Focal Ratio Calculator
Calculate a telescope or camera lens focal ratio (f-number) from focal length and aperture diameter, free and instant 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 focal length and aperture (same units).
- Read the focal ratio.
How it works
Focal ratio = focal length ÷ aperture. A 1200 mm scope with 150 mm aperture is f/8.
Worked example
1200 mm / 150 mm. = f/8.
Common mistakes
- Mixing units between focal length and aperture.
- Thinking a bigger f-number is 'better' — lower is faster/brighter.
Frequently asked questions
Why does focal ratio matter?
Lower f-ratios give brighter, wider fields (good for deep-sky); higher f-ratios give more magnification per eyepiece (good for planets).
Related tools
- Light Travel Time Calculator
- Telescope Magnification Calculator
- Telescope Max Magnification Calculator
- Angular Size Calculator
- Escape Velocity Calculator
- Light-Year Converter
Explore more in Astronomy & Navigation.
Cite this tool
Zerdly. (2026). Focal Ratio Calculator. Retrieved from https://www.zerdly.com/tools/focal-ratio-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.