🦉 UnitOwl

Inches of Mercury to mmHg

1 Inches of Mercury (inHg) = 25.4 mmHg (mmHg)

Result
25.4 mmHg
1 inHg = 25.4 mmHg

How Many mmHg in Inches of Mercury?

One inch of mercury (inHg) equals exactly 25.4 millimeters of mercury (mmHg). To convert inHg to mmHg, multiply the inHg value by 25.4. This conversion mirrors the fundamental inch-to-millimeter relationship and is particularly important in meteorology and aviation. In the United States, barometric pressure is reported in inches of mercury — weather forecasts say things like "the barometer is falling from 30.15 to 29.85 inches." In scientific and international contexts, the same pressure would be expressed in mmHg (or equivalently, torr). Standard atmospheric pressure is 29.9213 inHg or 760 mmHg. Pilots in the US set their altimeters using inHg (the "altimeter setting" in ATIS broadcasts), while some international airports use hectopascals. Understanding the inHg-to-mmHg conversion helps weather enthusiasts, pilots, and scientists translate between American and international pressure reporting conventions.

How to Convert Inches of Mercury to mmHg

  1. Start with your pressure value in inches of mercury (inHg).
  2. Multiply the inHg value by 25.4 to get mmHg.
  3. For example, 30.00 inHg x 25.4 = 762.0 mmHg.
  4. This conversion uses the same factor as inches to millimeters, because both units measure the height of a mercury column — just in different length units.
  5. Standard atmosphere: 29.9213 inHg = 760 mmHg (29.9213 x 25.4 = 760.00).

Real-World Examples

A US weather report says barometric pressure is 30.12 inHg. What is that in mmHg?
30.12 x 25.4 = 765.0 mmHg. This is above the standard 760 mmHg, indicating a high-pressure system.
A pilot receives an altimeter setting of 29.85 inHg. What is this in mmHg?
29.85 x 25.4 = 758.2 mmHg. This is slightly below standard pressure, indicating a low-pressure system.
A laboratory instrument reports vacuum level as 5 inHg below atmosphere. What is that in mmHg?
5 x 25.4 = 127 mmHg of vacuum. Absolute pressure: 760 - 127 = 633 mmHg.
A weather archive lists a hurricane's central pressure as 27.17 inHg. Convert to mmHg.
27.17 x 25.4 = 690.1 mmHg. This extremely low pressure (Hurricane Katrina's central pressure was about 902 mbar / 677 mmHg) indicates a powerful storm.
A barometric pressure map shows isobars from 29.50 to 30.50 inHg. What is this range in mmHg?
29.50 x 25.4 = 749.3 mmHg to 30.50 x 25.4 = 774.7 mmHg. A range of 25.4 mmHg (1 inHg).

Quick Reference

Inches of Mercury (inHg) mmHg (mmHg)
1 25.4
2 50.8
5 127
10 254
25 635
50 1,270
100 2,540
500 12,700
1,000 25,400

History of Inches of Mercury and mmHg

Both inHg and mmHg trace their origin to Torricelli's barometer experiment of 1643, where he demonstrated that atmospheric pressure supports a mercury column about 30 inches (760 mm) high. The choice of mercury was deliberate — its high density (13.6 times that of water) meant the barometer tube could be a practical length. A water barometer would need to be over 10 meters (33 feet) tall. The inch-based measurement became standard in the United States and United Kingdom, where the inch was the dominant small-length unit. Continental Europe, using the metric system, naturally expressed the same measurement in millimeters. Both units describe the same physical phenomenon — the height of a mercury column supported by pressure — but in different length systems. The exact relationship (1 inHg = 25.4 mmHg) was established when the inch was defined as exactly 25.4 mm in 1959. Mercury barometers are now being phased out due to environmental concerns about mercury toxicity, but the units persist in digital instruments that simulate the mercury measurement.

Common Mistakes to Avoid

  • Confusing inHg with mmHg in weather data. US reports use inHg (values around 29-31), while international science uses mmHg (values around 740-780). If you see a pressure value near 30, it is almost certainly inHg. If it is near 760, it is mmHg.
  • Forgetting that this conversion is exact, not approximate. Because 1 inch = exactly 25.4 mm (by definition), 1 inHg = exactly 25.4 mmHg. There is no rounding error.
  • Confusing mmHg with mbar or hPa. While 1 atm = 760 mmHg = 1013.25 mbar, the mmHg-to-mbar conversion is NOT a simple factor. 1 mmHg = 1.33322 mbar. Do not treat mmHg and mbar as interchangeable.

Frequently Asked Questions

Why does the US use inches of mercury while most countries use millibars or hectopascals?
Historical convention. American weather services adopted inHg from the British tradition of mercury barometers calibrated in inches. When the World Meteorological Organization standardized on hectopascals (millibars), most national weather services switched, but the US National Weather Service retained inHg for public forecasts. Aviation in the US also uses inHg for altimeter settings, while ICAO recommends hectopascals.
What is a "normal" barometric pressure reading in both units?
Standard atmospheric pressure: 29.92 inHg = 760 mmHg. Normal range for weather: 29.70-30.20 inHg (754-767 mmHg). Strong high pressure: above 30.40 inHg (772 mmHg). Strong low pressure (storms): below 29.40 inHg (747 mmHg). Hurricane-level low: below 28.00 inHg (711 mmHg).
How does barometric pressure affect weather?
Rising pressure (above 760 mmHg / 29.92 inHg) generally indicates improving weather with clearer skies. Falling pressure (below 760 mmHg / 29.92 inHg) typically signals approaching clouds, rain, or storms. Rapid pressure drops (more than 2 mmHg or 0.08 inHg per hour) can indicate severe weather approaching. Pilots use barometric pressure to calibrate altimeters — incorrect settings cause altitude errors of about 30 feet (9 meters) per 0.01 inHg of error.
Quick Tip

A handy benchmark: standard atmospheric pressure is almost exactly 30 inHg (the precise value is 29.9213 inHg = 760 mmHg). So when you hear a weather report saying the barometer is "falling through 30 inches," that means pressure is crossing the standard atmosphere mark. For quick mental conversion, remember that 1 inch of mercury = about 25 mmHg, so each 0.1 inHg change equals about 2.5 mmHg.