View source for Inch of mercury

{{Short description|Measurement unit for pressure}}
{{Also|Inch of water}}
{{refimprove|date=May 2016}}
{{Infobox unit
| bgcolor      = 
| name         = Inch of mercury
| image        = Rare American Barometer, Lyman King, Clifton Springs, New York, c. 1860 - Museum of Science and Industry (Chicago) - DSC06342.JPG
| caption      = Early American barometer calibrated in inches of mercury
| standard     = 
| quantity     = [[Pressure]]
| symbol       = inHg
| symbol2      = ″Hg
| namedafter   = 
| extralabel   = 
| extradata    = 
| units1 = [[SI units]]
| inunits1     = {{convert|1.000000|inHg|disp=out|lk=out}}
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| units2    = [[United States customary units]]
| inunits2  = {{convert|1.000000|inHg|psi|disp=out|lk=out}}
| units6       = 
| inunits6     = 
| units_imp1   = 
| inunits_imp1 = 
<!--...-->
| units_imp6   = 
| inunits_imp6 = 
| units_us1    = 
| inunits_us1  = 
<!--...-->
| units_us6    = 
| inunits_us6  = 
}}
'''Inch of mercury''' ('''inHg''', '''″Hg''', or '''in''') is a non-[[SI]] [[unit of measurement]] for [[pressure]]. It is used for [[barometric pressure]] in [[Weather forecasting|weather report]]s, [[refrigeration]] and [[aviation]] in the [[United States customary units|United States]].

It is the pressure exerted by a column of [[mercury (element)|mercury]] {{convert|1|in|sigfig=3}} in height at the [[standard gravity|standard acceleration of gravity]].  Conversion to metric units depends on the density of mercury, and hence its temperature; typical conversion factors are:<ref name = NIST811/>

{| class="wikitable"
|-
! Conditions
! Pressure
|-
| conventional
| 3386.389 pascals
|-
| {{Convert|32|F}}
| 3386.38 pascals
|-
| {{Convert|60|F}}
| 3376.85 pascals
|}

In older literature, an "inch of mercury" is based on the height of a column of mercury at {{convert|60|F|sigfig=3}}.<ref name="NIST811">Barry N. Taylor, ''Guide for the Use of the International System of Units (SI),'' 1995, [[NIST]] Special Publication 811, Appendix B [http://physics.nist.gov/Pubs/SP811/appenB8.html#I]</ref>
:1&nbsp;inHg<sub>60&nbsp;°F</sub> = {{convert|3376.85|Pa|hPa|}}

In Imperial units: 1&nbsp;inHg<sub>60&nbsp;°F</sub> = 0.489 771&nbsp;[[Pounds per square inch|psi]], or 2.041 771&nbsp;inHg<sub>60&nbsp;°F</sub> = 1&nbsp;psi.

==Applications==
===Aircraft and automobiles===
Aircraft [[altimeter]]s measure the relative pressure difference between the lower ambient pressure at altitude and a calibrated reading on the ground.  In North America<ref>From the Ground Up – 29th Edition {{ISBN?}} {{page?|date=December 2022}}</ref> and Japan,<ref>{{cite web |title=Full automation of aeronautical meteorological observations and reports at aerodromes |url=https://www.jma.go.jp/jma/kishou/know/kouku/2_kannsoku/27_jidoka/jidoka_gaiyo_en.pdf |publisher=[[Japan Meteorological Agency]] |access-date=7 May 2025 |archive-url=https://web.archive.org/web/20250507030748/https://www.jma.go.jp/jma/kishou/know/kouku/2_kannsoku/27_jidoka/jidoka_gaiyo_en.pdf |archive-date=7 May 2025 |language=English |date=August 2016}}</ref> these altimeter readings are provided in inches of mercury, but most other nations use [[hectopascal]]s. Ground readings vary with weather and along the route of the aircraft as it travels, so current readings are relayed periodically by air traffic control. Aircraft operating at higher altitudes (at or above what is called the [[Flight level#Transition altitude|transition altitude]], which varies by country) set their barometric [[altimeter]]s to a standard pressure of 29.92&nbsp;inHg (1&nbsp;atm = 29.92&nbsp;inHg) or 1013.25&nbsp;[[pascal (unit)|hPa]] (1&nbsp;hPa = 1&nbsp;[[mbar]]) regardless of the actual sea level pressure. The resulting altimeter readings are known as [[flight level]]s.

[[Piston engine]] [[aircraft]] with [[constant-speed propeller]]s also use inches of mercury to measure [[manifold pressure]], which is indicative of engine power produced in engines equipped with a [[supercharger]] or [[turbocharger|turbosupercharger]] (naturally aspirated engines measure manifold vacuum instead). In automobile racing, particularly [[United States Auto Club]] and [[Champ Car]] [[Indy car racing]], inches of mercury was the unit used to measure [[turbocharger]] inlet pressure. However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum or pressure within the engine's intake manifold. This can be seen on "boost gauges (forced induction) or vacuum gauges (natural induction), which give a rough indication of the relative power being produced at any given time.

===Cooling systems===
In [[air conditioning]] and refrigeration, inHg is often used to describe "inches of mercury vacuum", or pressures below ambient atmospheric pressure, for recovery of refrigerants from air conditioning and refrigeration systems, as well as for leak testing of systems while under a vacuum, and for dehydration of refrigeration systems. The low-side gauge in a refrigeration gauge manifold indicates pressures below ambient in "inches of mercury vacuum" (inHg), down to a 30&nbsp;inHg vacuum.

Inches of mercury is also used in automotive cooling system vacuum test and fill tools. A technician will use this tool to remove air from modern automotive cooling systems, test the system's ability to hold vacuum, and subsequently refill using the vacuum as suction for the new coolant. Typical minimum vacuum values are between 22 and 27&nbsp;inHg.

===Vacuum brakes===
Inches of mercury was the usual unit of pressure measurement in railway [[vacuum brake]]s.

==See also==
*[[Torr]] (millimeters of mercury)
*[[Bar (unit)]]
*[[Mercury barometer]]
*[[Millimetre of mercury]]

==References==
{{reflist}}

{{United States Customary Units}}

[[Category:Units of pressure]]
[[Category:Mercury (element)]]
[[Category:Customary units of measurement in the United States]]