Flash Guide Number
Aperture for a flash distance, or the reach at an aperture, from the guide number at your ISO and power — metres or feet, with the inverse-square falloff.
The aperture a flash at a distance needs, or how far it reaches at an aperture, the effective guide number at your ISO and power, and the distances one stop darker and two stops brighter.
Example: A guide number 36 (m) flash at ISO 400 and half power has an effective GN of 50.9: a subject 4 m away needs f/12.7.
Distance times
aperture.
What a guide number means, how ISO and power scale it, how to solve for aperture or reach, and what the inverse-square law does across a scene.
The guide number
A flash’s guide number is the distance times the f-number that gives a correct exposure at ISO 100 and full power: GN 36 (metres) means f/9 at 4 m or f/4 at 9 m. So the aperture for a distance is GN ÷ d and the reach at an aperture is GN ÷ N. The unit matters — a GN in feet is 3.28 times the same flash’s GN in metres.
ISO and power
Light falls with the square of distance, so the guide number scales with the square root of the light: × √(ISO ÷ 100) for sensitivity and × √(power) for a partial-power setting. Half power is × 0.71; ISO 400 is × 2.
Falloff
Because light follows the inverse square, something √2 times further than the subject is a stop darker and something at half the distance is two stops brighter — why a flash-lit subject stands out against a dark background.
What the number does not settle
Guide numbers are for direct flash at the zoom-head setting the maker quotes; bouncing, diffusers and modifiers cost light the number does not include. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.