What the standard defines
Neither figure is defined by a standards organisation, and both are defined precisely enough by physics that the confusion between them is avoidable.
A guide number is a distance multiplied by an aperture: divide it by the f-number and you get the distance at which the flash correctly exposes, under the sensitivity and the head position the maker used as its reference. Three products here publish one — GN65, GN8, GN 16 — and the reference conditions behind those three numbers are not stated in any of the three titles.
A watt-second is energy. It describes what the capacitor holds before the tube fires, and two units of the same energy differ in output according to reflector, tube and efficiency. Two products here publish one: 400 Ws and 600 Ws.
What it does not promise
Neither is light on a subject. A guide number assumes a reference sensitivity and head position; an energy assumes nothing about what happens after the tube. What actually lands depends on the modifier, the distance and the surface in between.
No comparison across the two. There is no conversion. A listing quoting one cannot be ranked against a listing quoting the other, and this site does not attempt it.
No comparison within one, without its conditions. Two guide numbers are comparable only at the same sensitivity and the same head position. Where a maker omits those, the figure is a number rather than a measurement.
No recycle time. How quickly the unit is ready again is a separate published figure — one product here states 0.01 to 2.5 s — and it is unrelated to either output figure.
What it works with
Every flash on this site, and the triggers, softboxes and diffusers hung off them. A modifier changes what arrives without changing either published figure, which is precisely why neither number describes a lit subject. The other figure published on these boxes is a colour temperature, which describes the light’s colour and not its quantity.
What people ask before they commit
Can I convert watt-seconds into a guide number?
No, and the reason is worth keeping: watt-seconds describe energy stored in a capacitor before the flash fires, while a guide number describes illumination reaching a distance under reference conditions a maker chose. Between the two sits the reflector, the zoom head, the efficiency of the tube and the choices of sensitivity and focal length behind the guide number. Two units at 400 Ws with different reflectors are not two equal lights.
Why do guide numbers always look so large on zoom flashes?
Because a guide number rises with the focal length the head is zoomed to, and makers quote the figure at the longest setting where it flatters most. A number given without its reference focal length and sensitivity is not comparable with another given without them either. Where a listing states both, it is publishing something usable; where it states one number alone, that is what this site records.
Last reviewed 12 September 2026