Flash Guide Number Calculator
Manual flash exposure from the classic GN relationship.
Aperture to set
f/12
Effective GN at this ISO
GN 36
What is this tool?
A flash guide number (GN) summarizes flash power: GN = subject distance × f-number at a stated ISO (usually 100). Given any two of the three values this calculator solves the third — the aperture to set for a subject at a known distance, the maximum distance a flash can properly light at an aperture, or your flash's effective guide number from a test shot. Raising ISO scales the effective GN by √(ISO/100). Assumes direct, unmodified flash at full power.
How to use it
- Choose what to solve for: aperture, distance, or guide number.
- Enter the two known values, your ISO, and the distance unit.
- Read the solved setting and the effective GN at your ISO.
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