Free Flash Guide Number Calculator
Guide number is the measure of a flash's power: the distance at which it delivers correct exposure at f/1.0, ISO 100. 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.
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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).
Related tools & guides
Exposure Calculator
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Open →Background Blur Calculator
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Open →Aperture Explained
What f-stops really mean for light and depth.
Open →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.
Guide number chart: how far the flash reaches
Short answer: distance = guide number divided by f-number. A flash with GN 60 m at ISO 100 reaches 7.5 m at f/8 and 21.4 m at f/2.8. Double the ISO and the guide number rises by 1.4 times.
| Flash | f/2.8 | f/4 | f/5.6 | f/8 |
|---|---|---|---|---|
| GN 30 m | 10.7 m | 7.5 m | 5.4 m | 3.8 m |
| GN 40 m | 14.3 m | 10.0 m | 7.1 m | 5.0 m |
| GN 50 m | 17.9 m | 12.5 m | 8.9 m | 6.2 m |
| GN 60 m | 21.4 m | 15.0 m | 10.7 m | 7.5 m |
Guide numbers are quoted at ISO 100 and a specific zoom position, usually the widest the head covers. Zooming the head concentrates the light and raises the effective guide number; a diffuser spreads it and cuts it by roughly a stop. Bounce flash loses two to three stops because the ceiling is a poor reflector.
At ISO 400 the guide number doubles, so the same flash reaches twice as far at the same aperture. That is why high ISO plus a small aperture is the usual way to light a large room from one speedlight.