Flash Guide Number Calculator (Distance and Aperture)
How far a flash reaches at a given aperture and ISO, or the aperture to use for a subject at a given distance, from its guide number.
Results
How far the flash reaches at that aperture
6.43
m
f-number needed for that distance
9.0
Guide number at your ISO
36.0
m
What this tool does
The guide number (GN) is the flash's reach times the f-number at full power, quoted for ISO 100: a GN of 36 m lights a subject 6.4 m away at f/5.6. Raising the ISO four times doubles the reach, because the number scales with the square root of the ISO, and half power cuts it to about 70 %. Manufacturers quote the GN for a particular zoom position, in metres or in feet; the figure in feet is about 3.3 times bigger. It applies to direct flash with no modifiers, and bounced light or a diffuser needs more power.
Formula
distance = GN ÷ f-number × √(ISO ÷ 100) · f-number = GN × √(ISO ÷ 100) ÷ distance
Variables
| Symbol | Meaning | Unit |
|---|---|---|
gn | Guide number (metres, at ISO 100) | m |
iso | ISO | — |
n | f-number | — |
dst | Distance to the subject | m |
DI | How far the flash reaches at that aperture | m |
AP | f-number needed for that distance | — |
GI | Guide number at your ISO | m |
Worked example
- Guide number (metres, at ISO 100)36 m
- ISO100
- f-number5.6
- Distance to the subject4 m
- How far the flash reaches at that aperture6.43 m
- f-number needed for that distance9.0
- Guide number at your ISO36.0 m
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.