Flash Guide Number Exposure Calculator
Estimate direct-flash aperture or distance from an entered guide number, ISO and relative manual power.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Scale the entered guide number by square roots of ISO ratio and relative energy, then use guide number=distance×f-number.
Effective guide=reference guide×√(ISO/reference ISO)×√(power fraction)
Worked example
Guide number 40 m at ISO 100 becomes 80 m at ISO 400 and full power. At distance 5 m, the corresponding aperture is f/16.
Assumptions and input details
Ideal direct-flash guide-number arithmetic using an entered measured/manufacturer value for a fixed beam setting. Bounce, diffusion, TTL metering, scene reflectance, high-speed sync and ambient light are not modeled. No flash catalogue or available camera stop rounding. Relative energy must be positive, up to 100%.
References: Nikon: guide number, ISO and aperture
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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