Vacuum Pressure Conversion Chart: Common Units and Formulas

Vacuum readings are not always easy to compare. One gauge may use inHg, while an equipment specification lists Torr, microns, psi, mbar, or pascals. A vacuum pressure conversion puts those readings into the same unit, but there is one detail to check first: Is the pressure absolute, gauge-based, or expressed as vacuum below atmospheric pressure? That distinction changes what the number represents.

Common Vacuum Pressure Units

The most common vacuum pressure units are:

• Torr: Common in vacuum technology and laboratory work.
• Micron: One-thousandth of a Torr. A reading of 500 microns equals 0.5 Torr.
• Millimeters of mercury (mmHg): Numerically close to Torr and often treated as equivalent for ordinary industrial calculations.
• Inches of mercury (inHg): Often shown on mechanical vacuum gauges.
• Pounds per square inch (psi): May be expressed as absolute pressure, gauge pressure, or a vacuum differential.
• Pascals (Pa) and kilopascals (kPa): SI pressure units.
• Bar and millibar (mbar): Common in European equipment specifications.
• Atmosphere (atm): A reference unit based on standard atmospheric pressure.

Always check the unit and pressure reference printed beside a gauge reading or listed in the equipment documentation. The number alone is not enough.

Absolute Pressure vs. Gauge Pressure

Absolute pressure uses a perfect vacuum as its zero reference. Gauge pressure uses local atmospheric pressure as zero.

This relationship is commonly written as:

Absolute pressure = atmospheric pressure + gauge pressure

Gauge pressure below atmospheric pressure is negative. At standard sea-level conditions, a gauge reading of −5 psig corresponds to:

14.696 psia + (−5 psi) = 9.696 psia

Some vacuum gauges display the pressure difference below atmosphere as a positive vacuum reading. For those readings, use:

Absolute pressure = atmospheric pressure − indicated vacuum

Standard atmospheric pressure is approximately 14.696 psia, but local atmospheric pressure changes with elevation and weather. Use the actual atmospheric pressure when the application requires more than a rough estimate.

Vacuum Conversion Chart

The following vacuum conversion chart shows standard atmospheric pressure expressed in several units.

These values are absolute-pressure equivalents at one standard atmosphere. They do not mean that a 20 inHg vacuum gauge reading equals 20 inHg absolute.

Common Vacuum Pressure Conversion Formulas

The following pressure conversion factors provide quick manual conversions without a vacuum conversion calculator:

• Torr × 1,000 = microns
• Microns ÷ 1,000 = Torr
• Torr × 133.322 = Pa
• Torr ÷ 760 = atm
• PSI × 6.89476 = kPa
• Bar × 14.5038 = PSI
• inHg × 0.49115 = PSI
• mbar × 0.75006 = Torr

Example: Torr to Microns

To convert 250 Torr to microns:

250 Torr × 1,000 = 250,000 microns

A lower absolute-pressure reading indicates a deeper vacuum. For example, 250 microns represents a deeper vacuum than 250 Torr.

Example: inHg to PSI

A vacuum gauge shows 20 inHg below atmospheric pressure:

20 inHg × 0.49115 = 9.82 psi

The 9.82 psi value is the pressure difference below atmosphere. Using standard atmospheric pressure, the estimated absolute pressure is:

14.696 psia − 9.82 psi = 4.88 psia

This extra step is easy to miss when comparing an inHg vacuum reading with a specification written in psia.

Note: Before using a converted value, confirm that both readings use the same pressure reference. Also account for local atmospheric pressure and use enough decimal precision for the application.

A Conversion Does Not Size Vacuum Equipment

A converted pressure value helps compare specifications, but it does not determine the required vacuum pump or blower. Equipment selection can also depend on:

• System volume
• Required pump-down time
• Gas flow
• Leakage
• Operating elevation
• Process temperature
• Target absolute pressure
• Pipe diameter and length
• Fittings and other pressure losses

Contact DXP for Help Selecting Vacuum and Blower Equipment

Pressure conversion is only one part of equipment selection. DXP can help evaluate operating pressure, target vacuum, gas flow, system volume, piping losses, leakage, elevation, and pump-down requirements. Our team supports industrial facilities with vacuum and blower equipment, pumping solutions, and rotating equipment services for new systems and existing installations.

Need help comparing equipment options or reviewing system requirements? Contact DXP today.