What does the 36 on a flash spec sheet actually tell you? It says how far the flash can light a subject at each aperture. At f/4 and ISO 100, a guide number of 36 in meters reaches exactly 9 meters.
This guide explains the one-line formula behind that number. You will see how ISO, power settings and the zoom head change it. You will also learn why light fades so fast behind your subject.
- Guide number equals flash-to-subject distance times f-number, quoted at ISO 100 in meters or feet.
- Distance equals guide number divided by f-number, so GN 36 at f/5.6 reaches about 6.43 m.
- The guide number grows with the square root of ISO over 100, so ISO 400 doubles it.
- Light falls with the square of distance, so twice as far gets one quarter of the light.
The Number Printed on Every Flash
A guide number, or GN, rates how much light a flash puts on a subject. It is the product of two things: the flash-to-subject distance and the f-number. A Michigan Tech camera guide defines it at ISO 100 in exactly that way.
Because it is a product, one number covers every pairing. GN 36 works as f/4 at 9 m, f/8 at 4.5 m, or f/2.8 at about 12.86 m. Each pair multiplies back to 36.
This works because two squares cancel out. Flash light on the subject falls with the square of distance. Light passing through the lens falls with the square of the f-number. So doubling the distance and doubling the f-number balance each other.
Meters or Feet?
Every guide number carries a hidden unit. A spec sheet may list “36 m” and “118 ft” for the same flash, since 36 x 3.28084 is about 118.1. Always divide by a distance in the same unit as the guide number.
A bigger number means a stronger flash. Twice the guide number reaches twice as far at the same aperture. It also allows an f-number twice as large at the same distance.
How Do You Turn a Guide Number Into a Distance?
Divide the guide number by the f-number you plan to use. The answer is the distance the flash lights correctly at full power.
- Read the rated number. Note the guide number and its unit. Our example flash is GN 36 in meters at ISO 100.
- Adjust for your ISO. Multiply by the square root of ISO divided by 100. At ISO 100 the factor is 1, so GN stays 36.
- Pick the aperture. Choose the f-number your shot needs. Here it is f/5.6.
- Divide. 36 / 5.6 is about 6.43 m, or about 21.1 ft. Place the subject at that distance.
- Or solve for aperture. Divide the guide number by a known distance instead. A subject 6 m away needs 36 / 6, or about f/6.
This matches the example on our tool page. Raise the same flash to ISO 400 and the guide number becomes 72. At f/5.6 it now reaches about 12.86 m. Stop down to f/8 at ISO 100, and the reach shrinks to 4.5 m.
Feet work the same way. Michigan Tech gives a flash of GN 90 in feet lighting a subject 20 feet away. The aperture is 90 / 20, or f/4.5, at ISO 100.
Your aperture choice also sets how much of the scene is sharp. Our guide to how aperture controls depth of field covers that side of the trade.
The Flash Guide Number Calculator takes your guide number, aperture and ISO and returns the working distance in meters and feet.
Why Does ISO Scale the Guide Number by a Square Root?
Doubling ISO doubles the sensor’s response to light, but distance acts through a square. So the guide number rises by the square root of the ISO ratio, not by the ratio itself.
The rule is GN at your ISO = GN at ISO 100 x the square root of (ISO / 100). Each two stops of ISO doubles the guide number. The table shows GN 36 and its reach at f/5.6.
| ISO | Factor | Adjusted GN | Reach at f/5.6 |
|---|---|---|---|
| 50 | 0.71 | 25.5 | 4.55 m |
| 100 | 1.00 | 36.0 | 6.43 m |
| 200 | 1.41 | 50.9 | 9.09 m |
| 400 | 2.00 | 72.0 | 12.86 m |
| 800 | 2.83 | 101.8 | 18.18 m |
| 1600 | 4.00 | 144.0 | 25.71 m |
Michigan Tech gives the same pattern for a GN 80 flash. It becomes about 113 at ISO 200 and 160 at ISO 400. Our guide to the exposure triangle explains the noise cost of higher ISO.
Power Settings Follow the Same Root
Manual flashes cut power in steps such as 1/2, 1/4 and 1/16. Wikipedia’s guide number entry gives reduced GN = full GN x the square root of the power ratio. So GN 36 at 1/4 power acts like GN 18, and at 1/16 power like GN 9.
That gives you a close-up tool. At 1/16 power and f/4, GN 9 lights a subject just 2.25 m away.
