What is hectopascal?
A hectopascal is a pressure unit equal to 100 pascals, numerically identical to the millibar. It is the standard unit for atmospheric pressure reports in modern meteorology and aviation weather data.
Real-world uses
Hectopascals are the modern meteorological standard for atmospheric pressure reporting, used by the World Meteorological Organization and most national weather services. Standard atmospheric pressure is 1013.25 hPa. Altimeter settings in aviation outside the US use hPa (QNH).
History
The hectopascal was promoted by the WMO beginning in 1982 as a replacement for the millibar, aligning meteorology with the SI system. Since 1 hPa equals exactly 1 mbar, the transition required no changes to numerical readings or instruments.
Common mistakes
Not realizing that 1 hPa = 1 mbar exactly. The switch from millibars to hectopascals changed only the name, not the numerical values. Also, some people confuse hPa with kPa; 1 kPa = 10 hPa.
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 hectopascal 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.
Worked examples
1 hectopascal = 0.02952998 inch of mercury
1 inch of mercury = 3,386.389 pascal
How to convert hectopascal to inch of mercury
To convert hectopascal to inch of mercury, multiply the value by 0.02952998.
To convert inch of mercury back to hectopascal, multiply by 33.86389.
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: hectopascal to inch of mercury
| hectopascal | inch of mercury |
|---|---|
| 0.1 | 0.002953 |
| 0.5 | 0.01476499 |
| 1 | 0.02952998 |
| 2 | 0.05905996 |
| 5 | 0.1476499 |
| 10 | 0.2952998 |
| 25 | 0.7382495 |
| 50 | 1.47649901 |
| 100 | 2.95299802 |
| 250 | 7.38249504 |
| 500 | 14.76499008 |
| 1,000 | 29.52998016 |
Common values
| hectopascal | inch of mercury | |
|---|---|---|
| Car tyre | 2,200 hectopascal | 64.96595636 inch of mercury |
| Standard atmosphere | 1,013.25 hectopascal | 29.9212524 inch of mercury |
| Blood pressure (systolic) | 160 hectopascal | 4.72479683 inch of mercury |
| Deep-sea submersible | 1,100,000 hectopascal | 32,482.97818118 inch of mercury |
| Bicycle tyre | 6,200 hectopascal | 183.08587702 inch of mercury |