Calculator

Flash guide number calculator

Correct aperture for any flash power, ISO and distance.

Correct aperture—
Effective guide number—
Max distance at f/8—

How it works#

Aperture = GN ÷ distance (GN in metres at ISO 100). Each halving of power divides GN by √2; each doubling of ISO multiplies it by √2.

Last updated October 03, 2026 · How we calculate · Sources

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