Calculate flash guide number

Solve flash aperture or subject distance from guide number and the other input.

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ToolFlash Guide Number Calculator
Input
Output
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Preview

How it works

Guide number equals aperture × distance, so the missing aperture or distance is found by division. Units must match the guide-number rating.

  • The blank identifies whether aperture or distance is solved.
  • Guide number is supplied by the flash or test setup.

Worked example

GN40 flash at 5 metres
Aperture left blank to solve for it
Input
											Guide number: 40
Distance m: 5
Output
												Guide number: 40
Aperture: 8
Distance m: 5

When to use this

Photographers estimate flash distance, studio worksheets select aperture, and lighting lessons show aperture-reach tradeoffs.

Edge cases

  • Doubling distance needs roughly double guide number or a two-stop aperture change.
  • Bounce, diffusion, and modifiers reduce effective reach.
  • Guide numbers at different ISO values need normalization before comparison.