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About this tool
Solve aperture, distance or guide number from GN = f-number × distance, adjusted for ISO and flash power fraction.
Flash Guide Number Calculator solves the relationship GN = f-number × distance, so entering any two of guide number, aperture and flash-to-subject distance returns the third. It scales the guide number by the square root of ISO ÷ 100 and by the square root of the manual power fraction, which is why quadrupling the ISO doubles the reach and quarter power halves it. Written for manual flash and studio strobe work where no TTL metering is involved.
Open Flash Guide Number Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Aperture, distance or guide number — whichever one you are missing.
Both follow square-root laws, applied correctly instead of by rule of thumb.
Guide numbers are published in both; the result is shown in each so nothing is misread.
It is the product of the f-number and the flash-to-subject distance that gives a correct exposure, quoted at ISO 100 for a stated distance unit. A GN of 32 in metres means f/8 at 4 m, f/4 at 8 m, or f/16 at 2 m.
The guide number scales with the square root of ISO ÷ 100, so ISO 400 doubles it and ISO 1600 quadruples it. A GN 32 flash behaves like GN 64 at ISO 400, letting you shoot the same distance two stops smaller.
Two stops of light, which halves the guide number because GN scales with the square root of the power fraction. A GN 32 head at 1/4 power behaves like GN 16, so the working distance at a given aperture halves.
Published figures assume bare direct flash with the head zoomed to its longest setting, and manufacturers tend to quote optimistically. Bouncing off a ceiling, adding a diffuser or zooming wide can each cost one to three stops — measure your own guide number from a test frame instead.
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