Translate a documented or tested flash guide number to a target ISO, then solve a nominal direct-flash f-number or same-unit distance.
Choose your path
Translate a condition-specific manual-flash guide number between rating and target ISO, then solve one nominal direct-flash aperture or distance without declaring correct exposure.
Estimate a starting f-number from one documented direct-flash rating and measured distance.
Copy GN, rating ISO, unit and conditions; choose aperture mode and enter direct distance.
Gets nominal math that remains subject to a meter/test capture.
Keep the guide-number unit and subject distance consistent.
Select the explicit unit and review the retained rating record with the result.
Avoids the roughly 3.28× unit error and can reproduce the calculation.
Use an effective tested GN rather than an unmatched maximum bare-head rating.
Enter the measured GN for exact power, zoom/coverage and modifier conditions, then solve distance mode.
Keeps bounce, modifier and equipment variation outside the nominal formula until measured.
Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.
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At the guide number's documented rating ISO and conditions, nominal f-number multiplied by direct subject distance in the rating's unit equals GN. Meter and foot ratings are different numbers for the same flash.
GN = f-number × distanceThe calculator scales the entered rating from its explicit ISO to the target ISO using a square-root relationship. It does not assume the rating was made at ISO 100 or that every camera matches the nominal response.
GN₂ = GN₁ × √(ISO₂ / ISO₁)Aperture mode divides effective GN by the entered direct distance. Distance mode divides effective GN by the entered f-number. The unused field is retained only for switching modes and is not silently combined.
f = GN / distance; distance = GN / fUpdated: August 2026
For a documented GN 60 ft at ISO 100 and a direct distance of 10 ft, aperture mode returns nominal f/6. Meter or test the exact subject, room, flash and camera before using a supported setting.
Do not reuse a bare-head direct-distance rating. Measure an effective guide number under the exact power, zoom, bounce or modifier test condition, or use a flash meter and controlled test captures.
A technician can enter an effective GN measured for one head, power, reflector or softbox, placement, ISO and unit, then use distance mode to document a nominal comparison point.
Direct-path GN math does not model bounce geometry and scatter. Use a flash meter or derive an effective GN from a controlled test under the exact bounce setup.
Select the rating's unit and keep distance in that same unit. Preserve the unit beside every saved guide-number record.
Guide-number math can document a nominal direct-flash starting point only when the rating conditions are explicit. Record the rating ISO, meter or foot unit, power, zoom or coverage, modifier/test condition, and the value used by the selected solve mode. Meter or test the real setup before accepting exposure.