RedAngle is a calculator that works out Dimensional and Angular Field of View (FoV)

Enter the Focal length of your lens, the focal distance, and select the sensor size.
RedAngle will provide you with the the angular and dimensional field of views for that setup.

RedAngle provides Vertical, Horizontal and Diagonal measurements.

RedAngle has four presets with different sensor size settings (in mm):
35mm (22 x 12.375), RED 2K (11.06 x 6.22), RED 3K (16.59 x 9.33), and RED 4K (22.12 x 12.44)

You can change the Aspect ratio of the sensor by going to “More Settings”, and selecting the desired aspect.
Apect ratio only affects presets, not custom sensor sizes.

You can create your own custom sensor size by going to “More Settings”, turning on the “Custom Sensor Size” switch and entering the W x H (in mm).

Similarly if your lenses were designed for another sensor size than the camera you are using them on (i.e Nikon lenses on a RED) you can add a custom Lens Magnification Factor (LMF) in “More Settings” too.
A Nikon lens LMF on RED is around 1.52.