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)

## 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 = ?