From Kekus Digital:
fovCalculator is a handy utility for photographers needing to calculate the horizontal, vertical and diagonal field of view of their lenses. If you have a professional 35 mm based digital camera, the field of view given in your lens manual is incorrect for your camera. fovCalculator takes your camera's focal length multiplier (found on sites such as www.dpreview.com) or the size of your image sensor and calculates the field of view, number of photos to make a panorama and the overlap of your images. You can also calculate the field of view for any reasonable lens and film size combination.