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Telescope Magnification Calculator

Calculate telescope magnification from focal length and eyepiece focal length to know what power you're viewing at, free online.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

How to use this calculator

  1. Enter the telescope's focal length and the eyepiece focal length (both in mm).
  2. Optionally add the aperture (objective diameter) for the focal ratio, exit pupil and max useful magnification.
  3. Read the magnification and optics figures.

How it works

The Telescope Magnification Calculator works out the power of a telescope-and-eyepiece combination and a few related optics figures.

Magnification = telescope focal length ÷ eyepiece focal length. The focal ratio is the focal length ÷ aperture. The exit pupil (aperture ÷ magnification) is the width of the light beam leaving the eyepiece, and a practical ceiling for useful magnification is roughly 2× the aperture in mm.

Worked example

1200 mm scope, 10 mm eyepiece, 150 mm aperture. Magnification = 1200 ÷ 10 = 120×, at f/8, with a 1.25 mm exit pupil. The max useful magnification is about 300×.

Frequently asked questions

Can I just keep using shorter eyepieces for more power?

Up to a point. Beyond about 2× the aperture (in mm) the image gets dim and fuzzy, and atmospheric 'seeing' usually limits you well before the theoretical maximum.

What is the exit pupil?

It's the width of the beam of light leaving the eyepiece. If it's wider than your eye's pupil (about 7 mm in the dark) some light is wasted; very small exit pupils make the image dim.

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Cite this tool

Zerdly. (2026). Telescope Magnification Calculator. Retrieved from https://www.zerdly.com/tools/telescope-magnification-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.