What is the standard altimeter setting?

The standard datum is 29.92 inHg, equal to 1013.25 hPa. Above the transition altitude, pilots set this value so everyone shares a common pressure reference for flight levels.

Why does the US use inHg while other countries use hPa?

It is largely historical. US altimeters and ATC phraseology are built around inches of mercury, while ICAO and most other states standardized on hectopascals (millibars), so international flying requires switching between them.

How do I convert inHg to hPa?

Multiply inches of mercury by 33.8639 to get hectopascals. For example, 29.92 inHg is about 1013.2 hPa.

inHg ↔ hPa Converter

Convert between inches of mercury and hectopascals (millibars). The US uses inHg for altimeter settings while most of the world uses hPa.

From

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Formula

1 inHg = 33.8639 hPa
1 hPa = 0.02953 inHg

Multiply inHg by 33.8639 to get hPa, or hPa by 0.02953 to get inHg.

Reference Table

inHg	hPa	Context
28.00	948	Very low pressure — significant weather
29.00	982	Below standard — PA above field elevation
29.92	1013.25	Standard atmosphere setting
30.50	1033	High pressure system
31.00	1050	Near maximum typical pressure

Aviation Context

In the US, altimeter settings are given in inHg (e.g., 29.92). When flying internationally, pilots must set the altimeter in hPa (QNH). At the transition altitude, the altimeter is set to standard (29.92 inHg / 1013.25 hPa) for flight level operations. Understanding this conversion is essential for international flight planning.

Why This Matters in Aviation

An incorrect altimeter setting puts the aircraft at the wrong true altitude, and the units differ by region: the US issues altimeter settings in inches of mercury while most of the world uses hectopascals (QNH). When crossing into airspace that uses hPa, a pilot must convert and reset the altimeter or risk a significant altitude error near terrain and other traffic. At the transition altitude, pilots set the standard datum of 29.92 inHg (1013.25 hPa) for flight-level operations, a procedure described in the AIM. Understanding the conversion keeps vertical separation intact during international operations.

Understanding This Conversion

Barometric pressure sets your altimeter, and it is reported in two units depending on where you fly: inches of mercury in the United States and Canada, and hectopascals (equivalent to millibars) across most of the rest of the world. Setting the wrong value, or misreading one unit as the other, puts every altitude you fly in error, so the ability to convert confidently between inHg and hPa is a genuine safety skill. This converter provides that bridge.

The standard sea-level pressure of 29.92 inHg equals 1013.25 hPa, and pilots who fly internationally must be equally comfortable dialing either value into the altimeter. Understanding the relationship also helps when a foreign weather report gives a pressure your instruments expect in the other unit.

Why the Unit Matters in Aviation

The altimeter is fundamentally a barometer, and it converts sensed static pressure into an altitude by assuming a standard relationship between pressure and height. Setting the current altimeter setting corrects for the day's pressure so the instrument reads true field elevation on the ground. Because a small pressure error translates into a real altitude error, using the correct value in the correct unit is essential.

In United States and Canadian airspace, altimeter settings are given in inches of mercury, so ATIS, tower, and weather reports all use inHg. Most of the world uses hectopascals, and the transition altitude procedures abroad require setting the standard 1013 hPa when climbing to the flight levels. A pilot crossing between these systems must convert fluently to avoid a gross altitude error at exactly the moment when altitude accuracy matters most.

Common Conversion Mistakes to Avoid

The most serious mistake is entering a hectopascal value into an altimeter expecting inches, or vice versa. A setting of 1005 hPa is not 10.05 or 1005 inHg; it is about 29.68 inHg. Entering the raw number in the wrong unit produces an altimeter error of thousands of feet.

A subtler error is neglecting the roughly 1,000-foot-per-inch sensitivity of the altimeter. Because each inch of mercury corresponds to about 1,000 feet, even a small setting error of a few hundredths of an inch shifts your indicated altitude by tens of feet, which accumulates in precise operations.

Quick-Reference Scenarios

Flying into metric airspace

Arriving in a country that uses hectopascals, you receive an altimeter setting of 1008 hPa. Converting to about 29.77 inHg lets you verify the value your instrument displays and catch any entry error.

Setting standard pressure

Climbing to the flight levels abroad, you set the standard 1013 hPa. Knowing this equals 29.92 inHg confirms you have made the correct transition from the local setting.

Cross-checking a foreign report

A weather product lists pressure in hectopascals while your training and instruments use inches. Converting keeps your altitude picture consistent with the reported conditions.

Related Standards & Regulations

The International Standard Atmosphere defines sea-level pressure as 1013.25 hPa, exactly equal to 29.92 inHg, the value used for pressure altitude and for the flight levels worldwide.

ICAO uses hectopascals for altimeter settings in most of the world, while the United States and Canada use inches of mercury. Because the two systems meet at international boundaries, correct conversion at the transition is a required airmanship skill.

Frequently Asked Questions

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