Flash Guide Number Calculator
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Solve flash exposure with GN = distance × f-number — find GN, distance, or aperture.
Flash exposure
Pick which value to solve for — the other two are used as inputs.
Guide number (GN)
40
10 m at f/4
- Formula
- GN = distance × f-number
Related calculators
Flash guide number
A flash's guide number (GN) rates its power for manual exposure calculations: GN = distance × f-number, using a fixed ISO (usually ISO 100) and often a specific unit of distance (meters or feet — check your flash's manual). A higher GN means a more powerful flash that can properly expose subjects farther away at a given aperture.
Because the formula has three variables, you can solve for whichever one you don't know: divide GN by the f-number to find the maximum flash distance, or divide GN by the distance to find the aperture needed to correctly expose a subject at that distance.
Example: a flash with a guide number of 40 (meters, ISO 100) can properly expose a subject 10 meters away at f/4 (40 ÷ 10 = 4), or a subject 5 meters away at f/8 (40 ÷ 5 = 8) — the same flash power, different apertures for different distances.
Common questions
Is guide number the same for every ISO?
No — guide numbers are usually published at ISO 100. Raising ISO effectively increases usable flash range or allows a smaller aperture, since higher ISO makes the sensor more sensitive to the same amount of light.
Do I need to know the distance in meters or feet?
Guide numbers are given in either meters or feet depending on the manufacturer — check your flash's specifications and keep your distance units consistent with the guide number's units.