Inch of Mercury (inHg) - Unit Information & Conversion
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What is a Inch of Mercury?
Inch of mercury (inHg) measures pressure by mercury column height in inches. US standard for barometric pressure, aviation altimeter settings. 1 inHg = 3,386.39 Pa ≈ 33.86 mbar. 29.92 inHg = 1 atmosphere. Used in US weather reports.
History of the Inch of Mercury
Imperial version of mmHg. Standard in US meteorology, aviation since early 1900s. Mercury barometer reads pressure as column height. Still used in US despite global SI adoption. NOAA weather reports use inHg.
Quick Answer
What is inHg? Inch of mercury (inHg) measures barometric pressure as mercury column height in inches. US standard for weather reports and aviation. 1 inHg = 3,386.39 Pa = 33.86 mbar. Normal sea level: 29.92 inHg. Use our pressure converter for conversions.
Key Facts: Inch of Mercury
| Property | Value |
|---|---|
| Symbol | inHg |
| Quantity | Pressure |
| System | Metric/SI Derived |
| Derived from | Pascal |
| Category | Pressure |
| Standard Body | NIST / ISO |
Definition
1 inHg = 3,386.39 Pa = 3.386 kPa = 33.86 mbar = 33.86 hPa = 0.491 PSI = 0.0334 bar = 0.0334 atm = 25.4 mmHg
Key relationship: 29.92 inHg = 1 atmosphere (exactly)
Origin: Mercury Column
The unit literally measures how high mercury rises:
- Historical: Barometers invented in 1600s used mercury column
- Measurement: Mercury rises/falls with atmospheric pressure
- 1 inHg: Mercury column exactly 1 inch high
- 29.92 inHg: Mercury rises 29.92 inches at sea level (1 atm)
- Why mercury?: Highest density liquid (narrow barometer), visible, stable
Common Uses
US Weather: Barometric pressure in weather forecasts, NOAA reports - THE US standard. Aviation: Altimeter settings (US standard, some international - critical for flight safety). HVAC: Vacuum measurements, duct pressure, system diagnostics. Historical meteorology: Traditional US measurement since early 1900s.
Real-World Examples
Weather System Pressures and Conditions (inHg)
Understanding barometric pressure is key to weather prediction:
| Pressure (inHg) | Condition | Weather | Visibility | Typical Duration |
|---|---|---|---|---|
| >30.5 | Very high | Clear, cold | Excellent | 12-36 hours |
| 30.2-30.5 | High pressure | Fair, pleasant | Very good | 24-48 hours |
| 30.0-30.2 | Good | Steady, pleasant | Good | Variable |
| 29.92 | Standard | Baseline condition | Baseline | Reference |
| 29.8-30.0 | Normal | Variable | Good | Variable |
| 29.4-29.8 | Low pressure | Cloudy, possible rain | Moderate | 12-24 hours |
| 28.5-29.4 | Very low | Rain, snow likely | Moderate-poor | 24-48 hours |
| 27.5-28.5 | Storm | Heavy rain/snow | Poor | 12-36 hours |
| 27.0-27.5 | Severe storm | Severe weather risk | Poor | 6-24 hours |
| <27.0 | Extreme | Hurricane/typhoon | Very poor | Hours |
Critical fact: Falling pressure = worsening weather; Rising pressure = improving conditions
Hurricane Categories by Central Pressure
Barometric pressure is the PRIMARY hurricane classification metric:
| Category | Central Pressure | Wind Speed | Typical inHg | Damage |
|---|---|---|---|---|
| Depression | >1,002 mb | <39 mph | >29.6 inHg | Minimal |
| Tropical Storm | 965-1,002 mb | 39-73 mph | 28.5-29.6 inHg | Moderate |
| Cat 1 | 945-964 mb | 74-95 mph | 27.9-28.5 inHg | Significant |
| Cat 2 | 930-944 mb | 96-110 mph | 27.5-27.8 inHg | Extensive |
| Cat 3 | 920-929 mb | 111-129 mph | 27.2-27.5 inHg | Devastating |
| Cat 4 | 900-919 mb | 130-156 mph | 26.6-27.2 inHg | Catastrophic |
