How do I calculate 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.

What do flash power fractions mean?

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.

Why should I use the lowest power possible?

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.

How does ISO interact with flash power?

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.

Flash Power Calculator — Required Flash Power

Calculate the flash power setting needed for correct exposure at a given aperture, distance and ISO.

Aperture

Your working f-number (e.g. f/8). A smaller aperture needs more flash power.

Flash-to-Subject Distance

Distance from the flash head to your subject — power rises with the square of distance.

ISO

Camera ISO. Doubling ISO halves the flash power you need (1 stop).

Flash Guide Number

Maximum guide number of your flash at its rated ISO (often ISO 100), in metres.

How We Calculate This

This calculator determines the flash power needed for correct exposure:

Required Guide Number

Required GN = (distance × aperture) ÷ √(ISO ÷ base ISO)

Base ISO is the rating ISO of your flash guide number (100 by default — change it under Advanced Options if your flash is rated at a different ISO).

Power Fraction

Power = (required GN ÷ flash GN)²

Power in Stops

Stops below full = −log₂(power fraction)

Light intensity is proportional to the square of the guide number, so the power fraction compares the GN you need against your flash’s maximum GN. The nearest manual setting (1/1, 1/2, 1/4 …) is found by rounding the stop count.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.