Flash Guide Number Calculator
Flash reach from a guide number — where doubling the distance takes four times the power, and the feet figure is 3.28 times the metric one.
Manufacturers quote guide numbers optimistically: usually at the flash head's longest zoom setting, sometimes in feet, and always into a helpfully reflective room. A real one is often a stop below the printed figure. Take the number on the box as a ceiling to test rather than a promise, and meter if it matters. Nothing is uploaded.
A guide number is a distance times an f-number, and that is all of it
Reach is the guide number over the aperture, so a GN 40 flash at f/8 reaches exactly 5 m. Opening up two stops doubles it, because two stops is a factor of two in the f-number. And check the units before you trust any of it: the same flash carries two guide numbers, and the feet figure is 3.28× larger — a "GN 110" is a GN 34 in metres, which is a much less impressive flash than the box implies.
| Aperture | Reach (GN 40) |
|---|---|
| f/1.4 | 28.57 m |
| f/2 | 20.00 m |
| f/2.8 | 14.29 m |
| f/4 | 10.00 m |
| f/5.6 | 7.14 m |
| f/8 | 5.00 m |
| f/11 | 3.64 m |
| f/16 | 2.50 m |
To double the reach you need four times the power
Light goes as the square of the distance, so distance goes as the square root of the light — and that square root is everywhere here. Doubling the ISO multiplies the reach by 1.414, not two: 5.00 m becomes 7.07 m. Getting to 10.00 m takes ISO 400 — four times the sensitivity, which is two stops. Halving the power is the same arithmetic in reverse: it leaves 70.7% of the reach, not half. Quarter power is what halves it exactly.
| ISO | Guide number | Reach at f/8 | Stops of light |
|---|---|---|---|
| 100 | 40.0 | 5.00 m | 0 |
| 200 | 56.6 | 7.07 m | 1 |
| 400 | 80.0 | 10.00 m | 2 |
| 800 | 113.1 | 14.14 m | 3 |
| 1600 | 160.0 | 20.00 m | 4 |
| 3200 | 226.3 | 28.28 m | 5 |
Turning the power down runs the same way, and it runs out fast: at 1/32 power a GN 40 flash reaches 0.88 m at f/8 — close enough to touch the subject. Each click is a whole stop, and five of them cost you more than four fifths of the distance.
| Power | Guide number | Reach at f/8 | Stops down |
|---|---|---|---|
| Full | 40.00 | 5.00 m | 0 |
| 1/2 | 28.28 | 3.54 m | 1 |
| 1/4 | 20.00 | 2.50 m | 2 |
| 1/8 | 14.14 | 1.77 m | 3 |
| 1/16 | 10.00 | 1.25 m | 4 |
| 1/32 | 7.07 | 0.88 m | 5 |
How to use
- Enter the guide number from the flash specification.
- Enter your aperture and ISO.
- Enter the power fraction you are firing at.
- Read the reach, and how it changes across apertures.
Frequently asked questions
What is a guide number?
A distance multiplied by an f-number, and nothing more. So reach is the guide number divided by the aperture — a GN 40 flash at f/8 reaches exactly 5 metres, and at f/4 reaches 10.
How much further does a higher ISO get me?
Less than you would think. Doubling the ISO multiplies the reach by 1.414, not by two, because light goes as the square of the distance. Getting twice as far takes four times the ISO — two stops.
What does turning the power down cost?
The same square root in reverse. Half power leaves about 71% of the reach, and it is quarter power that halves it exactly. At a thirty-second, five stops down, a GN 40 flash reaches 0.88 m at f/8.
Why is my flash quoted as GN 110 elsewhere?
Because that figure is in feet. The same flash has two guide numbers and the feet one is 3.28 times larger, so a GN 110 is a GN 34 in metres — a much less impressive flash than the box implies.
Can I trust the printed guide number?
Treat it as a ceiling. Manufacturers quote at the flash head longest zoom setting, sometimes in feet, and into a helpfully reflective room. A real one is often a stop below the printed figure, so test it.
How do I work backwards from a distance?
Divide the guide number by the distance and you have the aperture. It is the same equation read a third way, which is why guide numbers survive at all — one multiplication covers every question you can ask of a flash.
Is power the same as ISO here?
Arithmetically, yes. Quarter power and quarter the ISO give the same guide number, because both are simply two stops less light. That equivalence is worth knowing when a flash is already at full power and cannot go further.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.