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Flash guide number calculator
Solve for guide number, distance or aperture with manual flash, adjust for ISO and power, and see how fast the light falls off behind the subject.
All tools- Aperture
- f/8Nearest marked stop f/8
- Effective GN (m)
- 401× the rated GN at this ISO and power
| Distance from flash | Light | Versus subject |
|---|---|---|
| 6.3 m | 64% | -0.6 stops |
| 7.5 m | 44% | -1.2 stops |
| 10 m | 25% | -2 stops |
| 15 m | 11% | -3.2 stops |
| 20 m | 6% | -4 stops |
How it works
The method behind the numbers.
A flash's guide number (GN) rates its output in one number. At full power and ISO 100, the right aperture is the GN divided by the flash-to-subject distance.
GN = distance × N
N = GN ÷ distance
d = GN ÷ N
GN(ISO, power) = GN(ISO 100, 1/1) × √(ISO ÷ 100) × √power- A guide number is tied to its unit. GN 40 in metres is about GN 131 in feet, so enter the figure in the unit you pick here. Makers usually quote it at ISO 100, full power and a stated zoom position.
- Four times the ISO or four times the power doubles or halves the GN. Each power step (1/1, 1/2, 1/4) is one stop of light, which is a factor of √2 in GN.
- Solving for guide number takes a distance and the aperture that exposed correctly, and gives the figure normalised to ISO 100 and full power.
- Light falls with the square of distance, so a background twice as far from the flash as the subject receives a quarter of the light, two stops less.
- Real flashes vary with zoom, modifiers and bounce; treat a calculated aperture as a starting point and check the result.