Calculate vacuum pressure or absolute pressure from local atmospheric pressure and one known pressure value in Pa, bar, psi, and more.

Vacuum here is the positive pressure below local atmosphere. Gauge pressure has the opposite sign. Use actual local atmospheric pressure when known.

Preset values are examples; altitude and weather change pressure.

Vacuum Pressure Formula

The following formula is used to calculate the Vacuum Pressure. 

Pvac = PL - PA
  • Where Pvac is the Vacuum Pressure (Pa)
  • PL is the local atmospheric pressure (Pa) 
  • PA is the absolute pressure (Pa) 

To calculate the positive vacuum-pressure magnitude, subtract absolute pressure from local atmospheric pressure. Signed gauge pressure is its negative. This vacuum-only model requires absolute pressure between zero and local atmosphere.

How to Calculate Vacuum Pressure?

The following two arithmetic examples use illustrative low reference pressures, rather than typical atmospheric pressures near Earth’s surface.

Example Problem #1:

  1. First, determine the local atmospheric pressure (Pa). In this example, the local atmospheric pressure (Pa) is given as 500.
  2. Next, determine the absolute pressure (Pa). For this problem, the absolute pressure (Pa) is given as 400.
  3. Finally, calculate the Vacuum Pressure using the equation above: 

Pvacuum = PL – PA

Inserting the values from above and solving the equation: 

Pvacuum = 500 – 400 = 100 (Pa)

Example Problem #2: 

Using the same process as above, first define the variables required by the formula. In this case, these values are:

local atmospheric pressure (Pa) = 234

absolute pressure (Pa) = 95

Entering these values yields:

Pvacuum = 234 – 95 = 139 (Pa)