Flash Guide Number Calculator
Calculate aperture, distance, or power settings for manual flash photography. The guide number formula: GN = Distance × Aperture.
Calculate flash exposure
Choose what you want to find
Adjusted GN at ISO 100: 43
Required aperture
f/8.6
Nearest standard: f/8
Common flash guide numbers
Guide numbers at ISO 100 in meters
| Flash | GN (m) | GN (ft) |
|---|---|---|
| Canon Speedlite 430EX (105 mm) | 43 | 141 |
| Canon Speedlite 580EX (105 mm) | 58 | 190 |
| Nikon SB-700 (120 mm) | 38 | 125 |
| Nikon SB-900 (35 mm) | 34 | 112 |
| Godox V860II (200 mm) | 60 | 197 |
| Vivitar 285HV (GN 120 ft) | 36.6 | 120 |
| Metz 45 CT-1 | 45 | 148 |
Understanding guide numbers
The guide number (GN) is a measure of flash power. It's defined at ISO 100 and relates aperture, distance, and flash power through a simple formula:
GN = Distance × Aperture
This can be rearranged to find any variable:
- Aperture = GN ÷ Distance
- Distance = GN ÷ Aperture
ISO adjustment
When shooting at higher ISOs, the effective guide number increases:
Adjusted GN = Base GN × √(ISO ÷ 100)
At ISO 400, your flash effectively has double the guide number.
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