Flash / Guide Number Calculator
Calculate the flash power you need to light a subject at any distance, or find your flash's maximum reach at a given aperture and ISO. The comparison table shows your effective range across all common f-stops.
Max Distance at Each Aperture
With a guide number of at , here's your maximum flash range at each f-stop.
| Aperture | Max Distance | Max Distance (ft) | Typical Use |
|---|---|---|---|
Understanding Flash Guide Numbers
A flash's guide number (GN) is basically a single number that tells you how powerful it is. It represents the farthest distance (in metres) the flash can properly expose a subject at f/1 and ISO 100.
Higher number, more power. Simple as that. A tiny built-in pop-up flash might sit around GN 12. A professional speedlight? That can hit GN 58 or more.
The Guide Number Formula
Here's the core relationship: Guide Number = Distance x Aperture. That's it.
Say your flash has a GN of 40 at ISO 100 and you're shooting at f/4. Your maximum flash distance is 40 / 4 = 10 metres. You can rearrange the formula depending on what you need to solve for: Distance = GN / Aperture, or Aperture = GN / Distance.
This calculator handles all that for you, including the ISO adjustments.
The Inverse Square Law
Light from a flash doesn't just get a little dimmer as it travels. It falls off fast.
The inverse square law says that when you double the distance between your flash and your subject, the light drops to one quarter of what it was. That's two full stops gone. Triple the distance and you're down to one ninth the light.
This is why flash has a hard ceiling on range, no matter how powerful it is. And it's why being even a metre or two closer to your subject can make a huge difference to your exposure.
ISO and Guide Numbers
Guide numbers are always quoted at ISO 100. But when you raise your ISO, your flash effectively reaches further because the sensor needs less light to get a proper exposure.
Doubling your ISO (say, 100 to 200) multiplies the effective guide number by about 1.4x. Jump from ISO 100 to 400 and the effective GN doubles. ISO 1600? It quadruples.
The calculator factors in your ISO automatically, so just set it and read the results.
Bounce Flash and Modifiers
Bouncing your flash off a ceiling or wall gives you gorgeous, soft lighting. But you pay for it in power.
Typically, bouncing costs you 1.5 to 2 stops, which cuts your effective range roughly in half. Diffusers, softboxes, and umbrellas eat into your output too, usually around 30-50%.
As a rough guide, cut your effective GN by about 50% when bouncing and 30-50% with a modifier. The light quality improves so much that it's almost always worth the trade-off.
Choosing the Right Flash
For casual indoor shooting and portraits within a few metres, a speedlight with a GN of 30-40 will do the job nicely.
Shooting events, weddings, or larger rooms? You'll want a GN 58+ speedlight that can actually reach subjects across the space.
Studio strobes with guide numbers of 60-80 and beyond are built for controlled setups where you need consistent power and fast recycle times.