Flash calculator
Flash guide number calculator
Turn a guide number, ISO and distance into the aperture you need, or work out the furthest distance at a given aperture. Guide numbers are quoted at ISO 100 in metres by default.
Required apertureβ
Max distance at that apertureβ
Common conversions
| GN | 2 m | 4 m | 6 m | 8 m |
|---|---|---|---|---|
| 12 | f/6 | f/3 | f/2 | f/1.5 |
| 16 | f/8 | f/4 | f/2.7 | f/2 |
| 20 | f/10 | f/5 | f/3.3 | f/2.5 |
| 24 | f/12 | f/6 | f/4 | f/3 |
| 28 | f/14 | f/7 | f/4.7 | f/3.5 |
| 32 | f/16 | f/8 | f/5.3 | f/4 |
| 36 | f/18 | f/9 | f/6 | f/4.5 |
| 40 | f/20 | f/10 | f/6.7 | f/5 |
A guide number is quoted at a specific ISO and zoom position; modifiers change the real value a lot. Test your own flash before a critical shoot.