Flash Guide Number Calculator

This tool links flash Guide Number (GN) with aperture and subject distance so you can build a reliable starting setup before shooting.

It is useful for portrait, event, and product scenarios where reducing trial-and-error saves time and keeps lighting consistent.

Core inputs are GN (often manufacturer-rated at ISO100), aperture (f-number), subject distance, and ISO. The calculator solves for the unknown based on selected mode.

GN values come from manufacturer documentation, while distance/aperture come from your scene. Output quality depends on correct catalog values and real setup conditions.

The base relation is GN = f x distance. Rearranging gives distance = GN / f or aperture = GN / distance, depending on what you need.

ISO scaling follows a square-root rule. Moving from ISO100 to ISO400 roughly doubles effective GN in ideal conditions, but real light loss may reduce that gain.

Output is a theoretical value from your entered GN, ISO, and selected solve mode. Treat it as a baseline, not an absolute guarantee.

Modifiers, bounce surfaces, flash zoom head settings, TTL behavior, and ambient reflections can all shift practical results.

Example: with GN 36 at ISO100 and aperture f/4, the theoretical working distance is 9 meters (36 / 4).

If you move to ISO400, ideal effective GN is around 72, so theoretical distance at f/4 becomes 18 meters, though real-world losses usually reduce it.

The tool supports fast technical planning in manual flash workflows and helps compare exposure options quickly.

This content is informational. Manufacturer GN test conditions and environmental factors vary, so validate critical decisions with official docs and on-set test shots.