Flash Guide Number Calculator — Manual Flash Power & Distance
Required Aperture
—
Max Distance at f/2.8
—
Adjusted Guide Number
—
Understanding Flash Guide Numbers and Exposure
The guide number (GN) is the standard measure of a flash unit power output and determines how far the flash can illuminate a subject at a given aperture. This calculator uses the fundamental flash exposure formula: Guide Number = Distance * f-stop. Rearranging this equation lets you solve for the maximum flash-to-subject distance at a given aperture, or the required aperture at a given distance. Guide numbers are specified at ISO 100 and a reference zoom position.
To calculate flash range at a specific aperture, divide the guide number by the f-stop: Maximum Distance = GN / f-stop. A flash with GN 40 (meters at ISO 100) can illuminate a subject up to 10 meters away at f/4, or 5 meters at f/8. Increasing ISO doubles the effective distance for each stop of added sensitivity. At ISO 400 (two stops above ISO 100), the GN 40 flash can reach 20 meters at f/4. The calculator handles these ISO adjustments automatically.
Understanding guide numbers is essential for manual flash photography, off-camera flash setups, and evaluating flash purchases. When comparing speedlights, the guide number tells you how much light the flash can produce. Higher guide numbers mean more power and greater reach, which matters for event photography in large venues and outdoor flash work. For fill flash in daylight, you need enough power to balance ambient light at small apertures like f/11 or f/16, which demands a high guide number or close flash-to-subject distance.
Frequently Asked Questions
What is a guide number?
A guide number (GN) describes flash power. It equals the product of distance and f-number for correct exposure at ISO 100. Higher GN means more powerful flash. GN 36 (meters) is typical for built-in speedlights.
How does ISO affect flash range?
Doubling ISO increases effective flash range by a factor of 1.4 (square root of 2). At ISO 400, a flash with GN 36 has an effective GN of 72, doubling its reach.
What is TTL vs manual flash?
TTL (Through The Lens) metering automatically adjusts flash power. Manual flash requires you to calculate exposure using the guide number. Manual gives more consistent results for controlled setups.