Free Flash Guide Number Calculator

The guide number rule: GN = aperture × distance. Give it two of the three (plus your ISO) and ShutterMath solves the rest — including scaling the GN when you change sensitivity.

Solve for

Guide number (at ISO 100)

Speedlight GN ≈ 30–60 (m, ISO 100, 50 mm zoom) · built-in ≈ 10–15

Flash distance (m)

Aperture (f/)

ISO

GN scales by √2 per stop: ISO 400 doubles the effective GN.

Result

Required aperture

—

at full power

Effective GN at ISO

—

GN scaled to your ISO

Distance (m)

—

working distance for this GN

💡 Reading: at full power and ISO 100, your flash covers . Open the aperture, raise ISO, or move closer to use less power / get faster recycle.

How guide numbers work

At a given ISO (traditionally 100) and zoom head setting, a flash's guide number is the product of aperture × distance that gives a correct exposure at full power: GN = f-stop × metres.

Same GN, bigger aperture → shorter reach: GN 32 at f/2.8 reaches 11.4 m; at f/8 it reaches 4 m.
ISO scaling: the effective GN multiplies by √2 for each ISO stop — ISO 100 → 400 (2 stops) doubles the GN, so GN 32 @ ISO 100 behaves like GN 64 @ ISO 400.
Power fractions: ½ power reduces GN by √2 (one stop); ¼ power by 2 (two stops).

Flash GN FAQ

What does "GN 60 at ISO 100" actually mean?

It means the flash produces correct exposure at full power when aperture × distance = 60 (metres): e.g. f/2 at 30 m, f/4 at 15 m, f/8 at 7.5 m. Real-world reach is always shorter once ceilings, modifiers, and shutter limits are factored in.

Do softboxes and bounce reduce effective GN?

Yes, a lot. A softbox typically costs 1–2 stops (GN ÷ 1.4–2), bouncing off a ceiling costs 2–4 stops. Add those losses before trusting the calculator for off-camera work.

Why does my camera meter differently than this?

TTL metering measures the actual reflected flash and is usually more accurate for close work. The GN math is the planning tool — it tells you if the flash can reach before you set anything up.