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Telescope Magnification Calculator

Find your telescope's magnification in a click

How it is calculated

The tool divides the focal length of the telescope's main mirror or lens by the focal length of the eyepiece: M = F_telescope / f_eyepiece. Both values are in millimetres and must be in the same unit, and both must be above zero. The result is rounded to a whole number and shown with an x, so a ratio of 36.4 is displayed as 36x. The calculation uses only focal lengths; it does not know the telescope's aperture, so it cannot tell you whether the image will still be sharp at that power. If you use a Barlow lens or focal reducer, multiply the telescope focal length by its factor before entering it.

Worked example

A telescope with a focal length of 900 mm is used with a 25 mm eyepiece. Magnification: 900 / 25 = 36x. Changing to a 10 mm eyepiece gives 900 / 10 = 90x, and a 6 mm eyepiece gives 150x. A smaller eyepiece focal length means a higher magnification.

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Common mistakes

  • Using the aperture (diameter) instead of the focal length of the telescope; they are different numbers, and the focal length is usually printed on the tube.
  • Dividing the eyepiece focal length by the telescope's, which inverts the answer.
  • Mixing units, such as a telescope in inches and an eyepiece in millimetres.
  • Forgetting a Barlow lens, which multiplies the actual magnification (a 2x Barlow doubles the result).
  • Assuming a higher number is a better view; the image gets dimmer and blurrier once the magnification is too high for the aperture.

About this calculator

Telescope magnification follows the simple ratio M = telescope focal length / eyepiece focal length, so a 1000 mm focal length telescope paired with a 10 mm eyepiece yields 100× magnification, and swapping in a 25 mm eyepiece drops it to 40×. The same telescope therefore produces a whole range of magnifications simply by changing eyepieces, without altering the telescope's aperture or light-gathering ability.

Amateur astronomers use this calculation to choose the right eyepiece for a given target — low magnification with a wide field for star clusters and galaxies, high magnification for planetary and lunar detail — while keeping in mind that a telescope's aperture sets a practical maximum useful magnification (roughly 50× per inch of aperture), beyond which the image just grows dimmer and blurrier rather than more detailed.

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