The GN spec is in the flash manuals, and if it has zoom, it will have many guide numbers... and a GN chart in the manual (for zooms and levels). The charts usually show GN as both meter and feet values (specified as meters/feet). There are 3.28 ft in 1 m, so the GN conversion is: GN in feet = GN in meters x 3.28, or GN in meters = GN in feet / 3.28. Use either feet or meters, so long as you are consistent with units (same units everywhere).
The charts are always for ISO 100, but we know the ISO 400 factor is x2. We seek a ISO 400 GN in this example, so to go the other way, we divide the ISO 400 value by this 2x to get the ISO 100 value (to be able to compare in the ISO 100 chart). All of this was just shown above.
Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400. So we obviously know this needs flash power of (f8 x 12 feet) = GN 96 (feet) at ISO 400.
Converting this example (f/8 at 12 feet, GN 96 at ISO 400) to ISO 100 is GN 96/2 = GN 48 (feet, ISO 100). Now we can search the guide number chart in the speedlight manual (ISO 100), and maybe we find this value at 24 mm zoom and 1/4 flash power to be GN 49 feet. More than close enough to 48. The point of course is that same chart value also applies to GN 96 at ISO 400. This SB-800 GN chart below (for an example) says that 24 mm flash head zoom and 1/4 flash power is GN 49, almost exactly the GN 48 that we seek. Measure the distance, set the settings (flash 24 mm zoom and manual 1/4 power, camera ISO 400 and f/8), press the shutter button, and Presto, you've got it. You're very close on first try for a 12 foot flash distance (direct flash).
Or, the chart includes several other combinations of power level and zoom which are near GN 49. It need not be exact, another GN value of about 12% difference is roughly within 1/3 stop (and 41% is one stop). We don't necessarily have to match flash zoom to lens zoom values, we can use any wider flash zoom - which may waste a little light if the flash is wider than the lens, which is no problem if not at maximum power (some flashes do not zoom anyway). FX flash on DX cameras is already 1.5x wider anyway. To adjust results of this method, you can simply adjust the power level by 1/3 stop, or adjust the aperture by 1/3 stop. Or zooming the flash head makes steps sometimes roughly about 1/3 stop (no bets, that is a very rough approximation).
Say you really wanted to use 35 mm zoom... or that you could use 35 mm zoom. You can of course simply select any nearby zoom and power cell near 48 there (say 35 mm, GN 44 at 1/8 power, that's close). Multiply it back to ISO 400 (44 * 2 = GN 88), and then compute a similar distance and aperture for that GN value (GN 88/12 feet = f/7.3) at ISO 400, 35 mm and 1/8 power. That is comparable to 12 feet at f/8. All that is left is to measure off the 12 feet. There is some situation that will work, and it will work (if using your own chart of course).
Or the GN calculator works: Choose any zoom in your GN chart, and enter its full power GN (GN 125 here for this 35 mm case) at ISO 100. Enter the new ISO 400 you want to use, and for this example, enter f/8 and 12 feet. Click Compute, and it will advise setting -2 2/3 stop power (1/3 higher than 1/8 power). Do that and click Compute again, and it verifies it is within 0.09 stop. And results should be pretty close.
Or if you have it, use the GN mode (next below), set ISO 400 and f/8 on the camera, and 12 feet in the GN menu. It will set the power for 12 feet, and it will work too (feet or meters, whichever you have the flash set to use).
This GN chart is from the Nikon SB-800 flash manual for regular speedlight mode (page 42 PDF manual). It is a full frame FX rated GN chart.
Regular speedlight flash mode exposure is independent of shutter speed (the flash duration is shorter than the shutter speed duration). But both sunlight and HSS flash are continuous light, so fast shutter speed of course reduces their exposure, both the same way. It's natural, we expect that. For either or both sun or HSS, we can open the aperture wider to compensate, which creates the concept of Equivalent Exposure (same as for sunlight). If shutter speed is twice as fast, we simply open aperture one stop to recover. Any equivalent exposure is still the same exposure, true for both sun and HSS flash. This is extremely different than speedlight flash mode.
The Nikon flash manuals have a separate Guide Number chart for HSS (GN is for bare direct flash.) Typically, the later models (SB-900, SB-700, spec chart in rear of manual) give 1/500 second shutter for HSS GN. Earlier models (SB-800 page 106, SB-600 page 76) give it as 1/300 second. Since HSS is continuous light, this means we can use any Equivalent Exposure to shift HSS away from this referenced shutter speed number (and the HSS GN remains the same for any equivalent exposure).
