How is telescope magnification calculated?

Magnification equals the telescope focal length divided by the eyepiece focal length. A 1000 mm telescope with a 25 mm eyepiece gives 40×.

What is the maximum useful magnification?

Roughly twice the aperture in millimetres. Beyond that the image grows dim and blurry, so the tool warns when you exceed it.

What is the focal ratio?

The focal ratio (f-number) is the telescope focal length divided by the aperture, for example a 1000 mm focal length and 200 mm aperture give f/5.

Telescope Magnification Calculator

Find your telescope magnification from focal length and eyepiece.

Telescope magnification calculator

Magnification is simply telescope focal length ÷ eyepiece focal length. A 1000 mm telescope with a 25 mm eyepiece magnifies 40×.

This tool also reports the focal ratio (f-number) and the maximum useful magnification — about twice the aperture in millimetres. Push past that and the image gets dim, so the calculator flags it. Everything runs in your browser.