Flash Guide Number Calculator
Calculate flash exposure using guide numbers.
Find the right aperture for any flash-to-subject distance.
Flash Exposure
Guide Number (GN) rates a flash unit’s power. It relates flash power, distance, and aperture.
The Guide Number formula:
GN = Distance × f-stop
f-stop = GN / Distance
Distance = GN / f-stop
Guide numbers are specified at ISO 100.
For other ISOs:
Adjusted GN = GN × sqrt(ISO / 100)
Example: Flash with GN 60 (meters, ISO 100):
- At 5m distance: f/12
- At 10m distance: f/6
- At ISO 400: GN becomes 120
Common guide numbers (meters, ISO 100):
- Pop-up flash: GN 12–15
- Budget speedlight: GN 30–38
- Pro speedlight: GN 40–60
- Studio strobe: GN 60–100+
Distance measured in feet vs meters: GN in feet = GN in meters × 3.28. Always check which unit your flash manual uses.