PhotoCalcs
Flash Guide Number Calculator — GN, Distance & Aperture
Calculate flash guide number, required aperture or maximum distance for correct flash exposure.
How We Calculate This
The guide number formula relates flash power, distance and aperture:
Guide Number Formula
GN = distance (m) × f-number
ISO Adjustment
Adjusted GN = GN₁₀₀ × √(ISO ÷ 100)
Rearranging: aperture = GN ÷ distance, or distance = GN ÷ aperture. Guide numbers assume direct, head-on flash; bouncing or adding a modifier loses 1–3 stops, so reduce GN accordingly with the Light Loss Compensation field.
Frequently Asked Questions
The guide number (GN) describes flash power. It relates aperture and distance: GN = distance (m) × f-number at ISO 100. A GN 58 flash at f/5.6 correctly exposes at 58 ÷ 5.6 = 10.4 metres. Higher GN means more powerful flash. Most speedlights have GN 36-60.
Guide numbers are specified at ISO 100. For other ISOs, multiply the GN by the square root of (ISO/100). At ISO 400, multiply GN by 2. At ISO 1600, multiply by 4. This effectively doubles the usable distance for each 2-stop ISO increase.
Flash follows the inverse square law — doubling the distance requires 4x the power (2 stops). If your subject is beyond the flash range (GN/aperture), increase ISO, open the aperture, or move closer. Also check if you are bouncing the flash, which significantly reduces effective power.
Compare guide numbers at the same ISO and zoom setting. A GN 60 flash is twice as powerful as a GN 42 flash (one stop difference, since 60/42 ≈ 1.4 and one stop = √2 = 1.414). Studio strobes are often rated in watt-seconds (Ws) — roughly, doubling Ws adds one stop.
Related Calculators
Flash Power Calculator
Calculate the exact flash power needed to achieve a target exposure at any distance and aperture. Balance strobe output with ambient light. Free calculator.
Flash Sync Speed Optimizer
Calculate guide number effectiveness, exposure at subject, ambient-to-flash ratio and HSS penalty. Optimize your flash photography settings.
Inverse Square Law Calculator
Calculate how light intensity falls off with distance using the inverse square law. Essential for positioning studio lights accurately. Free online calculator.
Flash Duration Calculator
Estimate flash duration at different power settings for freezing motion. Find t.1 and t.5 times to plan high-speed photography. Free online calculator.
Reflector Calculator
Estimate reflected light intensity based on reflector type, size and distance from subject. Choose the right reflector for fill lighting. Free online calculator.
Was this calculator helpful?
Last updated: February 2026
All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.