Enter the focal length (mm) and the aperture (mm^2) into the Focal Ratio Calculator. The calculator will evaluate and display the Focal Ratio. 

Focal Ratio Formula

The following formula is used to calculate the Focal Ratio. 

FR = FL / A
  • Where FR is the Focal Ratio
  • FL is the focal length (mm) 
  • A is the aperture (mm^2) 

To calculate the focal ratio, divide the foal length by the aperture area.

How to Calculate Focal Ratio?

The following example problems outline how to calculate Focal Ratio.

Example Problem #1:

  1. First, determine the focal length (mm).
    • The focal length (mm) is given as: 5.
  2. Next, determine the aperture (mm^2).
    • The aperture (mm^2) is provided as: 50.
  3. Finally, calculate the Focal Ratio using the equation above: 

FR = FL / A

The values given above are inserted into the equation below and the solution is calculated:

FR = 5 / 50 = .10

Example Problem #2: 

For this problem, the variables required are provided below:

focal length (mm) = 6

aperture (mm^2) = 70

Test your knowledge using the equation and check your answer with the calculator above.FR = FL / A = ?