What is a flash guide number?

A guide number (GN) expresses the light output of a flash as a single figure measured at ISO 100. It links aperture and distance through GN = distance × f-stop, so a larger GN means a more powerful flash that can light a subject farther away or at a smaller aperture.

How do I convert a guide number between metres and feet?

Multiply a metric guide number by 3.28084 to get the imperial value, or divide an imperial guide number by 3.28084 to get metres. Always check whether a manufacturer quotes GN in metres or feet, as the two differ by more than 3×.

Why does the guide number change with ISO?

Guide numbers are quoted at ISO 100. Effective GN scales with the square root of the ISO ratio: doubling ISO multiplies GN by about 1.41 (√2). This calculator applies GN = GN₁₀₀ × √(ISO ÷ 100) automatically.

Does bounce flash change the guide number?

Yes. The published guide number assumes a bare flash firing directly at the subject. Bouncing off a ceiling, adding diffusion, or using a softbox spreads and absorbs light, cutting the effective guide number — often by two stops or more, so you must reopen the aperture or move closer.

Is this calculator accurate for studio strobes?

The GN = distance × f-stop relationship holds for any flash, but studio strobes are usually rated in watt-seconds (joules) rather than guide numbers. If a strobe lists a guide number, this tool works; otherwise meter the light directly for studio work.

Flash Guide Number Calculator

A flash guide number condenses a flash’s power into one figure so you can quickly work out the aperture, distance, or output you need for a correct manual-flash exposure. This calculator solves the guide number equation in any direction and adjusts for ISO.

How it works

The exposure relationship for a direct flash is:

GN = distance × f-stop (aperture)

Guide numbers are quoted at ISO 100, in either metres or feet. Rearranging the equation gives the three things photographers usually need:

Distance = GN ÷ aperture
Aperture = GN ÷ distance
Guide number = distance × aperture (useful for measuring an unknown flash)

Because exposure depends on ISO, the effective guide number scales with the square root of the ISO ratio:

GN(ISO) = GN₁₀₀ × √(ISO ÷ 100)

So doubling the ISO from 100 to 200 multiplies the usable guide number by about 1.41, letting you stop down one extra stop or shoot one stop farther.

Worked example

A speedlight with a guide number of 56 (metres, ISO 100) shooting at f/8:

distance = 56 ÷ 8 = 7 metres. Raise ISO to 400 and the effective GN becomes 56 × √4 = 112, so the same f/8 now reaches 112 ÷ 8 = 14 metres.

Tips and notes

The published guide number is a best case: a bare head, full power, and a clean direct path.
Bounce, diffusion, softboxes and a wide zoom-head setting all reduce the effective GN — expect to open up one to three stops.
Use the metres-to-feet converter shown beside the input so you never mix unit systems by mistake.
All maths runs locally in your browser; nothing about your gear or settings is uploaded.