Guide number
GN = distance × aperture at a stated ISO. It depends on the coverage the maker chose, which is why a zoom head’s GN rises at its longer settings. The Godox TT600 states GN60 at ISO 100; the NEEWER TT560 GN38.
Watt-seconds
Energy stored before the flash. The Godox SK400III-V is a 400Ws head; the NEEWER Q6 600Ws.
Why they do not convert
Two heads of the same watt-seconds can put out different amounts of light, depending on reflector and efficiency.
How to compare
Compare guide numbers at the same ISO and coverage, and watt-seconds between similar heads only.
What people ask before they commit
What is a guide number?
Subject distance multiplied by the f-number for a correct exposure, at a stated ISO, usually 100, and in meters or feet.
What is a watt-second?
The energy stored in the flash capacitor, equal to a joule. It describes the input, not the light out.
Can I convert GN to Ws?
No. Reflector design and efficiency differ, so the conversion would be false.
Last reviewed November 2, 2026