Telescope Max Magnification Calculator
Work out your telescope's maximum useful magnification and exit pupil from aperture size with this free calculator, right in your browser.
100% FreeRuns in your browserBasic
Enter Values
Enter your values and press Calculate to see results.
How to use this calculator
- Enter the telescope's aperture in mm.
- Read the maximum useful magnification.
How it works
Rule of thumb: max useful magnification ≈ 2× the aperture in millimetres.
Worked example
150 mm aperture. ≈ 300× maximum.
Common mistakes
- Chasing the eyepiece's printed magnification beyond what the aperture supports.
- Forgetting that atmospheric 'seeing' often limits you well below the max.
Frequently asked questions
Why is there a maximum?
Beyond about 2× the aperture (mm), the image gets dim and blurry without revealing more detail.
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Cite this tool
Zerdly. (2026). Telescope Max Magnification Calculator. Retrieved from https://www.zerdly.com/tools/telescope-max-magnification-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.