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How to Use a Flash Guide Number
Heshan Fernando
Co-founder & COO
Guide number is a single figure describing how much light a flash produces. From it you can work out the aperture for a given distance, the distance for a given aperture, or how much you need to dial the power down.
It’s the arithmetic behind manual flash, and it rests on one relationship.
Aperture times distance is constant
GN = aperture × distance
Which rearranges to whatever you need:
aperture = GN ÷ distance
distance = GN ÷ aperture
A flash with a guide number of 58 (metres, ISO 100) at 4 metres gives 58 ÷ 4 = f/14.5. Move to 8 metres and it’s f/7.25 — halve the distance, double the aperture number.
That’s the inverse square law in disguise. Double the distance and the light spreads over four times the area, which is two stops — and f-numbers change by √2 per stop, so doubling distance halves the f-number.
Always check the units. Guide numbers are quoted in metres or feet, and the same flash has a guide number roughly 3.3 times larger in feet. Mixing them produces an answer wrong by more than three stops.
Scaling for ISO
Guide numbers are quoted at a reference ISO, almost always 100. At a different ISO:
GN_new = GN_100 × √(ISO_new ÷ 100)
The square root catches people. Four times the ISO doubles the guide number, not quadruples it — because four times the sensitivity is two stops, and each stop changes the f-number by √2.
GN 58 at ISO 100 becomes 116 at ISO 400, and 232 at ISO 1600.
| ISO | Multiplier | GN 58 becomes |
|---|---|---|
| 100 | 1 | 58 |
| 400 | 2 | 116 |
| 800 | 2.83 | 164 |
| 1600 | 4 | 232 |
Why quoted guide numbers are optimistic
Manufacturers quote at the flash’s narrowest zoom setting, where the light is concentrated into the tightest beam. That produces the largest number for the marketing material and a coverage angle suitable only for a long lens.
Set the flash to cover a wide-angle lens and the same energy spreads over a much larger area, giving a substantially lower effective guide number.
Test your own flash. Photograph a grey card at a measured distance at full power, find the aperture that exposes correctly, multiply, and you have your real guide number at that zoom setting. It will be lower than the box says.
Modifiers change everything
Guide number describes a bare flash. Put anything in front of it and the figure no longer applies.
A softbox or umbrella spreads and absorbs light, typically costing one to two stops depending on size, fabric and how much of the light misses the panel. Bouncing off a ceiling costs more, and depends entirely on the ceiling’s height and colour.
There’s no formula for this. Measure the loss for each modifier once — same method as testing the bare flash — and note it on the bag.
Common mistakes to avoid
- Mixing metre and foot guide numbers.
- Scaling ISO linearly instead of by square root.
- Using the quoted number without checking the zoom setting it assumes.
- Applying a bare-flash guide number through a modifier.
- Forgetting that fractional power settings reduce output by stops — half power is one stop, quarter is two.
How to do it with Flash Guide Number Calculator
The Flash Guide Number Calculator solves in any direction with ISO scaling handled.
- Enter the guide number with the ISO and units it’s quoted for.
- Enter whichever two of aperture, distance and power you know.
- Read the third — and check it against the power setting you can actually reach.
- Subtract your measured modifier loss for anything that isn’t bare flash.
Other photography tools are in the tools directory.
Frequently asked questions
Why does guide number scale with the square root of ISO?
Because aperture is a square-root scale too — each stop doubles light while the f-number changes by √2. Four times the ISO is two stops, which doubles the guide number.
Is a manufacturer’s guide number reliable?
Treat it as optimistic. Guide numbers are usually quoted at the flash’s narrowest zoom setting, which concentrates light and inflates the figure. Test your own flash.
Does guide number apply with modifiers?
Not directly. A softbox or umbrella absorbs and spreads light, typically costing one to two stops. Measure the loss for each modifier you use.
Final thought
Test your own flash once and write the real number on it. The figure on the box was measured in conditions you’ll never shoot in.