It tells you a distance once you divide it by an aperture — but only under the sensitivity and zoom setting the maker chose as its reference, and those are usually left unstated. It is not a quantity of light, and it does not compare with an energy in watt-seconds.
A guide number is a distance multiplied by an f-number. Divide it by the aperture you are using and you have the distance at which the flash exposes correctly — under conditions the maker picked.
The conditions that make it a measurement
Two of them, and both are usually missing from the box: the sensitivity the figure assumes, and the zoom head position. Change either and the number changes. Three products in this catalogue publish a guide number — GN65, GN8, GN 16 — and none of the three titles states the conditions behind it.
The other figure on the same boxes
Two products here publish an energy instead: 400 Ws and 600 Ws. One publishes both at once, three characters apart: Studioblitz 400Ws, GN65 5600K. They answer different questions, and neither is the light landing on a subject — what arrives depends on the modifier, the distance and the surface in between.
The flash figures that survive scrutiny
For comparing two flashes of the same kind, the figures that survive scrutiny are the ones with their conditions attached, plus the recycle time if it is published — one unit here states 0.01 to 2.5 s. Where a listing gives a bare number, this site records the number and says the conditions are unstated.
Where this page and the maker’s own manual disagree, the manual decides. Every focal length, aperture, guide number, stop of stabilisation, load rating and sealing claim on this page is reported from the manufacturer’s own documentation, with its unit written as published, and named as such — none of it is measured here. An engraved figure is reported as the maker engraves it, and is never restated in another convention, because the two are not the same number. And where two light sources are set beside each other: a guide number and an energy in watt-seconds describe different quantities and cannot be compared with one another. A guide number depends on the sensitivity and the reference coverage the maker chose before printing it; a watt-second figure describes the energy a capacitor holds before the tube fires. Neither is the light that reaches a subject, which depends on the modifier, the distance and the room.
What people ask before they commit
Can I compare a guide number with a watt-second figure?
No, and there is no conversion between them. Watt-seconds describe energy stored in a capacitor before the tube fires; a guide number describes illumination arriving at a distance under reference conditions. Between the two sit the reflector, the zoom head and the efficiency of the tube.
Why do zoom flashes publish such large guide numbers?
Because the figure rises with the focal length the head is zoomed to, and the number quoted is usually the one at the longest setting. Two guide numbers are comparable only at the same sensitivity and the same head position, and most listings state neither.
Last reviewed 12 September 2026