A flash guide number is a single figure that lets you solve for aperture without a light meter. Divide it by your flash-to-subject distance and you have the f-stop that gives a correct exposure. That is the whole idea — and almost every problem people have with guide numbers comes from the conditions attached to the number rather than the arithmetic.
The formula
GN = aperture × distance, which rearranges to the two forms you actually use on a shoot: aperture = GN ÷ distance and distance = GN ÷ aperture.
A flash with a guide number of 32 lighting a subject 4 metres away wants f/8. Move to 8 metres and it wants f/4. Move to 2 metres and it wants f/16. Notice that doubling the distance costs two full stops, not one — that is the inverse square law doing its work, and it is the single most useful intuition the guide number encodes.
The conditions attached to the number
A guide number is meaningless without the conditions it was measured under, and manufacturers do not always print them next to the headline figure. Four things have to match before the number means anything:
- ISO. Guide numbers are quoted at ISO 100 by convention. A number quoted at ISO 200 will look 1.4× better for no real reason.
- Units. Metres or feet. The same flash is GN 32 in metres or GN 105 in feet. Mixing them up produces a five-stop error.
- Zoom head position. This is the big one. A speedlight with a zoom head concentrates its output into a narrower beam as it zooms in, so the guide number rises without a single extra watt-second being produced.
- Full power. The quoted figure assumes 1/1 output with fresh batteries.
Why zoom head position inflates the headline number
Marketing departments quote the guide number at the longest zoom setting, because that is the biggest number. A typical speedlight advertised at GN 60 is usually GN 60 at 200mm zoom — at 35mm, which is where you will actually use it for a normal scene, the same flash might be GN 36. That is not a defect, it is a beam angle. But it means the GN on the box is not the GN you will be dividing by.
This is also why guide numbers are close to useless for comparing two speedlights from different brands unless you can find the number at a matched zoom setting. Watt-seconds are a better comparison for studio strobes; for speedlights, a GN quoted at 35mm is the honest figure to look for.
Scaling for ISO
Guide number scales with the square root of the ISO ratio, because doubling ISO buys one stop and a stop is a factor of √2 in aperture. The correction is:
GN at new ISO = GN₁₀₀ × √(ISO ÷ 100)
| ISO | Multiplier | GN 32 becomes |
|---|---|---|
| 100 | 1.0 | 32 |
| 200 | 1.41 | 45 |
| 400 | 2.0 | 64 |
| 800 | 2.83 | 90 |
| 1600 | 4.0 | 128 |
Read that table the useful way round: raising ISO from 100 to 400 doubles your effective guide number, which doubles your working distance at the same aperture. Flash range is one of the few places where cranking ISO buys you something concrete rather than just a brighter, noisier file.
Power fractions
Cutting flash power in half costs one stop, so it divides the guide number by √2, or about 1.41. A GN 32 flash at 1/2 power behaves like GN 23; at 1/4 power, GN 16; at 1/8 power, GN 11. Each click down the power scale multiplies the guide number by 0.707.
Worth knowing because the useful end of a speedlight is rarely full power. Full power means the longest recycle time and the fewest frames per battery, so most people run at 1/4 or below and then wonder why the range is nothing like the box promised.
Modifiers change everything
Every modifier eats light, and the guide number does not know about them. Rough figures for what to subtract:
| Modifier | Typical loss | Effect on GN |
|---|---|---|
| Bounce off white ceiling | 2 to 3 stops | ÷ 2 to ÷ 2.8 |
| Small softbox on a speedlight | 1.5 to 2 stops | ÷ 1.7 to ÷ 2 |
| Shoot-through umbrella | 1.5 to 2 stops | ÷ 1.7 to ÷ 2 |
| Dome diffuser | 1 to 1.5 stops | ÷ 1.4 to ÷ 1.7 |
| CTO gel (full) | About 1 stop | ÷ 1.4 |
Bounce flash is the case where guide number arithmetic breaks down completely, because the effective distance is now flash to ceiling to subject, not flash to subject, and the ceiling absorbs and scatters an unpredictable amount. For bounce, meter it or chimp it.
Where guide numbers still earn their keep
TTL metering has made the guide number optional for most shooting, and that is fine. But it remains genuinely useful in three situations. In manual flash work, it gets you to a correct exposure on the first frame instead of the third. When you are deciding whether a flash can even reach — lighting a group at the back of a hall, say — GN divided by your working aperture tells you the maximum distance immediately. And when you are buying, knowing how to normalise a quoted GN to ISO 100 at 35mm zoom is the only way to compare two units honestly.
The deeper point is the one the diagram shows: flash falls off with the square of distance. Moving a light a metre closer to your subject does far more than another half-stop of power, and the guide number is the simplest tool for making that trade-off in your head.
Run the numbers
Solve GN = aperture × distance in any direction, factoring in ISO, power fraction and modifier loss.
Related reading
The falloff behind the formula is covered in The Inverse Square Law, and the aperture side in Aperture and F-Stops Explained. For how speedlights compare to bigger sources see Speedlights and Studio Strobes, and for shaping what comes out of them, Light Modifiers.