Flash Guide Number Calculator
A flash guide number ties together aperture and distance through one rule: guide number equals aperture times flash-to-subject distance. Enter any two of guide number, aperture and distance and this calculator solves the third. Set a shooting ISO different from the rated reference ISO and it scales the effective guide number by the square root of the ISO ratio, since two stops more ISO doubles the flash reach.
How to use the flash guide number calculator
- Choose what to solve for with the buttons: Aperture, Distance or Guide number.
- Enter the two values you are not solving for; the target field is disabled automatically.
- Optionally set the shooting ISO and the rated Reference ISO (both default to 100).
- Read the aperture, distance and effective guide number cards below.
- Click Copy to grab the GN, aperture, distance and ISO line.
Examples
Aperture from guide number and distance
GN 56 (ISO 100), distance 7
f/8
Higher ISO extends reach
GN 56, distance 7, ISO 400
effective GN 112, so f/16
Guide number from aperture and distance
f/8 at distance 10 (ISO 100)
GN 80
Frequently asked questions
What is a flash guide number?
A guide number rates flash power as the aperture times the subject distance that yields a correct exposure at a reference ISO. A GN of 56 in metres means f/8 at 7 metres, because 8 times 7 is 56.
How do I find the aperture from a guide number?
Divide the guide number by the flash-to-subject distance. With a GN of 56 in metres and a subject 7 metres away, the aperture is 56 divided by 7, which is f/8. Set the calculator to solve for Aperture and enter the other two values.
How do I find the working distance?
Divide the guide number by your aperture. A GN of 56 at f/8 reaches 56 divided by 8, which is 7 metres. Solve for Distance and the tool does the division for you.
How does ISO change the guide number?
The effective guide number scales with the square root of the ISO ratio. Going from ISO 100 to ISO 400 is two stops, so the multiplier is the square root of 4, which is 2. The effective guide number doubles from 56 to 112 and the flash reaches twice as far.
What is the reference ISO field for?
Manufacturers rate a guide number at a stated ISO, almost always 100. Enter that as the Reference ISO and your actual shooting ISO separately, and the tool scales between them. If both are equal there is no scaling.
Are guide numbers in metres or feet?
Either, but the unit must match your distance. A guide number quoted in metres needs distance in metres; the same flash carries a larger guide number when quoted in feet, roughly 3.28 times higher.
Does the guide number assume full power?
Usually yes, at a stated head zoom and reference ISO. Cutting flash power lowers the effective guide number, while zooming the flash head narrower concentrates the beam and raises it.
Why does my real exposure not match the number?
Guide numbers assume direct flash in the open. Bouncing off a ceiling, shooting into a large room that swallows light, or a subject that reflects unusually all shift the result, so treat the number as a strong starting point and confirm with a test frame.
Is anything sent to a server?
No. The calculation runs in your browser, so your numbers stay on your device.
Learn more
- The flash guide number formula explained
How GN equals aperture times distance, why raising ISO by two stops doubles reach, and the real-world limits of the number.
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