For example, if the HSS GN is 36 at 1/300 second (from the SB-800 HSS GN chart next below, 24 mm ISO 100, feet), then HSS exposure for 10 feet is 1/300 at 36/10 = f/3.6. Then any equivalent exposure of 1/300 f/3.6 works the same at 10 feet for HSS, for example 1/1200 f/1.8 is the same GN, same distance, same exposure (you can get more GN and more flash range and higher equivalent shutter speeds by converting to higher ISO, ISO 400 is 2x GN). Both sunlight and HSS are continuous light, so both work this same way (equivalent exposures are the same exposure). The beauty is that changing equivalent exposures won't affect your flash or sun exposure, or your HSS fill ratio. The problem is that HSS guide numbers are quite low.
Equivalent Exposure concept of HSS, for both flash and sunlight. This is a big deal. It's a D800 and SB-800 at 5.5 feet, at ISO 400 and 24 mm, hot shoe direct flash. Flash is at manual Full power level. The Guide Number is GN 72 at reference 1/300 second if at ISO 400, so f/16 1/320 worked for the ambient alone (Auto FP 1/250 mode). So at f/16, HSS GN distance is 72/16 = 4.5 feet. Using 5.5 distance instead of computed 4.5 was a flash reduction to become a lower fill level in daylight (computes 122% distance, roughly -0.6 stop). Both flash and sunlight have the same results at f/16 1/320 second and also same at f/4 1/5000 second with HSS flash (equivalent exposures). This is a Big Deal, HSS works with the same Equivalent Exposures as Daylight (because both are continuous light). Specifically, this means the same one HSS GN works at any Equivalent Exposure (up and down the camera P mode range), which is very useful to know when using HSS flash.
This is the SB-800 HSS GN chart (page 106 PDF manual). Older models used this 1/300 second reference, newer models use 1/500 second (which is about 2/3 stop difference). The 1/300 is an awkward shutter number, so I call it 1/320 second. The idea is to compute HSS GN settings for that referenced shutter speed, and then any equivalent exposure works the same (for HSS or sunlight).
So repeating, HSS GN is different, and easy, but a little tricky. Shutter speed does not affect speedlight GN, but shutter speed of course does greatly affect HSS GN. This older GN chart says it is for 1/300 second (1/500 second is more common to see today). HSS has Guide Numbers, for the specified conditions stated. Here 24 mm ISO 400 is GN 72 specified at 1/300 second. So for 1/320 f/16, maximum TTL power is range GN72/f16 = 4.5 foot range (because, in this case, the shutter speed matches the stated situation). This tree is a foot farther, so it's down about -0.6 EV here (let's call it fill flash). But this same 4.5 distance is applied to Any Equivalent Exposure, so 1/5000 f/4 is the same 4.5 foot range.
For HSS flash GN, compute HSS range for the specified shutter speed, and then any Equivalent Exposure is equivalent, with same range. Careful though, unlike flash GN, this says 1/300 second, and it does NOT mean 1/5000 f/4 has 72/4 = 18 foot range. It does mean that 1/5000 second is -4 EV from 1/320 second (1/16 exposure), so for 1/5000 second, we open the aperture 4 stops to f/4, with same range of the same Equivalent Exposure.
However in sunlight, our exposure also has to match the suns ambient exposure. Then any Equivalent Exposure responds the same for both sunlight and for HSS flash.
But assuming bright sun (if we are using HSS), then fill flash level only needs to be a stop or two less than full illumination level, and if assuming -2 EV fill, that fill concept doubles the GN distance. That range becomes 15.6 feet for -2 EV fill, which is 20% flash. A -1 EV flash compensation is 1.4x GN distance, and 33% flash. This level at higher ISO can be usable for f/1.8 portraits at reasonable range, but sports action may be too distant.
(More about HSS here). If the ambient were dimmer, and if we can underexpose ambient a couple of stops, I would never consider HSS. Speedlight mode can run circles around it, power, range, and motion stopping, but it is limited to maximum shutter sync speed... which won't matter if the ambient is underexposed enough to not be a factor.
We should realize that neither TTL metering nor Guide Number calculations are always precisely correct. Things happen, but at least GN is independent of the subject colors which do fool camera meters. But BOTH methods are pretty close first attempts, tremendously better than having no clue at all. The standard used to be that flashes that zoom always advertised GN at 35 mm, considered a probable useful working value for full frame cameras. Nowadays, the Chinese flashes advertise the GN at maximum zoom (the biggest number), an inflated representation, not a likely probable common actual use. You will want to see the GN Chart in the flash manual, instead of the advertised claims. Flash power levels can only be compared at the same zoom value for both.
