What Is Inch of Mercury (inHg)?
An inch of mercury, written as inHg, is a unit for measuring pressure. It shows how much force air or gas puts on a surface by how high it can push liquid mercury in a tube.
If the pressure is higher, the mercury rises higher in the tube. If the pressure is lower, the mercury level falls. One inch of mercury means the pressure needed to raise a column of mercury exactly one inch high at normal gravity and temperature.
Definition
In science and engineering, inch of mercury is defined using the weight of a column of mercury that is one inch tall.
In modern terms, one inch of mercury is equal to about:
- 3,386 pascals (Pa)
- 3.386 kilopascals (kPa)
- 0.03386 bar
- 0.491 pounds per square inch (psi)
The exact value can change a tiny bit with temperature, because mercury expands and shrinks, but for most everyday uses the values above are close enough.
History / Origin
The idea of measuring pressure with a column of liquid started in the 1600s. Scientists wanted to measure air pressure and understand how the atmosphere behaves.
The Italian scientist Evangelista Torricelli built one of the first mercury barometers. He filled a long glass tube with mercury, flipped it into a dish, and saw that some mercury stayed up in the tube. The height of the mercury column showed the air pressure.
At first, the height was measured in millimeters of mercury and in other length units. In countries that used inches, it became common to describe pressure using inches of mercury. This made it easy to read barometers and talk about weather and air pressure before SI units like the pascal were created.
Symbol & Abbreviation
The common symbol for inch of mercury is:
- inHg pronounced inch of mercury
Here is what each part means:
- in stands for inch
- Hg is the chemical symbol for mercury from its Latin name hydrargyrum
Write it like this when giving a pressure value:
- 29.92 inHg
- 15 inHg
Current Use Around the World
Inch of mercury is not an SI unit, so it is not the main scientific unit today. Still, it is used in several important areas, mainly in countries that use inches and pounds.
Common uses include:
- Aviation pilots and air traffic controllers in the United States and some other countries use inHg to set altimeters and report air pressure at airports
- Weather reports some weather stations and TV weather reports in the United States still show air pressure in inHg
- Vacuum systems mechanics and technicians sometimes use inHg to describe how strong a vacuum pump is in engines or air conditioning systems
- Older equipment some older gauges and meters are marked in inHg, so technicians keep using this unit for those tools
Most scientific work today uses the pascal or kilopascal, but knowing inHg is still helpful for flying, weather watching, and working with some machines.
Example Conversions
Here are some simple examples of how inch of mercury compares to other pressure units. Values are rounded to make them easy to remember.
From inHg to other units
- 1 inHg ≈ 3,386 Pa
- 1 inHg ≈ 3.386 kPa
- 1 inHg ≈ 0.03386 bar
- 1 inHg ≈ 0.491 psi
- 1 inHg ≈ 25.4 mmHg
- 1 inHg ≈ 0.0334 atm
From other units to inHg
- 1 atm ≈ 29.92 inHg
- 1 bar ≈ 29.53 inHg
- 101.3 kPa (about 1 atm) ≈ 29.9 inHg
- 1 psi ≈ 2.036 inHg
- 100 mmHg ≈ 3.94 inHg
Everyday style examples
- A normal sea level air pressure for weather reports is about 29.9 inHg
- A strong storm might bring air pressure down to about 28.5 inHg or lower
- A car engine vacuum reading might be around 18 to 22 inHg
Related Units
Inch of mercury is closely related to many other pressure units. Here are some you will often see with it.
- Pascal (Pa) the main SI unit for pressure, used in science and engineering
- Kilopascal (kPa) 1,000 pascals, often used in weather and engineering
- Bar a convenient pressure unit equal to 100,000 pascals, near normal air pressure
- Standard atmosphere (atm) a unit based on average sea level air pressure, equal to about 29.92 inHg
- Millimeter of mercury (mmHg) also called torr in many situations, widely used in medicine to measure blood pressure
- Pound per square inch (psi) a common unit for tire pressure and many tools in the United States
- Torr a pressure unit closely linked to mmHg, often used in vacuum science
FAQs
What is an inch of mercury in simple words?
It is a way to measure how hard air or gas pushes by seeing how many inches it can lift a column of liquid mercury in a tube.
Why is mercury used for this unit?
Mercury is very heavy, so a short column can show a lot of pressure. It also stays liquid over a wide range of temperatures and does not wet the glass tube, so the level is easy to see.
Is inch of mercury an SI unit?
No, inch of mercury is not an SI unit. The official SI unit for pressure is the pascal Pa. InHg is a traditional unit that is still used in some fields.
Where is inHg used today?
It is used mostly in aviation altimeter settings and airport reports, in some weather reports, and in some vacuum and engine measurements, mainly in countries that use imperial units.
What is the difference between inHg and mmHg?
The difference is in the length unit. InHg uses inches while mmHg uses millimeters. One inch is 25.4 millimeters, so 1 inHg is equal to 25.4 mmHg.
How do I convert from inHg to kPa?
Multiply the value in inHg by about 3.386. For example, 30 inHg × 3.386 ≈ 101.6 kPa.
Why do pilots still use inHg?
Many aviation rules, instruments, and training systems were built around inches and psi. Changing all altimeters and habits worldwide is hard, so inHg remains standard in some regions, especially the United States.
Is measuring with mercury safe?
Mercury is poisonous if it leaks, so new devices often avoid it. Many modern barometers and sensors use electronic methods to measure pressure and then show the result in inHg or other units on a digital display.