Convert Pressure from pounds per square inch to Inches of mercury (psi to inHg)

What is a pound per square inch (psi)?

A pound per square inch (PSI) is a unit of pressure commonly used in the imperial system (which is standard in the United States).

It measures the amount of force applied over a single square inch of area.

---

Common Examples of PSI (Where is PSI Used?)

PSI is a critical measurement for safety and performance in many common items, including:

• Vehicle Tires: Proper tire pressure (PSI) ensures optimal handling, fuel efficiency, and tire life. You can find the recommended level on a sticker inside the driver's door.

• Home Water Pressure: Affects everything from shower performance to how well appliances like dishwashers and washing machines function.

• Sports Equipment: Items like footballs, basketballs, and soccer balls require a specific PSI to perform correctly and safely.

• Tools: Air compressors, pressure washers, and paint sprayers all have specific PSI ratings for safe and effective operation.

---

Converting PSI to Metric Units like Pascals and Bar

While PSI is standard in the imperial system, scientific and international contexts use metric (SI) units like the Pascal (Pa) and the bar.

• PSI to Pascal (Pa): The official SI unit for pressure.
  1 PSI ≈ 6,894.76 Pascals (or 6.895 kilopascals, kPa).

• PSI to Bar: A common unit in automotive and weather contexts. 1 bar is roughly equal to atmospheric pressure at sea level.
  1 PSI ≈ 0.0689 bar.

---

Understanding the Types of PSI: PSIG, PSIA, and PSID

The term "PSI" can be specified further to give more context:

• PSIG (Pound-force per square inch gauge): Measures pressure relative to the surrounding atmospheric pressure. This is the most common measurement, used by a standard tire gauge.

• PSIA (Pound-force per square inch absolute): Measures pressure relative to a perfect vacuum (absolute zero pressure).
  The formula is: PSIA = PSIG + local atmospheric pressure.

• PSID (Pound-force per square inch differential): Measures the difference in pressure between two separate points in a system, such as across a filter or valve.

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)