Measured GN sometimes may seem to vary a little from the manufacturer's chart value. I have wondered at times, but prefer to believe the chart numbers normally are carefully prepared. One factor is that our own procedures can vary. A near wall surface can provide reflected fill (affecting exposure), so GN can increase in a small room where reflections from the near walls/ceiling combine, whereas GN is lower in wide open spaces with no reinforcement. How much the ambient light is also contributing can be a factor. Flash capacitors do of course have a capacity tolerance (typically -10%, +20%. new), so individual flash units might vary slightly (but it's small in terms of stops of power). Another possibility is that the capacitors in an old flash may have deteriorated somewhat, not still full capacity. My 25 year old Nikon SB-24 still performs to GN spec however.
The GN charts generally seem to specify GN for meters always in whole numbers, and then GN for feet is simply m/3.28. Just my assumption, but if every zoom value on every brand and model has a rounded GN value in meters (no decimal points), it suggests the published GN precision is ± 0.5 (meters) or ± 1.6 (feet), which is possibly a noticeable variation if subject is close. But if at usual distances, GN can be surprisingly accurate, an excellent first try. Just try it, especially if your flash has the GN mode (next below). I don't have much trouble with guide numbers, but there certainly are variations in different situations..
But I really think (in my own case) the most likely reason for GN differences is that our own perception of "correct exposure" is not always precise, so check multiple tries in different situations. And actually do take the picture to check it. My own experience is that the Nikon guide number seems accurate. If I measure the distance and take the picture, I get fairly good exposures, at least a good starting point. One beauty of it is that GN is not affected by subject reflectance in reflected meters. But if I just meter the direct flash, it may not always match GN expectations precisely? That's a contradiction, and is probably my procedural error? Metering flash is a good thing to do, and if you can meter your lights, that's great. But if you have no other means to adjust manual direct flash, you ought to try guide numbers.
One special case: If you attempt to verify your speedlight's guide number at maximum power level, don't use your maximum sync speed (at full power). Unless you are metering it, then at full power, back off to maybe 1/160 second. The speedlight becomes slow at maximum power, so if at the fastest shutter speed, the shutter can close on it, and the sensor can't see it all. If metering it with a handheld meter, and flash is on the hot shoe, the fastest shutter speed can quench the flash off too, so an external meter can't see it all either. This is only a slight effect and only occurs at maximum flash power and at fastest sync shutter speed. Flashes vary, but I do see my SB-800 slightly reduce maximum flash exposure at 1/320 second, but it seems negligible at 1/250 second. Speedlight lower power levels are greatly faster of course. Most studio monolights are the opposite, slowest at lower power levels.
SB-700 manual page C-11
SB-800 manual page 44 (shown at right)
SB-900 manual page D-11
SB-910 manual page C-12
Nikon calls this GN mode "Distance Priority Manual Flash". When on the hot shoe, CLS flash already knows ISO, aperture, zoom, and the guide number chart, so these flash models have a GN Mode option where all you do is set the distance into the flash menu (ten feet shown here). The flash computes and sets the flash power level automatically, to be correct for the distance and camera settings (bare direct flash). Automatic computation in that sense, but it is a Manual flash mode - we enter the distance manually (The D-lens focus distance is not used by this mode, because it is not accurate or complete enough for this).
The details are that we know f/8 at 10 feet is 8x10 = GN 80 at this ISO 200. That's 80/1.4 = GN 57 at ISO 100. This is a SB-800, so looking at its ISO 100 GN chart above, we know 50 mm at GN 57 must be set at a bit more than 1/8 power in this case. GN Mode simply knows how to do all that, and does it for you when you enter ten feet.
Camera Flash Compensation is not operative in GN mode (camera metering is not involved). If you want the flash a bit brighter, tell it the distance is a bit more (1/3 stop more is 12% more distance).
Guide Numbers are about direct flash, but this GN mode can surprise you if you will try it. We do have to know the distance, but one really wonderful GN advantage is that unlike TTL metering, GN Mode is independent of the subject colors which do affect TTL metering according to how well they reflect light. GN mode is independent of the subjects reflectance (like incident metering is also independent of seeing the subject). It just sets the right light level for any subject (at that one distance), and black things will come out black, and white things will come out white. So this mode would be fabulous, except that it is direct flash only, and we have to know distance. GN mode is for camera mode A or M (aperture is set and does not keep changing), and for direct flash only with the flash head straight ahead (the Nikon GN mode simply disappears from the menu if the flash head is tilted or rotated).
