Flash Power Calculator — Required Flash Power

First calculate the required guide number: GN = distance × aperture (adjusted for ISO). Then compare to your flash maximum GN. Power fraction = (required GN ÷ flash max GN)². If the required GN exceeds your flash capacity, you need to increase ISO, open the aperture, or move closer.

1/1 = full power, 1/2 = half power (1 stop less), 1/4 = quarter power (2 stops less), 1/8 = 3 stops less, etc. Each step halves the light output. Most speedlights offer 8 power levels (1/1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128), which is a 7-stop reduction range from full to minimum.

Lower power settings offer: faster recycle times (important for events), shorter flash durations (better motion freezing), longer battery life, less heat buildup, and more consistent output. Only use full power when you absolutely need the reach.

Doubling ISO is equivalent to doubling flash power (1 stop). If your flash at 1/4 power gives correct exposure at ISO 100, you can use 1/8 power at ISO 200 for the same result. Increasing ISO lets you use lower flash power or reach greater distances.