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.

What is the Flash Guide Number Calculator?

Free flash guide number calculator. Solves GN = distance × f-stop for any unknown — distance, aperture, or guide number — and scales for ISO. Converts guide numbers between metres and feet. Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Flash Guide Number Calculator

Name: Flash Guide Number Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Guide numbers and what they tell you

A guide number is a compact way to compare flash power across different units. If two speedlights list GN 40 and GN 58 (both in metres at ISO 100), the second delivers roughly (58/40)² ≈ 2.1× as much light — just under one stop more power. This scaling works because guide numbers encode the square-root relationship between output and the f-stop/distance trade-off.

Manufacturers typically quote GN at maximum power and zoom head at the longest tele position. Real-world GN at wide zoom is noticeably lower because the flash spreads its output over a larger area. Always check whether the stated GN is at 50mm, 85mm, or 105mm zoom equivalent to know if you are comparing like for like.

Practical shooting scenarios

Determining where to stand: If your flash has GN 40 (metres, ISO 100) and you want to shoot at f/5.6, position yourself at 40 ÷ 5.6 ≈ 7.1 m. Go closer and you will overexpose unless you stop down or reduce flash power.

Setting aperture for a known distance: Flash unit GN 56, subject 4 m away, ISO 100 — correct aperture is 56 ÷ 4 = f/14. Most photographers would round to f/11 or f/16 and verify with a test shot.

Measuring an unlabelled flash: Set up a controlled target at a known distance, meter the exposure, and multiply: distance × f-stop = your flash’s effective GN. Useful for vintage or unbranded flash units without data sheets.

Matching power across multiple units: If you need two speedlights to deliver equal output at different distances, adjust the power settings until GN × power-fraction is equal at each placement.

Bounce flash and effective guide number

A bare flash firing directly at the subject uses its full guide number. Bouncing off a ceiling or using a modifier scatters and absorbs a significant fraction of the light. A typical bounce off a white ceiling at normal room height costs about two stops — effectively halving the guide number twice, reducing it to about one quarter of the rated value. In practice this means:

GN 56 bare → effective GN approximately 14 when bounced off a white ceiling at 2.5 m
At f/5.6, that reaches only 14 ÷ 5.6 ≈ 2.5 m instead of 10 m

This is why portrait photographers often open to f/2.8 or f/4 for bounce flash in standard-height rooms.