You're looking at a weather report that reads "29.92 inches of mercury," while the spec sheet for your pressure sensor lists its range in kilopascals. Same air, same force, completely different numbers. If that mismatch has ever left you guessing, this guide will clear it up in plain language.

Pressure in Two Languages

Pressure is simply force spread over an area. Squeeze a balloon and the air inside pushes back on every square centimeter of its surface. That push can be described in many units, and which one you see usually depends on where you are and who built the gauge.

Inches of mercury grew out of early barometer design and still dominates American aviation and weather reporting. Kilopascals belong to the modern metric system and appear in science, engineering, and international standards worldwide. Converting between them isn't optional if you work across those worlds — it's essential.

What Is an Inch of Mercury (inHg)?

An inch of mercury is the pressure exerted by a column of liquid mercury exactly one inch tall. When scientists in the 1600s built the first barometers, they measured air pressure by how high it could push mercury up a glass tube. The unit stuck.

Today, the symbol inHg appears on aircraft instruments, U.S. weather broadcasts, and vacuum equipment labels. If you've heard a pilot say "altimeter two-niner-niner-two," that's 29.92 inches of mercury — roughly the pressure of a standard day at sea level.

What Is a Kilopascal (kPa)?

The pascal (Pa) is the official SI unit of pressure, defined as one newton of force pressing on one square meter. A kilopascal is simply 1,000 pascals. It's named after Blaise Pascal, the French physicist whose experiments helped establish how atmospheric pressure works.

Because it's part of the metric system, the kilopascal is the default in most countries, most textbooks, and nearly every international engineering document. Tire pressures, boiler ratings, and material strength data frequently use it.

How the Conversion Works

Both units describe the same physical reality, so the relationship between them is fixed by physics, not by anyone's preference. One inch of mercury equals approximately 3.3864 kilopascals. This figure comes from the weight of a one-inch column of mercury under standard gravity — a constant, measurable quantity.

That gives you two simple formulas:

• To convert inHg to kPa: multiply the inHg value by 3.3864
• To convert kPa to inHg: divide the kPa value by 3.3864

Example: A calm day produces a barometer reading of 28.50 inHg. Multiply 28.50 × 3.3864 and you get about 96.51 kPa. One multiplication, and the reading now speaks metric.

A Note on Precision

Mercury expands slightly when warmed, so ultra-precise laboratory conversions specify the temperature of the mercury column. The 3.3864 factor reflects the conventional standard based on mercury at 0 °C. For everyday aviation, weather, and workshop use, this level of precision is more than enough.

Where You'll Actually Use This Conversion

Aviation

Pilots set their altimeters using barometric pressure reported in inches of mercury across North America. Flight into international airspace or onto foreign charts often means encountering hectopascals and kilopascals instead. A quick conversion keeps altitude readings accurate and everyone on the same page.

Weather and Meteorology

U.S. forecasts quote sea-level pressure in inHg, while global meteorological data uses metric units. Comparing a storm's intensity across sources means translating between the two. For reference, standard sea-level pressure is about 29.92 inHg, which works out to roughly 101.33 kPa.

Vacuum Systems and HVAC

Vacuum pumps and refrigeration equipment are commonly rated in inches of mercury, especially in North America. European-made components and technical manuals typically specify the same performance in kilopascals. Technicians who service mixed fleets rely on this conversion daily.

Engineering and Calibration

Sensor datasheets, calibration certificates, and test procedures mix unit systems depending on their origin. Verifying that a gauge reading in inHg falls within a tolerance written in kPa requires exactly this calculation — done correctly, every time.

Quick Reference: inHg to kPa Conversion Table

The table below uses the standard factor of 3.3864 kPa per inHg. Values are rounded to two decimal places for practical use.

Tips for Accurate Conversions

1. Multiply first, round last. Carry extra decimal places through your calculation and round only the final answer to avoid compounding errors.
2. Don't confuse inHg with mmHg. Millimeters of mercury is a different unit entirely — one inch holds 25.4 millimeters, so mixing them up throws results off dramatically.
3. Know whether you're dealing with absolute or gauge pressure. Gauge readings exclude atmospheric pressure; absolute readings include it. The conversion factor stays the same, but interpreting the result correctly matters.
4. Match the context. Aviation and weather values are almost always absolute pressure. Vacuum measurements are often quoted relative to atmosphere, so read the label before converting.

The Bottom Line

Inches of mercury and kilopascals are two dialects of the same language: pressure. The bridge between them is a single, reliable multiplier — 3.3864 — and once you know it, readings from a cockpit, a forecast, or a pump manual all become interchangeable. Whether you're a pilot cross-checking an altimeter, a technician commissioning a vacuum system, or a student checking homework, the process takes seconds.

Ready to skip the mental math? Use the converter on this page to turn any inHg value into kilopascals instantly — accurately, every time.