Convert Pressure from Inches of mercury to kilopascals (inHg to kPa)

Inch of Mercury to Kilopascal Conversion Formula

To convert a pressure from inches of mercury (inHg) to kilopascals (kPa), multiply by 3.3864.

• kPa = inHg × 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.

Step-by-Step Example

An aviation weather report gives the altimeter setting as 30.12 inHg, and a performance calculation uses SI units:

1. Take the pressure: 30.12 inHg
2. Multiply by 3.3864: 30.12 × 3.3864 = 102.0 kPa

The day is slightly higher-pressure than standard (101.325 kPa), which the density-altitude calculation will credit as a small performance gain.

Quick Reference Table

The Barometer Unit of North American Skies

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.

How many inches of mercury are in one kilopascal? One kilopascal (kPa) contains 0.29529980572284863811 Inches of mercury (inHg) — the inverse of the factor above. Multiplying by 3.3863889532610878529 takes you from inches of mercury to kilopascals; multiplying by 0.29529980572284863811 brings you back.

Put in words: one inch of mercury equals 3.3863889532610878529 kilopascals, so the inch of mercury is the larger unit of this pair. Converting between them never changes the amount of pressure being measured — only the size of the unit you count it in.

What is an 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.

Where is inHg Used?

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.

How to Convert inHg to Other Pressure Units

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:

• 0.491154 pounds per square inch (PSI)
• 33.86 millibars (mbar)
• 3,386.39 pascals (Pa)
• 0.000491154 kilopound per square inch (ksi)

What is a kilopascal (kPa)?

A kilopascal (kPa) is a metric unit of pressure that equals 1,000 pascals.

It is one of the most common units for measuring pressure worldwide, used in both scientific and everyday applications.

The Kilopascal (kPa) Explained

The base unit, the pascal (Pa), is named after the French polymath Blaise Pascal.

As part of the metric system (also known as the SI system), the kilopascal is widely used in scientific, engineering, and industrial fields due to its standardization and ease of calculation.

Common Applications of Kilopascals (kPa)

The kPa unit is used to measure pressure in many everyday and technical contexts. Common examples include:

• Tire Pressure: A common measurement for vehicle tire pressure in many countries outside the US.
• Weather Forecasting: Used for reporting atmospheric pressure.
• HVAC Systems: Essential for measuring differential pressure in heating, ventilation, and air conditioning systems.
• Medical: Frequently used for measuring blood pressure.
• Manufacturing: Critical for monitoring pressure in various industrial processes.

Kilopascal Conversions (kPa to PSI and Bar)

For practical use, it's essential to understand how kPa converts to other common pressure units.

• kPa to PSI: 1 kPa is approximately 0.145 PSI (Pounds per Square Inch). PSI is the standard pressure unit used in the United States.
• kPa to Bar: 100 kPa is exactly equal to 1 bar. The bar is another metric unit frequently used for atmospheric and water pressure.

Inch of mercury to kilopascal Conversion Table

Here are some quick reference conversions from Inch of mercury (inHg) to kilopascal (kPa):

This conversion in other languages

Engineering datasheets across Europe write Inch of mercury to kilopascal several different ways. The calculation is the same in every one of them.