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Multiple conversions
To convert from Inch of mercury (inHg) to pascal (Pa), use the following formula:
pascal (Pa)
= 0.000491154 × 10.00014503768078 × 1000× Inch of mercury (inHg)
= 3386.3889532610878529× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert a pressure from inches of mercury (inHg) to kilopascals (kPa), multiply by 3.3864.
The inch of mercury is 25.4 mm of mercury, and each mmHg is 133.322 Pa, giving 3,386.39 Pa — or 3.38639 kPa — per inch. The scale's anchor is the standard atmosphere: 29.92 inHg = 101.32 kPa.
An aviation weather report gives the altimeter setting as 30.12 inHg, and a performance calculation uses SI units:
The day is slightly higher-pressure than standard (101.325 kPa), which the density-altitude calculation will credit as a small performance gain.
| inHg | kPa |
|---|---|
| 1 | 3.39 |
| 5 | 16.9 |
| 15 | 50.8 |
| 28.5 | 96.5 |
| 29.92 | 101.32 |
| 30.5 | 103.3 |
Inches of mercury persist in three North American strongholds, and each generates conversions to kPa. Aviation is the largest: US and Canadian altimeter settings are broadcast in inHg (the familiar "two-niner-niner-two"), while flight-planning physics, ICAO's standard atmosphere and most of the world's METARs use hectopascals — 1 kPa = 10 hPa away from this conversion. Weather broadcasting is the second: US barometric pressure is reported as "30.05 inches and falling," which a scientific or agricultural model must read as 101.8 kPa. Engine vacuum is the third: manifold vacuum gauges and HVAC service instruments read inHg below atmospheric, so "18 inches of vacuum" translates to about 61 kPa below ambient, or roughly 40 kPa absolute on a standard day. The multiplication by 3.39 is straightforward; what earns attention is the reference direction, since vacuum-gauge inches count downward from atmosphere while kPa-absolute counts upward from zero. Stating every converted value explicitly as gauge, vacuum, or absolute is what keeps the arithmetic honest.
To convert from Inch of mercury (inHg) to megapascal (MPa), use the following formula:
megapascal (MPa)
= 0.000491154 × 10.00014503768078 × 11000× Inch of mercury (inHg)
= 0.0033863889532610878529× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to hectopascal (hPa), use the following formula:
hectopascal (hPa)
= 0.000491154 × 10.00014503768078 × 10× Inch of mercury (inHg)
= 33.863889532610878529× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to millibar (mbar), use the following formula:
millibar (mbar)
= 0.000491154 × 10.00014503768078 × 10× Inch of mercury (inHg)
= 33.863889532610878529× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to bar (bar), use the following formula:
bar (bar)
= 0.000491154 × 10.00014503768078 × 1100× Inch of mercury (inHg)
= 0.033863889532610878529× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to torr (torr), use the following formula:
torr (torr)
= 0.000491154 × 10.00014503768078 × 760000101325× Inch of mercury (inHg)
= 25.400005965738236054× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to meter of water @ 4°C (mH2O), use the following formula:
meter of water @ 4°C (mH2O)
= 0.000491154 × 10.00014503768078 × 19.80665× Inch of mercury (inHg)
= 0.345315571908968695× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to millimeter of mercury (mmHg), use the following formula:
millimeter of mercury (mmHg)
= 0.000491154 × 10.00014503768078 × 10.133322× Inch of mercury (inHg)
= 25.400076155931413067× Inch of mercury (inHg)
With 10.00014503768078 is the ratio between the base units kilopascal (kPa) and kilopound per square inch (ksi).
To convert from Inch of mercury (inHg) to pound per square inch (psi), use the following formula:
pound per square inch (psi)
= 0.000491154 × 1000× Inch of mercury (inHg)
= 0.491154× Inch of mercury (inHg)
To convert from Inch of mercury (inHg) to kilopound per square inch (ksi), use the following formula:
kilopound per square inch (ksi)
= 0.000491154× Inch of mercury (inHg)
An inch of mercury, often abbreviated as inHg, is a unit of pressure.
It is not part of the standard metric (SI) system but is widely used in the United States and in specific fields like meteorology and aviation.
The unit is defined by its name: it is the amount of pressure exerted by a column of liquid mercury exactly one inch tall at a standard temperature.
You will most commonly encounter inches of mercury in a few key areas:
Weather Reporting: In the U.S., inHg is the standard unit for reporting barometric pressure (also known as atmospheric pressure).
A "high" pressure system indicating clear weather might be around 30.50 inHg, while a "low" pressure system associated with storms could be 28.50 inHg.
Aviation: This measurement is critical for pilot safety.
Pilots use the local barometric pressure (in inHg) to calibrate their altimeters—instruments that measure altitude—to ensure they are flying at the correct height.
Vacuum Systems: The unit is also widely used in science and industry to measure vacuum pressure.
This is common in systems like air conditioning, refrigeration, and for monitoring the manifold pressure in piston engines.
Because inHg is not a global standard, it's often necessary to convert it to other pressure units for calculations or international use.
Here are the most common conversions for 1 inHg: