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Photography Calculators

Flash Guide Number Calculator

Verified formula Updated Jun 2026 Private — runs on your device

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Verified formula Private

Aperture (f-number)

8.0

For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.

How to use the Flash Guide Number Calculator

The Flash Guide Number Calculator works out your aperture (f-number) in an instant. Enter guide number (gn) and distance to subject and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.

  1. Enter the guide number (gn).
  2. Enter the distance to subject.
  3. Read off your aperture (f-number) — the calculator updates automatically, with no button to press.

Formula

The Flash Guide Number Calculator uses the formula:

Aperture (f-number) = Guide number (GN) ÷ Distance to subject

Worked example

For example, with guide number (gn) of 40 and distance to subject of 5 m, the aperture (f-number) is 8.0.

Inputs used
Guide number (GN) 40
Distance to subject 5 m
Results
Aperture (f-number) 8.0

Results are estimates for educational use, not professional advice.

Frequently asked questions

Divide the guide number by the distance to get the aperture. A GN of 40 at 5 m calls for f/8.

It measures a flash's power for a given ISO, usually quoted in metres at ISO 100. Higher means more powerful.

Farther subjects need a wider aperture (smaller f-number) to gather the dimmer flash light. The GN ÷ distance rule handles this.

Yes. Guide numbers are quoted at a base ISO. Raising ISO effectively increases the guide number, allowing smaller apertures.

The Flash Guide Number Calculator uses the formula: Aperture (f-number) = Guide number (GN) ÷ Distance to subject. For example, with guide number (gn) of 40 and distance to subject of 5 m, the aperture (f-number) is 8.0.

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