Flash Exposure Distance Calculator
The flash guide number (GN) formula lets you calculate the correct flash-to-subject distance, required aperture, or effective guide number for any combination of flash power and ISO. The fundamental relationship is GN = aperture x distance. When you know the guide number and aperture, distance = GN / aperture. When you know the guide number and distance, aperture = GN / distance. This calculator also adjusts for ISO: guide numbers are typically published at ISO 100, so an ISO correction factor of sqrt(ISO / 100) scales the effective GN for other ISO settings.
Flash guide number formula
GN = aperture x distance
Distance (m) = GN_eff / aperture
GN_eff = GN_100 x sqrt(ISO / 100)
Example: flash GN 40 at ISO 100, shooting at f/8, ISO 400. GN_eff = 40 x sqrt(4) = 80. Distance = 80 / 8 = 10 meters.
Understanding flash guide numbers
- Guide numbers assume ISO 100 and a specific flash zoom head position. Check your flash manual for the published GN and the zoom setting it applies to.
- Opening aperture by one stop (e.g. f/8 to f/5.6) halves the required distance.
- Doubling ISO increases effective GN by a factor of sqrt(2), extending flash range by about 41%.
- Flash exposure follows the inverse-square law: doubling distance requires four times the flash power to maintain the same exposure.
- Guide numbers in feet are approximately 3.28 times the meter value.
Flash distance calculator: frequently asked questions
What is a flash guide number?
A guide number (GN) is a measure of flash power. It relates the aperture needed for correct exposure to the flash-to-subject distance: GN = aperture x distance. Manufacturers publish GN at a specific ISO (usually 100) and zoom setting.
How do I find the correct flash distance?
Rearrange the guide number formula: distance = GN / aperture. If your flash has GN 40 (meters, ISO 100) and you are shooting at f/8, the correct distance is 40 / 8 = 5 meters.
How does ISO affect flash distance?
Guide numbers are specified at a reference ISO (usually 100). To adjust for a different ISO, multiply GN by sqrt(ISO / 100). At ISO 400, multiply GN by sqrt(4) = 2, doubling effective flash range.
What if the GN is given in feet instead of meters?
Use the same formula but keep your distance in feet. To convert: GN in feet = GN in meters x 3.281. The formula works identically in either unit system as long as you are consistent.
Can I use this to find the required aperture?
Yes. Rearrange: aperture = GN / distance. If your subject is 3 meters away and your flash GN is 36 (meters, ISO 100), the required aperture is 36 / 3 = f/12, meaning you should use f/11 or f/16 depending on your lens.
Official sources
- ISO 12232:2019: Photography, Digital still cameras, Determination of exposure index. ISO.org.
- ISO 2827:1974: Photography, Electronic flash, Guide numbers for amateur use. ISO.org.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.