Flash Guide Number Calculator

In short A flash guide number is a single figure describing how much light the unit puts out at full power, and it works because guide number equals distance times f-number. A flash with a guide number of 60 in metres at ISO 100 needs f/20 at three metres, or f/10 at six. Rearrange it and you get reach: at f/8 that same flash covers 7.5 metres. The relationship that matters is the inverse square law. Light spreads over the square of the distance, so doubling the distance costs TWO stops, not one — which is why moving a subject a little further from a flash costs so much more than it seems it should, and why an on-camera flash that lights a person at three metres does nothing at all to a room behind them. ISO works the other way round and scales as a square root, because guide number is a distance while light goes as its square: four times the ISO doubles the reach. Two warnings about the number on the box. Manufacturers quote guide number at the flash head's narrowest zoom setting, where the beam is most concentrated, and at a wide zoom the real figure can be half of it — treat it as a ceiling rather than a specification. And a guide number in feet is 3.28 times the same flash in metres, so GN 58 in metres is GN 190 in feet, and confusing the two is a four-stop error. None of this applies to bounce flash, where the light travels further and scatters, typically costing two to three stops more with no formula to predict it.