Flash Guide Number Calculator

Background.

The guide number is the one piece of arithmetic that makes manual flash predictable, and it is far simpler than its reputation. A flash's guide number is the product of the aperture and the distance that produce correct exposure: GN = distance × f-number. Nikon states it in exactly those words in the terminology section of the SB-5000 Reference Manual — "there is a relation represented by an equation, GN = flash-to-subject distance (m or ft) × aperture f-number (ISO 100)". Give this calculator any two of the three and it returns the third, then applies the two corrections that trip people up: the ISO you are actually shooting at, and the manual power fraction the head is set to.

The reason the identity is so useful is that flash exposure, below the sync speed, does not depend on shutter speed at all. A speedlight fires for a few hundred microseconds; the shutter is open for thousands of times longer than that. Change the shutter from 1/60 to 1/200 and the flash-lit subject does not change brightness one bit — only the ambient light behind it does. That decoupling is what lets a single number describe a flash. Everything about the flash-lit part of the frame is captured by the guide number, the aperture, the distance and the ISO, and nothing else.

Two corrections turn a published rating into a working number. The first is sensitivity. Guide numbers are quoted at ISO 100, and light scales as the square of distance, so the guide number scales as the square root of the ISO ratio: √(ISO/100). Flash manuals print this as a lookup table — 0.71 at ISO 50, 1.4 at ISO 200, 2 at ISO 400, 4 at ISO 1600 — and every one of those entries is that square root rounded for print. This calculator uses the exact square root, so it will accept ISO 320 or ISO 5000 rather than forcing you onto a tabulated value.

The second is manual power. Guide number scales as the square root of the output fraction too, so quarter power is half the guide number, not a quarter of it. That is not a model this page invented: Nikon's own published guide-number table for the SB-5000 runs 34.5, 24.4, 17.3, 12.2, 8.6, 6.1, 4.3, 3.0, 2.1 metres from full power down to 1/256, and 34.5 × √(1/n) reproduces every one of those figures to within a unit of the last printed digit across nine stops. The calculator shows the stops you are giving up so you can see, for instance, that dropping to 1/4 power and pushing ISO from 100 to 400 leaves your reach exactly where it started — while cutting recycle time roughly fourfold.

Below the widget you will find the equation rearranged three ways, a worked example computed by hand against Nikon's own published figure, the inverse-square arithmetic that decides how far you can step back before you lose a stop, why a guide number quoted without a zoom-head position is close to meaningless, and an honest account of where the model stops working — bounce flash, high-speed sync, and rooms that are not the neutral grey box the rating assumes.

What is flash guide number calculator?

A guide number is a single figure that summarises how much light a flash puts out, defined so that it can be divided by an aperture to get a distance or by a distance to get an aperture. It exists because flash illuminance obeys the inverse-square law while aperture area scales as 1/N², and those two square relationships cancel: for correct exposure, N × d must be constant, and that constant is the guide number. ISO 1230:2007, Photography — Determination of flash guide numbers for electronic flash equipment, is the international standard that "specifies the definition and determination of the ISO guide number of electronic flash equipment", which is what makes a published guide number a comparable figure rather than a marketing claim; the companion standard ISO 2827 covers the light-output measurement itself. Three things are baked into any quoted guide number and must be quoted with it. The first is sensitivity: the convention is ISO 100, and a guide number at any other ISO scales by √(ISO/100). The second is units: a guide number in feet is 3.28 times the same rating in metres, which is why manufacturers publish the pair — the SB-5000 is "34.5/113 (ISO 100, m/ft)". The third, and the one most often omitted in conversation, is the zoom-head position and illumination pattern. A speedlight concentrates its light into the angle the lens needs, so the same flash rates 27 m at the 35 mm-format wide end and 55 m at 200 mm. Quoting "GN 60" without saying at what zoom setting is roughly as informative as quoting a car's top speed without saying downhill or up. What the guide number is not is a measure of exposure in the general sense. It describes direct flash on a subject of average reflectance in a space of average reflectance. Bounce the flash, put the subject against a black backdrop, or shoot through a softbox and the arithmetic still holds in principle but the effective guide number is no longer the published one — you are then measuring, not calculating.

How to use this calculator.

1. 01Choose what you need. "Distance" answers how far the flash reaches at a given aperture; "aperture" answers what f-number to set at a known distance; "guide number" answers how powerful a flash the shot requires. All three quantities appear in the results whichever mode you pick.
2. 02Enter your flash's published guide number at ISO 100 and full power, and pick the matching unit. Check the specification for the zoom-head position it was measured at — using the 200 mm figure while shooting at 24 mm will overestimate your reach by a stop or more.
3. 03Enter the aperture as the number after the slash, and the flash-to-subject distance in the unit you chose. Measure from the flash head, not the camera: as soon as the flash comes off the hot shoe those are different distances, and it is the flash's distance that counts.
4. 04Set the ISO you are actually shooting at. The calculator applies √(ISO/100) exactly, so any value works — you are not restricted to the nine sensitivities that appear in printed factor tables.
5. 05Set the manual output level. This is the fraction shown on the back of the flash. Watch the "stops below full power" output: it tells you directly how much light you have given up, which you can then compare against the stops of ISO you have gained.
6. 06Read the answer, then read the inverse-square line underneath. It gives the distances one stop either side of your setting, which is the practical question — how far can I move before the exposure is wrong?
7. 07Set the nearest marked aperture on the lens and shoot a test frame. The calculation is a starting point that assumes average reflectance; a bride in white and a graduate in black at the same distance need different apertures, and no guide number knows which one is in front of you.
8. 08For bounce flash, treat the result as a floor rather than an answer. Light bounced off a ceiling travels the longer path and loses energy to the surface, typically costing around two stops, and the loss depends on the ceiling's height and colour in a way arithmetic cannot capture.

