Studio Flash Power Calculator

Calculate the flash power needed to achieve your target aperture, or find what aperture a given strobe produces at any distance. Works for studio monolights, pack-and-head systems, and portable speedlights.

What do you want to find?
Find flash power
I know my target aperture
Find aperture
I know my flash power
Find distance
I know power and aperture
Camera settings
Flash and subject
From spec sheet in feet. Leave blank to use watt-seconds.

Please check your inputs.

Guide number needed
0 GN
f/8
Aperture
6ft
Distance
GN 48
Guide number
Power adjustment — stops from your strobe’s max
Common strobes — what aperture they give at your distance
StrobeGN (ISO 100, ft)ApertureStops diff
Target aperture
ISO
Flash-to-subject distance
Guide number needed (ISO 100)
With modifier loss
Formula
ISO adjustment factor
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.