Flash Guide Number Calculator
Calculate required aperture or distance for flash photography
About this calculator
The Flash Guide Number Calculator helps photographers determine the correct aperture setting or flash-to-subject distance for proper exposure in flash photography. By using your flash unit's guide number and known variables, this tool calculates the optimal camera settings to achieve perfectly exposed images. This calculator is essential for manual flash photography, studio work, and situations where you need precise control over flash exposure without relying on automatic flash metering.
How to use
Enter your flash unit's guide number (found in the manual or specifications) and either your desired aperture or subject distance. The calculator will compute the missing variable using the guide number formula (GN = Aperture × Distance). Use the result to set your camera's aperture or position your flash at the calculated distance for proper exposure.
Frequently asked questions
What is a flash guide number?
A guide number is a rating that indicates a flash unit's power output, representing the distance-aperture relationship needed for proper exposure at ISO 100.
How do I find my flash's guide number?
Check your flash unit's manual, specifications label, or manufacturer's website. Guide numbers are typically given for ISO 100 and measured in feet or meters.
Does ISO affect guide number calculations?
Yes, guide numbers change with ISO. If your flash's guide number is rated at ISO 100, multiply by the square root of your ISO divided by 100.