Your weather app says the barometer reads 29.92 inHg. Your lab notebook logs the very same afternoon as 760 mmHg. Nothing changed outside your window — only the unit did. If you have ever bounced between a blood pressure cuff, an aviation briefing, and a vacuum gauge, you already know the frustration: two units, one physical reality, and no obvious bridge between them. This guide builds that bridge.

Two Units, One Shiny Liquid

Both mmHg and inHg trace back to the same invention: the mercury barometer. Because mercury is extremely dense, a column of it just a few dozen centimeters tall can balance the weight of the entire atmosphere. Early scientists realized that the height of that column was a direct, readable measure of pressure.

The only disagreement was the ruler. Countries using the metric system measured the column in millimeters. Countries using the imperial system measured it in inches. Same liquid, same physics, different markings on the glass.

What Is a Millimeter of Mercury (mmHg)?

A millimeter of mercury is the pressure exerted by a mercury column exactly one millimeter tall under standard conditions. The symbol is mmHg, sometimes written as mm Hg.

You meet this unit most often in medicine. Every blood pressure reading on Earth — the familiar "120 over 80" — is expressed in mmHg. It also appears in vacuum science, where its close cousin, the torr, is nearly identical in value.

One honest caveat: mmHg is not part of the SI system. Scientists prefer the pascal. But medicine and industry have kept mmHg alive because it is intuitive, compact, and deeply embedded in equipment and standards.

What Is an Inch of Mercury (inHg)?

An inch of mercury is the same idea with an imperial ruler: the pressure produced by a mercury column one inch tall. Its symbol is inHg.

This unit dominates three fields, especially in the United States:

• Aviation: Pilots dial their altimeters to a setting given in inHg, such as 29.92, so the instrument reads true altitude.
• Meteorology: American TV forecasts and airport reports quote barometric pressure in inHg.
• HVAC and vacuum work: Technicians describe refrigeration evacuation and vacuum depth in "inches of mercury," often shortened to "inches of vacuum."

How mmHg and inHg Relate

Here is the elegant part. Both units measure the push of a mercury column, so the only thing separating them is the length scale. By international agreement, one inch equals exactly 25.4 millimeters. That single fixed fact locks the two pressure units together forever.

• 1 inHg = exactly 25.4 mmHg
• 1 mmHg = exactly 1 ÷ 25.4 inHg (about 0.0394 inHg)

No approximations, no temperature corrections, no guesswork. The relationship is exact because the underlying ruler relationship is exact.

The Formulas You Need

To go from metric to imperial, divide by 25.4:

inHg = mmHg ÷ 25.4

To go from imperial to metric, multiply by 25.4:

mmHg = inHg × 25.4

A Worked Example

Suppose a European station reports today's pressure as 752 mmHg, and you want to compare it with a U.S. forecast. Divide: 752 ÷ 25.4 ≈ 29.61 inHg. Now the two reports speak the same language.

Running the reverse check builds confidence: take 30.15 inHg and multiply by 25.4 to get roughly 765.8 mmHg. Both directions rely on nothing more than one multiplication or division.

mmHg to inHg Conversion Table

The table below covers the values people look up most often. Each result is rounded to four decimal places for clarity.

Notice how quickly the numbers grow. Five hundred millimeters of mercury sounds like a lot, yet it is still below typical sea-level atmospheric pressure — a reminder that these units describe modest, human-scale pressures.

Where This Conversion Earns Its Keep

Medicine and Blood Pressure

Medical literature, monitors, and clinical guidelines use mmHg universally. But if you encounter a historical document, an engineering spec for medical devices, or a pressure sensor datasheet quoting inHg, conversion keeps everything comparable. For perspective, a systolic reading of 120 mmHg works out to roughly 4.72 inHg.

Aviation and Altimetry

Pilots in North America receive altimeter settings in inHg. Some legacy instruments, training materials, and foreign procedures still reference mmHg. A pilot who can move between the two instantly eliminates a whole class of altitude-setting confusion — and altitude errors are not the kind you want to discover at cruise.

Weather, Barometers, and Science

Much of the classic scientific record — chemistry texts, physiology papers, older meteorological tables — uses mmHg. Modern American media uses inHg. Researchers and hobbyists who read across both worlds convert constantly, usually without thinking twice.

Vacuum Systems and HVAC

Vacuum adds a twist worth knowing: technicians often quote inHg as a depth below atmospheric pressure, while lab documents quote mmHg as an absolute pressure. Before comparing any two figures, confirm whether each one describes pressure above zero or a drop from ambient. The arithmetic stays the same; the interpretation changes.

Three Tips for Flawless Conversions

1. Keep full precision until the last step. Round only your final answer. Chopping digits mid-calculation compounds small errors into visible ones.
2. Match your rounding to the job. Weather reporting lives happily at two decimals. Laboratory work may demand four or more. Choose deliberately.
3. Label gauge versus absolute. Write down which kind of pressure you are converting. Future-you will thank present-you.

The Bottom Line

Millimeters of mercury and inches of mercury are two dialects of the same pressure language, separated by a single exact number: 25.4. Divide mmHg by 25.4 to reach inHg; multiply inHg by 25.4 to return. Master that one relationship and every blood pressure chart, altimeter setting, barometer reading, and vacuum spec becomes instantly translatable.

Ready to skip the mental math? Run your next value through the converter above and get a precise answer in seconds.