Flash Guide Number Calculator
Calculate flash guide number, maximum flash distance, and required aperture.
Essential for manual flash photography.
Flash guide number (GN) is a measure of flash power that relates flash output, subject distance, and aperture (f-stop).
Guide number formula:
GN = Distance × f-stop
Rearranged:
Distance = GN / f-stop
f-stop = GN / Distance
Guide numbers are specified at ISO 100. For other ISOs:
Effective GN = Base GN × √(ISO / 100)
For example, a flash with GN 40 (at ISO 100) has an effective GN of 56.6 at ISO 200.
Common flash guide numbers:
| Flash Type | GN (meters, ISO 100) | GN (feet, ISO 100) |
|---|---|---|
| Built-in (pop-up) | 12–13 | 39–43 |
| Entry speedlight | 28–36 | 92–118 |
| Mid-range speedlight | 36–48 | 118–157 |
| Professional speedlight | 48–60 | 157–197 |
| Studio strobe (500W) | 64–76 | 210–250 |
Practical flash distance chart (GN 40m, ISO 100):
| Aperture | Max Distance |
|---|---|
| f/2.0 | 20 m (65.6 ft) |
| f/2.8 | 14.3 m (46.9 ft) |
| f/4.0 | 10 m (32.8 ft) |
| f/5.6 | 7.1 m (23.3 ft) |
| f/8.0 | 5 m (16.4 ft) |
| f/11 | 3.6 m (11.8 ft) |
| f/16 | 2.5 m (8.2 ft) |
Flash power fractions: Most speedlights allow fractional power output. Each step halves the light and reduces the effective GN:
- Full power: GN × 1.0
- 1/2 power: GN × 0.71
- 1/4 power: GN × 0.5
- 1/8 power: GN × 0.35
- 1/16 power: GN × 0.25
Inverse square law: Light intensity falls off with the square of the distance. Doubling the distance requires 4× the light (2 stops more).
Tip: GN specifications from manufacturers are often measured in ideal conditions (white room, zoom head at maximum telephoto). Real-world GN is typically 15–30% lower. Always test your flash to determine its actual guide number.