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Get the exact f-stop for manual flash from a GN of 36 in seconds. Distance, aperture and ISO are covered, with the nearest standard stop shown too.

📘 How to Use

  1. Pick whether to solve for aperture or distance under "Solve for"
  2. Enter the guide number, the distance (or aperture) and your ISO
  3. Read the f-stop, the nearest standard stop and the effective guide number

Flash Guide Number Calculator

m

The value on your flash spec sheet (rated at ISO 100)

m
ISO

Shooting ISO. Anything other than 100 auto-adjusts the effective GN

※ Formula: guide number = aperture × distance

※ Effective GN = GN × √(ISO ÷ 100). The GN is treated as rated at ISO 100

Aperture
f/ —
Nearest standard f-stop
f/ —
Effective GN at this ISO
— m
Guide number scaled for the chosen ISO
info

Results are a theoretical guide. Real exposure varies with the flash unit, light spread and reflective surroundings, so confirm with a test shot.

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Flash Guide Number Calculator

Turn a flash guide number and a flash-to-subject distance into the f-stop you need for manual flash. Built on the core relationship guide number = aperture × distance, it solves for either aperture or distance, corrects for ISO, and snaps the result to the nearest standard f-stop you can actually dial in.

💡 Dial in manual flash without guessing

Manual flash gives you repeatable exposures, but only if you know the math. The guide number (GN) describes a flash's maximum output: it is the product of f-stop and distance that yields a correct exposure at ISO 100. A bigger GN reaches further at the same aperture.

Rearrange that one relationship and you get everything you need. Fix the distance and the tool returns aperture = GN ÷ distance; fix the aperture and it returns distance = GN ÷ aperture. Flip the "Solve for" toggle to switch between the two. Because manufacturers rate GN at ISO 100, anything else is scaled to an effective GN = GN × √(ISO ÷ 100), so a GN of 36 used at ISO 800 climbs to roughly 102. The computed aperture is also snapped to the nearest full stop (…5.6, 8, 11…) so you can set it on the lens straight away.

🧐 Frequently Asked Questions

Is the distance from the camera or from the flash? Use the flash-to-subject distance, not camera-to-subject. With an off-camera flash placed to the side, measure from the flash head to the subject.

What if I shoot above ISO 100? Guide numbers are rated at ISO 100. Enter your real ISO and the tool applies effective GN = GN × √(ISO ÷ 100). Each doubling of ISO multiplies the reach by about 1.41.

Why is my real exposure darker than the calculation? Guide numbers assume bare, direct flash. Diffusers, softboxes and ceiling bounce all cut output, so treat the result as a starting point and confirm with a test frame.

Does the calculator handle feet as well as metres? The English version has a ft/m toggle. Switch units and it converts both the guide number and the distance, since the GN itself carries a length unit (GN 36 m ≈ 118 ft).

What is the "nearest standard f-stop"? The closest full-stop aperture (1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32) to the raw computed value. Because stops are logarithmic, the match is made on a log scale rather than by plain subtraction.

📚 Why the square-root rule keeps showing up

Guide numbers behave in a way that trips up beginners: doubling the ISO does not double the GN, it multiplies it by only √2 ≈ 1.41. The reason is that light falls off with the square of distance and the aperture also passes light in proportion to the square of the f-number, so the two square laws meet in a square root.

The same logic governs flash power. Drop from full power to half power and the guide number does not halve — it drops to about 0.71 of its value (√0.5). Quarter power gives roughly half the GN. That is why manual flash scales line up neatly with full stops: every halving of power is exactly one stop of light, which is precisely how photographers think about exposure in the first place.