Bounce is often the very good stuff, but Guide Numbers do not work so well for bounce, because the total distance up and down is greater than the direct path, and there is also reflection loss at the ceiling. TTL is wonderful to actually meter bounce (reflected metering), but bounce will need two or three stops more flash power then the direct distance would indicate. In the real world, we usually just guess at the direct distance, so the initial result might be off a little, but like TTL, it will be a close starting point. One advantage of digital cameras today is that we immediately see our result, and we can simply correct it and retry until we are happy. The camera body has a flash compensation menu, and the flash body has a flash compensation menu, and the camera will add these to a new total. Changing TTL compensation works, but changing the other values like aperture or ISO just recalculates the same TTL exposure goal.
Guide Numbers are used for direct bare flash, but it becomes tough and unknown for bounce and umbrellas, etc. Path distance has to be measured from the light source (the flash tube), via the reflection surface (NOT just from the fabric panel). In the old days (before TTL electronics), we used to approximate for ceiling bounce with the rule of thumb "open two stops for bounce" (from the direct values). Three stops was usually closer, which was sometimes adequately ballpark for negative film (much more latitude than digital), but of course, very crude and vague, because every situation was different (ceiling height and texture and reflectance, and flash head angle, etc). FWIW, the three stops might be reasoned this way: a 45 degree path was 1.4x longer which is one stop, the reflection at the ceiling was one stop loss, and half the light went elsewhere, another stop. The film had much latitude, and it usually worked OK. Digital is more critical, but TTL makes this easy today.
So TTL excels for bounce, it simply meters the actual light arriving via that path, whatever it is. However, regardless if bounce or direct, TTL accuracy is still always affected by the reflectivity of the various subject's colors (clothing, walls, etc) - which does not affect guide number, which does not even take the subject into account (only the distance and GN are considered). For direct flash, the beauty is, if we know GN and distance, we KNOW the exposure, independent of the subject colors. There is a good case for that, but frankly, measuring distance and doing division is more awkward than automatic TTL metering. We likely have to adjust it slightly either way (due to TTL reflectance, or GN distance).
So to repeat: Problems are, the guide number method needs to know a fairly precise distance from flash to subject, and this leaves out bounce flash. Or, we can always guess roughly at the distance, and get a rough trial answer, and then tweak that result better by trail and error. It is a good starting point, but we also need to know the guide number fairly precisely, which implies direct flash only. The guide number chart in the flash manual can differ a bit from our results, which could be due to added reflections from walls in a tiny room, could be flash capacitor aging, could be marketing exaggeration of specs. Or, often it is rather accurate. But it will be a constant after you know it.
But if we do know one precise exposure result, we can change distance and still know the right exposure. We have to do some division, but sometimes we can approximate this in our heads, or many flashes have (or used to have) guide number calculators, where we enter distance, and it tells us f/stop, or vice versa.
While guide number is a fundamental basic we ought to know (it handles the Inverse Square Law), and which is still dead on today, frankly, the method may seem old fashioned now. It is what we used back in the 1940s to 1960s - it was all there was for the flash bulb era. But by about 1970, we had electronic flashes with the photo sensors for the Auto modes that self-metered the reflection back from subject. We certainly liked that, and it worked for bounce too. The 1980s introduced TTL, metered and controlled by the camera computer. We liked that too, it was great to actually meter the flash. Reflective metering certainly can have issues, it often needs some correction (called Flash Compensation, which we add manually by trial and error and experience). But TTL is metered, and is generally always a pretty close starting point.
Many users use TTL flash today, but also many prefer manual flash mode, for the control it offers. Both modes must be watched and adjusted, frankly, both are just the first starting point for determining actual proper exposure. TTL may start closer, but frankly, there is much less difference than we may imagine, in that we adjust both for a final result. Manual flash users just quickly "know" (remember) that this familiar situation will need about 1/4 power, same as last time. Honest, neither method is difficult except first day.
But either way, guide number is really about the least we can know about flash. For example, you're at home wondering about the graduation picture tomorrow. You think you can sit with 50 feet of the stage. You have a suitable lens, say 105 mm for DX, but you're wondering about the flash. The zoom on your SB-600 maxes out at 85 mm, and the GN there is 131 (ISO 100), and x2 for ISO 400 is GN 262. At 60 feet (safety factor), GN 262 / 60 feet = f/4.4. Piece of cake, ISO 800 should not be necessary. Even if planning to use TTL, this is good to know before you get there. Take a couple shots of the empty stage before things start, to get setup right.
See a page about comparing power ratings of flash units using guide numbers (NOT the same page as Next page below).