β‘ Flash Guide Number Calculator
Solve for aperture, distance, or guide number with ISO-adjusted flash math.
Correct aperture
Assumes direct flash at full manual power, zoom head at rated position. Keep the distance unit (meters or feet) consistent with your flash\'s GN rating.
How to Use This Calculator
Every flash has a guide number, and this calculator turns that number into the aperture, distance, or flash power you actually need. Pick what you want to solve for: the correct aperture for a known distance, the maximum distance your flash can reach at a chosen aperture, or the guide number your flash needs to cover a given combination. Enter the other two values plus your ISO, and the answer updates instantly.
Choose what to solve for from the dropdown: aperture, distance, or guide number. Only enter the values you already know; leave the solved-for field at its default.
Type your flash\'s guide number as rated at ISO 100. You will find it in the flash manual or on the manufacturer\'s spec sheet, usually listed in meters or feet.
Enter the subject distance and your ISO. Make sure the distance unit matches the unit your guide number was rated in: meters with meters, feet with feet.
Read the result. When solving for aperture, the calculator also shows the nearest standard full f-stop so you can dial it in on the lens without guessing.
The Guide Number Formula, in Plain English
The guide number ties three things together: how powerful your flash is, how far away the subject is, and which aperture you need. Doubling the distance means the light spreads over four times the area, so the intensity drops to one quarter. To compensate you must either open the aperture by two stops or quadruple the flash power. That is why the relationship is multiplicative rather than additive, and why the calculator can solve the equation in any direction.
ISO enters through a square-root relationship, which is just another way of saying each doubling of ISO adds exactly one stop of flash reach. A GN 58 flash at ISO 100 becomes an effective GN 82 at ISO 200 and GN 116 at ISO 400. This is the trick behind shooting bounce flash in big rooms: instead of buying a bigger flash, raise the ISO a stop or two and the same unit reaches much farther. The trade-off, of course, is noise, so the calculator lets you compare the aperture options side by side before you commit.
Worked Example
Your speedlight is rated GN 58 at ISO 100 in meters. You are shooting a group portrait 5 meters away at ISO 400. First adjust for ISO: 58 x sqrt(400/100) = 58 x 2 = 116. Then the aperture is 116 / 5 = f/23.2. The nearest standard f-stop is f/22, so set f/22 and the flash exposure will be correct at full manual power. If f/22 gives you too much depth of field and you would rather shoot at f/11, the same math shows the maximum distance is 116 / 11 = about 10.5 meters.
Working the other way: you want to shoot at f/8 with the subject 8 meters away at ISO 200. The required guide number is 8 x 8 = 64 at the shooting ISO, which converts back to 64 / sqrt(200/100) = about GN 45 at ISO 100. A flash rated GN 45 or higher will do the job.
Practical Tips
Most photographers use this formula in reverse in the field: before an event, compute the maximum usable distance for your favorite aperture and ISO combination, then you know whether you need to move closer, raise ISO, or add a second flash. Taping a small cheat card with two or three distance/aperture pairs to your flash is a classic wedding-photographer move.
Remember the two big exceptions. First, the formula assumes full manual power; if you dial the flash down to 1/4 power, you lose two stops, which halves the effective guide number. Second, modifiers eat light: a softbox or bounce costs roughly 1 to 2.5 stops, so multiply your computed distance by 0.5 to 0.7 as a rough correction. When in doubt, meter with a test shot: digital costs nothing, and the histogram never lies.
Frequently Asked Questions
What is a guide number?
A guide number (GN) is a single number that rates how powerful a flash is, usually measured at ISO 100 with the flash zoomed to its standard position. The relationship is simple: Guide Number = distance x f-number. A higher guide number means a more powerful flash that can expose a subject correctly from farther away or through a smaller aperture.
How does ISO change the guide number?
The published guide number is almost always given at ISO 100, but raising the ISO makes the sensor more sensitive, which effectively increases the flash's reach. The adjusted guide number is GN(ISO) = GN(100) x sqrt(ISO / 100). So at ISO 400 the effective guide number doubles, which is exactly two stops of extra flash power.
Which distance unit should I use?
Use whichever unit your flash manufacturer used for its guide number rating, most commonly meters or feet. The calculator keeps whatever unit you enter: if you type the distance in feet, the answer applies to feet. Mixing meters and feet without converting will give a wrong answer, so check your flash manual first.
My flash is zoomed or bounced. Does the guide number still apply?
No, the basic formula assumes a direct flash with the zoom head in the position the guide number was rated for. Zooming the head concentrates the beam and raises the effective guide number; bouncing off a ceiling or wall loses 1 to 3 stops. In those situations, treat the calculator's answer as a starting point and verify with a test shot or your camera's histogram.
Why doesn't the answer match my TTL flash reading?
TTL metering adjusts the actual flash output up or down from full power, so the camera's exposure reading can differ from the manual guide-number math. The guide number formula describes the flash at full manual power. If you are shooting TTL, think of the calculator as telling you the baseline exposure the flash can reach, and let the camera trim from there.
What is a good guide number for a speedlight?
Typical hot-shoe speedlights land between GN 36 and GN 60 in meters at ISO 100. GN 36 handles family portraits and events in normal rooms; GN 58 to 60 covers large halls and outdoor fill from farther back. Cheap pop-up or phone flashes are often GN 10 to 15 in meters, which is fine for subjects a few feet away but not much more.