Zoom Heads, Bounce and the Inverse Square Falloff
The same flash tube delivers a larger guide number when its beam is narrow. Michigan Tech notes that total flash output is fixed. Spread it wide and it cannot reach as far.
One flash manual quoted by Michigan Tech shows GN 12 m at a 14 mm zoom setting and GN 26 m at 50 mm. At f/4, that is 3 m of reach versus 6.5 m. Our calculator’s notes point out that makers quote the headline number at full zoom, so check which setting it assumes.
How Fast Does Flash Light Fade?
HyperPhysics states the rule plainly: light reaching 1 meter is one fourth as much at 2 meters. That is the inverse square law. Each doubling of distance costs two stops.
Picture a subject 3 m from the flash and a wall 6 m away. The wall gets 25 percent of the subject’s light, two stops darker. A wall 9 m away gets 11.1 percent, about 3.17 stops darker. That is why flash portraits often show a dark background.
Bounce flash changes the geometry. Aim the head at a ceiling 1.2 m above it, with the subject 3 m away at head height. The light now travels about 3.84 m, not 3 m. The ceiling’s color also changes how much light comes back, and our calculator does not model bounce.
Guide Number Slips and Better Habits
Flash misses often trace back to a unit, an ISO, or a zoom setting that does not match the rating. The table pairs each slip with a fix.
| Mistake | Better approach |
|---|---|
| Dividing a feet guide number by meters | Match units. GN 118 ft at f/4 is about 29.5 ft, which equals 9 m. |
| Multiplying GN by the full ISO ratio | Use the square root. ISO 400 doubles GN 36 to 72, not 144. |
| Doubling distance with double the power | Double distance needs four times the light, or two stops wider. |
| Trusting the headline rating with a wide zoom | Check the GN at your zoom setting. It can drop from 26 to 12. |
| Using full-power GN at reduced power | Scale by the square root of the power ratio. At 1/4 power, GN 36 becomes 18. |
A guide number assumes a bare flash in an average room. Light walls add bounce light, and a diffuser takes some away. Treat the result as a starting point, then check the first frame.
Try it with your own flash. Enter its rating, your aperture and ISO in the guide number calculator to see the working distance.
Flash Guide Numbers: Frequently Asked Questions
What Is a Flash Guide Number?
It is a rating of flash power equal to flash-to-subject distance times f-number. Makers usually quote it at ISO 100 and full power, in meters or feet.
How Do You Calculate Flash Distance From a Guide Number?
Divide the guide number by the f-number. GN 36 in meters at f/4 reaches 9 m, and at f/5.6 it reaches about 6.43 m.
How Does ISO Change the Guide Number?
Multiply the guide number by the square root of ISO divided by 100. GN 36 becomes 72 at ISO 400, so its reach at f/4 grows from 9 m to 18 m.
How Do You Convert a Guide Number From Meters to Feet?
Multiply by 3.28084. GN 36 in meters is about GN 118 in feet. The f-number part stays the same, so only the distance unit changes.
Why Does the Guide Number Change With Flash Zoom?
A narrow beam packs the same output into a smaller area, so it reaches farther. One flash manual lists GN 12 m at 14 mm and GN 26 m at 50 mm.
What Guide Number Does a Flash Have at Half Power?
Multiply the full-power guide number by the square root of 1/2, about 0.707. GN 36 at half power acts like GN 25.5, and at quarter power like GN 18.
Why Is the Background Dark in Flash Photos?
Flash light follows the inverse square law. A wall twice as far away as the subject gets one quarter of the light, which is two stops darker.
Does a Guide Number Work for Bounce Flash?
Only roughly. Bounced light travels a longer path, and the ceiling’s color changes how much comes back. Our calculator assumes a bare, direct flash, so check a test frame.
Sources and Further Reading
References Used in This Article
This article explains flash guide number math for learning and manual flash practice. Real output varies with the flash head, modifiers and the room. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 27, 2026.
Author
Shakeel Muzaffar is the Founder and Editor-in-Chief of MultiCalculators.com, bringing over 15 years of experience in digital publishing, product strategy, and online tool development. He leads the platform's editorial vision, ensuring every calculator meets strict standards for accuracy, usability, and real-world value. Shakeel personally oversees content quality, formula verification workflows, and the platform's commitment to publishing tools that are genuinely useful for students, professionals, and everyday users worldwide.
