Flash Guide Number Calculator
Turn a flash's guide number into real shooting distance for any aperture and ISO.
What is a flash guide number?
A guide number (GN) describes how powerful a flash is. It equals the aperture multiplied by the distance at which the flash correctly lights a subject, quoted at ISO 100. A higher guide number means the flash reaches further. Rearranged, it tells you the working distance for any aperture you choose.
Working out distance and aperture
Divide the guide number by your aperture to get the maximum flash distance: a GN of 36 at f/4 reaches about 9 metres. Or divide the guide number by the distance to find the aperture you need. The calculator does either direction so you can set power confidently in manual mode.
How ISO extends flash range
Raising the ISO makes the sensor more sensitive, so the same flash reaches further. Range grows with the square root of the ISO ratio: quadrupling the ISO doubles the distance. Enter your ISO and the tool scales the reach automatically, which is handy in large or dark venues.
How this calculation works
Formula: distance = GN / N × √(ISO / 100)
where:
GN= guide numberN= aperture (f-number)ISO= ISO sensitivity
Sources:
FAQ
How does ISO change flash range?
Range scales with the square root of the ISO ratio: quadrupling ISO doubles the distance your flash can reach.
Is guide number in metres or feet?
Both are used. This tool assumes metres at ISO 100 - the most common way manufacturers quote it.
What is a guide number?
It is a measure of flash power equal to aperture times distance at ISO 100; a higher guide number means the flash reaches further.
How do I find the aperture for a given distance?
Divide the guide number by the distance. With GN 36 at 4 m, you would use about f/9.