What it's for
The guide number (GN) describes a flash's power: the higher the GN, the more light the flash delivers at a given distance. This calculator uses the classic relationship between GN, aperture and distance to find whichever value is missing — aperture, distance or the GN itself — and automatically adjusts for the ISO you're using.
Formula used
The basic guide number relationship is GN = aperture × distance, so: aperture = GN ÷ distance, distance = GN ÷ aperture and GN = aperture × distance — depending on which value you want to find. Manufacturers always state the guide number at ISO 100; for other ISOs, we adjust with effective_GN = GN_at_ISO100 × √(ISO/100), since ISO sensitivity follows a square-root relationship with the amount of light needed.
Worked example
A flash with GN 30 (ISO 100) used at 5 meters requires an aperture of f/6 (30÷5). Raising the ISO to 400 pushes the effective GN to 60 (30×√4), letting you use f/12 at the same distance or double the distance while keeping f/6.
Limitations
This is the theoretical guide number relationship, which assumes an environment with no significant reflections and the flash firing directly at the subject (not bounced off a ceiling or wall) — in real conditions, light-colored walls, low ceilings and light modifiers change the result.
Frequently asked questions
Why is the guide number always stated at ISO 100?
Because it's the standard manufacturers use so guide numbers of different flashes can be fairly compared, regardless of the ISO the photo will actually be taken at.
Why does the ISO adjustment use a square root instead of being linear?
Because doubling ISO sensitivity is only one stop of light, and the relationship between stops and the guide number (which is proportional to aperture, not to the amount of light) follows a square-root scale, not a linear one.
Does this work with bounce flash?
Not directly — the formula assumes direct flash light hitting the subject. Bounce flash loses light in the reflection and needs additional adjustments this calculation doesn't capture.