Flash Guide Number Calculator
Estimate flash distance from guide number, ISO, aperture and power setting.
About this tool
A flash exposure calculator for photographers. Enter guide number, ISO, aperture and optional power fraction to estimate maximum flash distance in meters and feet.
Frequently asked questions
What is a flash guide number?
A guide number expresses flash power: at ISO 100, distance equals guide number divided by f-number.
How does ISO affect guide number?
Doubling ISO increases effective guide number by the square root of two, because sensor sensitivity rises by one stop.
How does flash power affect distance?
Reducing flash power reduces guide number by the square root of the power fraction. Quarter power gives half the guide number.