⚡ Flash Guide Number Calculator

Calculate the required aperture, distance, or guide number for manual flash photography. Includes inverse square law reference and common speedlight guide numbers.

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Common Speedlight Guide Numbers (ISO 100)

Flash Model	GN (m @ ISO 100)	GN (ft @ ISO 100)	Notes
Canon 430EX III-RT	33m	108ft	Good mid-range speedlight
Canon 600EX II-RT	60m	197ft	Canon's top speedlight
Nikon SB-700	28m	92ft	Versatile mid-range
Nikon SB-910	34m	111ft	Nikon flagship speedlight
Godox V860III	60m	197ft	Excellent value; li-ion battery
Godox TT685	60m	197ft	Budget option, same power
Sony HVL-F60RM2	60m	197ft	Sony flagship; weather sealed
Generic hotshoe flash	~32m	~105ft	Typical entry-level speedlight

Inverse Square Law

📐 Inverse Square Law: Light intensity falls off with the square of distance. Double the distance → 1/4 the light (–2 stops). Move the flash from 1m to 2m and you lose 2 stops of exposure.

💡 Bounce flash: Add 1–2 stops for bounce off white ceiling. Rule of thumb: add 1 stop for low white ceiling, 2 stops for high or off-white ceiling. Bounce direction affects the natural-looking shadow placement.

📊 GN scales with ISO: GN at ISO 400 = GN_100 × √(400/100) = GN_100 × 2. If your flash has GN40 at ISO 100, at ISO 400 it effectively becomes GN80.