Flash Guide Number Calculator

Calculate flash power, working distance, or required aperture using the guide number formula: GN = distance x f-stop.

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How It Works

A flash guide number (GN) describes its power: GN = distance x f-stop at ISO 100. A GN 36 flash can light a subject at 6.4 meters at f/5.6. The guide number scales with the square root of ISO, so doubling ISO increases effective GN by about 1.4x. Flash output follows the inverse square law, meaning double the distance requires 4x the power.