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Flash Guide Number Calculator

Aperture needed for correct flash exposure at a given distance.

Guide number (ISO 100, metres)

Flash to subject distance

ISO

How it works

Guide number = distance × f-number at ISO 100, so aperture = GN ÷ distance. Raising ISO by 4× doubles the effective guide number. Manufacturers quote GN for a specific zoom position, so a wider zoom setting reduces it. Bouncing the flash off a ceiling loses a stop or more, because the light travels farther. Example: A flash with GN 40 at 5 m needs f/8 at ISO 100.

FAQ

Is this calculator free?

Yes. No signup is needed and nothing you enter is stored.

Is the result exact?

Results are rounded for display. For financial, medical, or legal decisions, confirm with a professional.

Do my numbers leave my device?

No. The calculation runs in your browser.