| Cat 5 | <900 mb | >157 mph | <26.6 inHg | Catastrophic |
| Record | 26.05 inHg | - | Typhoon Tip (1979) |
Record holders:
- Typhoon Tip (1979): 26.05 inHg (strongest ever recorded)
- Hurricane Wilma (2005): 26.05 inHg (tied)
- Hurricane Maria (2017): 26.11 inHg
Aviation Altimeter Settings by Pressure Level
Pilots MUST set correct altimeter based on local pressure:
| Scenario | Pressure (inHg) | Altitude Impact | Action |
|---|---|---|---|
| High pressure system | >30.50 | Altimeter UNDER-reads | Set to actual pressure |
| Normal day | 29.92 | Standard (ISA) | Standard setting |
| Low pressure system | <29.50 | Altimeter OVER-reads | Reduce announced altitude |
| Sea level standard | 29.92 | Baseline | International standard |
| Denver area | ~24.6 | High altitude | Adjust for local pressure |
| Extreme low | 28.0 | Significant error | Major safety issue |
| Extreme high | 31.0 | Significant error | Safety concern |
Pilot rule: Set altimeter at each airport to ensure safe clearance from terrain
Geographic Variation in Barometric Pressure
Elevation dramatically changes barometric pressure:
| Location | Elevation | Sea-Level Pressure | Local Pressure (inHg) | Notes |
|---|---|---|---|---|
| Sea level | 0 ft | Standard | 29.92 | Reference point |
| New York City | 10 ft | Normal | ~29.90 | Nearly sea level |
| Denver | 5,280 ft | - | ~24.65 | "Mile High City" |
| Mexico City | 7,382 ft | - | ~22.65 | High altitude |
| La Paz, Bolivia | 11,975 ft | - | ~19.75 | Extreme altitude |
| Death Valley | -282 ft | - | ~30.25 | LOWEST point in US |
| Mount Whitney | 14,505 ft | - | ~16.88 | Highest in US |
| Mount Everest | 29,032 ft | - | ~9.23 | World's highest |
Weather reports: Always report sea-level pressure (not local) for standardization
Pressure Changes and Weather Prediction
Barometric TRENDS predict weather better than absolute values:
| Trend | Rate | Weather Prediction | Reliability |
|---|---|---|---|
| Rising rapidly | +0.10+ inHg/hour | Clearing, improving | Very high |
| Rising steady | +0.05-0.10 inHg/hour | Fair weather | High |
| Steady | ±0.03 inHg/hour | No change | High |
| Falling steady | -0.05-0.10 inHg/hour | Rain developing | High |
| Falling rapidly | -0.10+ inHg/hour | Heavy rain/storms | Very high |
Meteorologist's insight: A FALLING barometer is more important than a LOW barometer
Inch of Mercury Conversion Formulas
To Pascal:
To Kilopascal:
To Megapascal:
To Hectopascal:
To Bar:
To Millibar:
To Atmosphere:
To Technical Atmosphere:
To Torr:
To Millimeter of Mercury:
To Pound per Square Inch:
To Kilopound per Square Inch:
To Kilogram-force per Square Centimeter:
To Kilogram-force per Square Meter:
To Millimeter of Water Column:
To Inch of Water Column:
Frequently Asked Questions
Formula: PSI = inHg × 0.491154 Examples:
- 29.92 inHg = 14.7 PSI (sea level)
- 30 inHg = 14.74 PSI
- 28 inHg = 13.75 PSI (hurricane)
- 25 inHg = 12.28 PSI
- 20 inHg = 9.82 PSI inHg to PSI converter →
Convert Inch of Mercury
Need to convert Inch of Mercury to other pressure units? Use our conversion tool.
Inch of Mercury Quick Info
Related Pressure Units
Popular Conversions
- Inch of Mercury to PascalConvert →1 inHg = 3386.389 Pa
- Inch of Mercury to KilopascalConvert →1 inHg = 3.386389 kPa
- Inch of Mercury to MegapascalConvert →1 inHg = 0.003386 MPa
- Inch of Mercury to HectopascalConvert →1 inHg = 33.86389 hPa
- Inch of Mercury to BarConvert →1 inHg = 0.033864 bar
- Inch of Mercury to MillibarConvert →1 inHg = 33.86389 mbar