Solve flash exposure with the guide number: required aperture, max flash distance or guide number, adjusted for ISO.
Guide numbers are quoted for one distance unit (m or ft) and at ISO 100. The effective GN above already accounts for your chosen ISO.
About this tool
Flash Guide Number Calculator is a free, in-browser tool that works the guide-number equation three ways: solve for the aperture to use, the maximum flash-to-subject distance, or the flash's guide number itself. Nothing is uploaded — the math runs locally as you type.
The guide number relates aperture and distance by GN = aperture × distance (rated at ISO 100). So aperture = GN ÷ distance and distance = GN ÷ aperture. Because sensitivity changes reach, the effective guide number at any ISO is GN × √(ISO ÷ 100): doubling ISO multiplies the guide number by about 1.41. Guide numbers are quoted for a specific distance unit — a GN in metres and a GN in feet are different numbers for the same flash.
Use it in manual flash: enter your flash's rated GN and the subject distance to get the aperture for a correct exposure, or enter aperture and distance to reverse-engineer a flash's true output.
Frequently asked questions
What is a flash guide number?
It is a single number describing a flash's power: guide number = aperture × distance for a correct exposure at ISO 100. A GN of 56 (m) means f/8 correctly exposes a subject at 7 m (56 ÷ 7 = 8).
How does ISO change the result?
Higher ISO extends reach. The effective guide number scales with the square root of the ISO ratio: GN × √(ISO ÷ 100). Going from ISO 100 to 400 doubles the effective guide number, letting you close down two stops or shoot twice as far.
Why does the distance unit matter?
Guide numbers are unit-specific. A flash rated GN 56 in metres is roughly GN 184 in feet (56 × 3.28). Always match the unit selector to the unit your flash's GN is quoted in, then read the distance output in that same unit.
Is this an exact exposure?
The guide number gives a strong starting point, but bounce, diffusion, zoom head position and reflective surroundings all affect real output. Treat the result as a baseline, then chimp the histogram. All math runs in your browser.