Flash Guide Number Calculator
Turn a flash guide number into a real working distance and the aperture you need — with an ISO adjustment for the light you actually have.
How it works
Distance = guide number ÷ f-number · Effective GN = GN × √(ISO ÷ 100) · f-number = GN ÷ distance
A guide number packages a flash’s power into one figure: the guide number equals the correct aperture multiplied by the subject distance, at ISO 100. Rearranging gives the two things you actually want on location — the farthest a subject can be for a given aperture, or the aperture to dial in for a fixed distance. Because raising ISO makes the sensor more sensitive, the effective guide number grows with the square root of the ISO ratio, so doubling reach means quadrupling ISO.
Worked example
A GN 40 (m, ISO 100) flash shot at f/5.6 reaches 40 ÷ 5.6 = 7.1 m. Push ISO to 400 and the effective guide number becomes 40 × √4 = 80, so the same f/5.6 now reaches 14.3 m.
Frequently asked questions
Are guide numbers in metres or feet?
Both are used. This calculator works in metres; a guide number in feet is about 3.28× the metric value, so GN 40 m ≈ GN 131 ft. Always check which unit the flash spec quotes before comparing two units.
Why does my guide number assume ISO 100?
Guide numbers are standardised at ISO 100 so different flashes can be compared. At any other ISO the effective guide number scales by the square root of ISO ÷ 100, which is why the ISO field changes the reach.
Does zoom head position change the guide number?
Yes. Narrowing the flash zoom concentrates the beam and raises the guide number, while a wide zoom spreads light and lowers it. Manufacturers usually quote the guide number at the longest (most concentrated) zoom setting.
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This calculator is for educational purposes. Double-check important results before acting on them.