Flash Guide Number Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026Flash Guide Number Calculator
Manual flash photography without a light meter? This calculator finds the correct aperture based on your flash's guide number and subject distance.
Required Aperture = Guide Number / Distance(m)
Example
A flash with guide number 40 (at ISO 100), subject 5 meters away:
Required Aperture = 40 / 5 = f/8.0
What Guide Number Actually Represents
A flash's guide number describes its total light output at ISO 100, expressed as the product of aperture and distance for correct exposure - it's a single number that lets photographers calculate correct exposure at any distance without a separate light meter reading.
Why the Same Flash Needs Different Apertures
As subject distance increases, the same fixed flash output is spread across a wider area, requiring a wider aperture (smaller f-number) to let in enough light - this is exactly the tradeoff this formula calculates directly.