Find the active width
Start with the camera's active imaging area. If the data sheet gives 1,920 active columns on a 3.45 µm pitch, their nominal span is 6.624 mm. A crop or region of interest needs its own active column count.
Read the direction on the specification
A quoted diagonal angle uses the sensor diagonal. Compare a horizontal specification with a calculation using sensor width. Optical format labels such as 1-inch type describe a format class. Get the actual width in millimeters from the sensor data sheet.
Check the lens model
For rectilinear projection at infinity, image height equals focal length times the tangent of the field angle. At finite working distance or with significant distortion, use the lens's field mapping or a calibrated measurement.