inch of mercury to torr
- Convert inch of mercury to megapascal