Guide Number Calculator

Calculate flash guide number from test shots or convert between ISO values.

Enter the aperture used (f/) value for this calculation.
f-stop
Linear measurement between two points.
meters
Enter the test iso value for this calculation.
ISO
Desired or goal value.
ISO

Results

Guide Number @ Test ISO:
Number of items needed or calculated.
-m
Guide Number @ Target ISO:
Number of items needed or calculated.
-m
Max Distance at f/4:
Maximum allowed or calculated value.
-meters
Reference: Industry Standards.