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 bar?
A bar is a pressure unit approximately equal to standard atmospheric pressure, or 100,000 pascals. It is widely used in meteorology, hydraulics, diving equipment, and industrial pressure gauges.
Real-world uses
Bars are widely used in European tyre pressure specifications, compressed gas cylinder ratings, scuba diving depth gauges, and industrial process controls. Espresso machines operate at 9–15 bar. Atmospheric pressure is approximately 1.013 bar.
History
The bar was introduced by Norwegian meteorologist Vilhelm Bjerknes in 1909, derived from the Greek word "baros" meaning weight. Though not an SI unit, it is widely accepted in industry and was retained in the EU Directive on units of measurement.
Common mistakes
Confusing bar with atm—they are close (1 atm ≈ 1.01325 bar) but not identical. Also, gauge pressure (barg) versus absolute pressure (bara) is often mixed up; gauge pressure reads zero at atmospheric pressure.
When is this conversion used?
Converting between inch of mercury and bar 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 = 0.03386389 bar
1 bar = 100,000 pascal
How to convert inch of mercury to bar
To convert inch of mercury to bar, multiply the value by 0.03386389.
To convert bar back to inch of mercury, multiply by 29.52998016.
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 bar
| inch of mercury | bar |
|---|---|
| 0.1 | 0.00338639 |
| 0.5 | 0.01693195 |
| 1 | 0.03386389 |
| 2 | 0.06772778 |
| 5 | 0.16931945 |
| 10 | 0.3386389 |
| 25 | 0.84659725 |
| 50 | 1.6931945 |
| 100 | 3.386389 |
| 250 | 8.4659725 |
| 500 | 16.931945 |
| 1,000 | 33.86389 |
Common values
| inch of mercury | bar | |
|---|---|---|
| Car tyre | 64.96595636 inch of mercury | 2.2 bar |
| Standard atmosphere | 29.9212524 inch of mercury | 1.01325 bar |
| Blood pressure (systolic) | 4.72479683 inch of mercury | 0.16 bar |
| Deep-sea submersible | 32,482.97818118 inch of mercury | 1,100 bar |
| Bicycle tyre | 183.08587702 inch of mercury | 6.2 bar |
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