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Below the transition altitude (18,000 feet in the US), pilots use local pressure readings to calibrate their altimeters.

Above the transition altitude, pilots set a standard pressure to calibrate the altimeter. The standard pressure is 29.92 inches of mercury or 1013 hPa.

Where did this value come from?

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    $\begingroup$ Good question. Why use inches of mercury, makes about as much sense as burnt villages per meadow. $\endgroup$ – Koyovis Jun 16 '17 at 1:45
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    $\begingroup$ @Koyovis, inches of mercury because the early barometers and manometers used a column of mercury to balance the measured pressure. You might still find the that type in medical offices in use for measuring blood pressure. $\endgroup$ – Jan Hudec Jun 16 '17 at 17:15
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It is called "standard pressure" because 29.92 In-Hg (or 1013.25 hPa) is the standard atmospheric pressure at sea level according to both the ISA (International Standard Atmosphere) and the US 1976 Standard Atmosphere.

Below 18,000ft, local altimeter settings are used because you need to know how high you are above terrain, or whether or not you're at the proper altitude for an instrument approach, or at pattern altitude, etc. In cruising flight above 18,000ft, except for a few places in the world, you are well above any terrain, so errors in actual altitude of even a couple thousand feet is unlikely to cause a problem. In this case, having all airplanes use a common altimeter setting is useful because it 1. doesn't require you to change it frequently as you pass through changes in pressure, and 2. it helps ATC ensure separation without having to inform flights every 10 minutes of a new setting. It makes sense to use standard pressure in this case because it represents a good baseline average, and you may have noticed that reported altimeter settings tend to average around 29.92 anyway.

That being said, there are instances where you may still need to know the en-route altimeter settings for flight planning purposes. Suppose the minimum safe altitude along your route is 19,000 ft MSL and current pressure is 29.42. If you decided to cruise at FL190, you would actually be flying at about 18,500ft MSL, and would be below your minimum safe altitude. Colder than standard temperature can have similar effect as low pressure, and will result in the airplane being lower than the indicated altitude.

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    $\begingroup$ Note: the 18,000 ft is valid for USA; in Europe we have lower Transition Altitudes - in Germany it is 5000 ft above sea level but at least 2000 ft above ground (the higher one).. $\endgroup$ – Carlos Heuberger Aug 30 '16 at 4:43
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It is called "standard pressure" because at an altitude of mean sea level (mean of all tides low to high) averaging the air pressure across equator to pole, winter to summer, over land and sea and day to night you get an average pressure that will register on the barometer at around 29.92 inches of mercury.

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    $\begingroup$ Yours is the only answer that actually addresses the question in my opinion. The others answers address the usage of standard pressure, but not how it is arrived at. $\endgroup$ – Terry Jun 16 '17 at 2:07
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"standard day" model of the atmosphere is defined at sea level, with certain present conditions such as temperature and pressure. But other factors, such as humidity, further alter the nature of the atmosphere, and are also defined under standard day conditions":

These are the numbers for a standard day.

1013.25 hPa (14.7 lb/ in2) (pressure)

(T): 15 °C (59 °F) (Temperature)

(μ): 17.3 µPa·s (3.62 × 10−7 lb s/ft2) (viscosity)

(ρ): 1.225 kg/m³ (0.00237 slug/ft3 (density)

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