Telescopes and microscopes typically use two lenses. The user looks through the ocular lens, or eye piece, while an objective lens on the opposite end of the device further magnifies the object under observation. Though the two devices work similarly, the process for calculating their magnification is different.
Magnification in Microscopes
The formula for the total magnification of a microscope is the objective lens magnification times the ocular lens magnification. Almost all adjustable microscopes have the magnification of each lens written on the side of the lens. If you can't find it, look in the user's manual for the figures. Most microscopes have an ocular lens magnification of 10. In a microscope with an ocular magnification of 10 and an objective magnification of 20, the total magnification would be 200x.
Magnification in Telescopes
The formula for total magnification of a telescope is the focal length of the objective lens divided by the focal length of the eyepiece lens. The focal length is the distance between the center of the lens and the point where it is in focus. As with microscopes, you can find this data in the owner's manual or on the labeling of the specific device. In a telescope with an objective lens focal length of 1000 mm and an eyepiece focal length of 10mm, the total magnification would be 100x.