The formula.

Start from the physics, because it explains why the identity is a product rather than something messier.

Illuminance from a point source falls as 1/d². The light a lens delivers to the sensor scales as the aperture area, which goes as 1/N². Correct exposure requires the product of those to be constant, so 1/(N²d²) is constant, so N·d is constant. That constant is the guide number:

GN = N × d

Nikon's SB-5000 Reference Manual gives it in those words and supplies both rearrangements directly:

Aperture f-number = GN(ISO 100) × ISO factor ÷ distance Distance = GN(ISO 100) × ISO factor ÷ aperture f-number

The ISO factor. A guide number is quoted at ISO 100. Doubling the sensitivity doubles the light collected, which is worth one stop, and one stop of aperture is a factor of √2 in N — so the guide number scales by √2 as well:

k_ISO = √(ISO / 100)

The factor table printed in flash manuals is exactly this: 0.5 at ISO 25, 0.71 at ISO 50, 1.4 at ISO 200, 2 at ISO 400, 2.8 at ISO 800, 4 at ISO 1600, 5.6 at ISO 3200, 8 at ISO 6400. Note that ISO 3200's printed 5.6 is √32 = 5.6569 rounded the same way the f/5.6 marking is — the tables use marked-stop precision, this calculator uses the exact value.

The power factor. The same argument applies to manual output. Halving the power costs one stop, which is a factor of √2 on the guide number:

k_power = √(1 / n)

This is confirmed by measurement, not assumed. Nikon publishes a guide-number table for the SB-5000 by output level (FX format, standard illumination pattern, 35 mm zoom head, ISO 100): 34.5, 24.4, 17.3, 12.2, 8.6, 6.1, 4.3, 3.0 and 2.1 metres from 1/1 down to 1/256. The model 34.5 × √(1/n) gives 34.5, 24.40, 17.25, 12.20, 8.63, 6.10, 4.31, 3.05 and 2.16 — agreement to within one unit of the last printed digit across a 16× range. Nikon states the relation independently for high-speed sync too: changing the shutter from 1/500 to 1/1000 s "decreases the guide number 1 step (approx. 1/1.4)", and 1/√2 = 0.707 ≈ 1/1.4.

Putting them together, GN_eff = GN₁₀₀ × k_ISO × k_power, and then GN_eff = N × d solves for whichever quantity you did not supply.

Units are exact: 1 ft = 0.3048 m by definition (NIST SP 811), so a guide number of 34.5 m is 34.5 / 0.3048 = 113.19 ft, which is the 113 printed on the specification sheet.

What the model deliberately does not include: bounce and modifier losses, which depend on the geometry and colour of surfaces the arithmetic cannot see; subject reflectance, since the rating assumes an average scene; ambient light, which adds to the flash exposure and is a separate calculation; high-speed sync, which converts the flash into a quasi-continuous source and collapses the guide number by two to three stops; and colour temperature, which changes nothing about the quantity of light but a great deal about the picture.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Nikon Corporation. SB-5000 Reference Manual (English). The primary source for every relationship on this page. Terminology, p. A-8: 'Guide number (GN) — The amount of light generated by a flash unit; as the number increases, the light extends further. There is a relation represented by an equation, GN = flash-to-subject distance (m or ft) × aperture f-number (ISO 100).' Section H-6/H-7 gives the ISO sensitivity factor table (0.5, 0.71, 1, 1.4, 2, 2.8, 4, 5.6, 8 for ISO 25–6400), the rearrangements for aperture and distance, and the worked example 'When ISO sensitivity is 100 and aperture f-number is f/8, the illumination of the SB-5000 reaches 4.31 m (14 ft)'. Specification p. H-21: 'Guide number (at 35 mm zoom head position, in FX format, standard illumination pattern): 34.5/113 (ISO 100, m/ft)'. Guide-number table p. H-27 gives the output-level figures used to verify the √(1/n) power scaling.
2. [2]International Organization for Standardization (2007). ISO 1230:2007, Photography — Determination of flash guide numbers for electronic flash equipment. Scope: 'ISO 1230:2007 specifies the definition and determination of the ISO guide number of electronic flash equipment.' Third edition, published 2007, confirmed 2025. This is the standard that makes a published guide number a comparable measurement rather than a claim.
3. [3]International Organization for Standardization (1988). ISO 2827:1988, Photography — Electronic flash equipment — Determination of light output and performance. The companion standard to ISO 1230, specifying methods of measurement for light output, angle of coverage, recycle time and number of flashes. ISO 1230 deliberately excludes light-output measurement, which lives here.
4. [4]National Institute of Standards and Technology. NIST Special Publication 811, Appendix B.9, Factors for Units Listed Alphabetically. Gives 'foot (ft) → meter (m): 3.048 E-01' as an exact factor, the basis of the metre/foot guide-number pair and of the dual distance output on this page.