What is a camera flash guide number?

A guide number expresses nominal flash output for specified conditions. At ISO 100, guide number equals aperture f-number multiplied by flash-to-subject distance, using the same metre or foot unit as the rating.

How does ISO change flash guide number?

Multiply the ISO 100 guide number by the square root of ISO divided by 100. ISO 400 therefore doubles the working guide number, while ISO 200 multiplies it by about 1.414.

Can I mix a guide number in feet with distance in metres?

No. The guide-number unit and distance unit must match. This calculator converts both values together when you switch between metres and feet.

Does guide-number math work for bounced or modified flash?

Only if the guide number already represents that exact setup. Bounce, umbrellas, softboxes, grids, gels, zoom position, power level, and high-speed sync can change effective output substantially.

Is guide number the same at every flash zoom setting?

No. A narrower zoom setting usually produces a higher on-axis guide number. Use the value listed for the actual flash-head zoom, power level, and coverage mode.

Are my camera settings uploaded?

No. Calculations, copying, and CSV generation happen locally in the browser.

Camera Flash Guide Number Calculator — Aperture, Distance & ISO

Solve for aperture, maximum direct-flash distance, or required ISO using a guide number rated at ISO 100. Match metre or foot units and see the nearest practical camera setting.

Flash rating and known settings

Calculated flash setting

Calculated aperturef/8Nearest common one-third-stop marking: f/8

Guide number at working ISOGN 36 m

Aperturef/8

Flash distance4.5 m

ISOISO 100

What this means

A GN 36 m flash at ISO 100 gives a nominal direct-flash exposure of f/8 at 4.5 m.

Guide numbers are planning values. Confirm with a flash meter, histogram, or test frame.

How to use the camera flash guide number calculator

Choose the setting to find. Calculate aperture, maximum distance, or ISO.
Find the correct guide number. Use the GN stated at ISO 100 for the flash power, zoom position, and coverage mode you intend to use.
Match the unit. A guide number in metres must be paired with distance in metres; a guide number in feet must use feet. Changing the selector converts both values together.
Enter the known settings. Distance is measured from the flash to the subject, not from the camera when the flash is off-camera.
Round and test. Start with the nearest marked aperture or ISO, then check the real exposure. Small changes in placement, modifiers, and subject reflectance matter.

Flash guide number, aperture, distance, and ISO formulas

At ISO 100, guide number GN₁₀₀ is the aperture f-number N multiplied by flash-to-subject distance d. Guide numbers are unit-dependent, so GN metres pairs with metres and GN feet pairs with feet.

GN₁₀₀ = N × d

For another ISO sensitivity S, first adjust the guide number:

GN_S = GN₁₀₀ × √(S ÷ 100)

The calculator modes rearrange those relationships:

aperture: N = GN₁₀₀ × √(S ÷ 100) ÷ d
distance: d = GN₁₀₀ × √(S ÷ 100) ÷ N
ISO: S = 100 × (N × d ÷ GN₁₀₀)²

Raising ISO by one stop doubles sensitivity but increases effective guide number by √2, about 1.414. Raising ISO from 100 to 400 is two stops and doubles the effective guide number.

Formula reference: Nikon SB-910 Speedlight manual — Guide Number, Aperture and Flash-to-subject Distance.

Quick ISO guide-number reference

ISO	ISO change	GN multiplier	Example: GN 36 m	Distance at f/8
25	−2 stops	0.5×	GN 18 m	2.25 m
50	−1 stop	0.707×	GN 25.46 m	3.18 m
100	Reference	1×	GN 36 m	4.5 m
200	+1 stop	1.414×	GN 50.91 m	6.36 m
400	+2 stops	2×	GN 72 m	9 m
800	+3 stops	2.828×	GN 101.82 m	12.73 m
1600	+4 stops	4×	GN 144 m	18 m

Guide number accuracy and real-world limits

This calculator estimates nominal manual, direct-flash exposure. It assumes the guide number is accurate for the selected full or partial power, head zoom, beam pattern, and coverage format. It also assumes unobstructed light and compatible metre or foot units. Manufacturer ratings are normally measured under controlled conditions and may not match every room, subject, battery state, or individual unit.

Bounce flash lengthens the light path and loses energy at the reflecting surface. Softboxes, umbrellas, diffusion, grids, gels, domes, wide panels, and snoots also change output and distribution. Auto FP/high-speed sync can reduce effective guide number as shutter speed rises. TTL flash meters the scene automatically, so simple guide-number maths is most directly useful for manual flash, planning, and troubleshooting. For exposure-critical work, confirm with a calibrated flash meter or test frame and histogram.

Camera flash guide number FAQ

What does GN 36 mean?

GN 36 m at ISO 100 means aperture multiplied by distance equals 36 under the stated conditions. Examples include f/8 at 4.5 m, f/4 at 9 m, or f/2 at 18 m. GN 36 ft is a different, much lower output rating, so always check the unit.

How do I convert a guide number from metres to feet?

Multiply by 3.28084. GN 36 m is approximately GN 118.11 ft. Converting the guide number and distance by the same factor leaves the calculated aperture unchanged.

How does ISO affect flash range?

Flash range grows with the square root of the ISO ratio. Doubling ISO increases nominal range by about 1.414× at the same aperture; quadrupling ISO doubles the range.

Does reducing flash power change guide number?

Yes. Guide number changes with the square root of the power fraction. Relative to full power, half power is about 0.707× GN, quarter power is 0.5× GN, and one-sixteenth power is 0.25× GN. Use the guide-number table in the flash manual when available.

Why is the calculated aperture between marked f-stops?

The formula returns an exact mathematical f-number, while cameras use stepped settings. The calculator shows the closest common one-third-stop marking. Real guide-number accuracy usually does not justify finer precision.

Does shutter speed appear in the formula?

Below the camera's normal flash sync limit, shutter speed mainly changes ambient exposure because a conventional flash pulse is much shorter than the open-shutter time. Auto FP/high-speed sync is different: effective flash output depends strongly on shutter speed, so use the flash manufacturer's HSS data instead.

Does this page upload my flash settings?

No. All calculations, copying, and CSV generation run locally in your browser.