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Flash Aperture Calculator

Calculate the correct f-stop for flash photography from guide number, distance, and ISO. Instantly see the nearest standard f-stop and the correct distance for each aperture.

Input

Enter your external flash's guide number (GN), the subject distance, and the ISO to calculate the aperture (f-stop) for correct exposure. This is a guideline assuming manual (full) flash output.

GN
m

ISO sensitivity

ISO sensitivity
ISO

Result

Recommended aperture

F12.0

Nearest standard f-stop:F11.0

Effective guide number

GN 36

Shooting distance

3 m

ISO sensitivity

ISO 100

Correct distance for each standard f-stop (effective GN 36)

Aperture FCorrect shooting distance
F1.036 m
F1.425.7 m
F2.018 m
F2.812.9 m
F4.09 m
F5.66.4 m
F8.04.5 m
F11.0 (nearest)3.3 m
F16.02.3 m
F22.01.6 m
F32.01.1 m
F45.00.8 m

How it works

  • The guide number (GN) is a value referenced to ISO 100, defined as 'f-stop x shooting distance (m)' for correct exposure. The base formula is GN = F x distance.
  • From this relationship, the aperture is found as F = GN / shooting distance. This tool calculates the recommended aperture using that formula.
  • When the ISO differs from ISO 100, the effective guide number increases by about a factor of the square root of 2 for each doubling of sensitivity. We correct it as effective GN = GN x sqrt(ISO / 100) before calculating.
  • We also show the standard f-stop closest to the calculated value (one-stop steps such as F1.4, F2.8, F5.6, F8) so it's easy to match your actual lens setting.
  • Results assume manual (full) flash output and are a guideline only. Actual exposure is affected by reflectance, bounce, diffusers, and ambient light, so we recommend fine-tuning with test shots.