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Flash Guide Number Calculator

Find the correct flash distance from its guide number.

How it is calculated

A flash guide number links flash-to-subject distance and aperture: GN = distance × f-number. The tool rearranges this in two ways. The maximum distance is d = GN ÷ N and, if you also give a distance, the aperture needed is N = GN ÷ d, shown as f/N. GN is the manufacturer's guide number in metres, which the tool treats as given for ISO 100; there is no ISO or zoom setting, so use the figure for your own setting. Guide number and aperture must be above zero. The distance is optional; without it the aperture line stays empty. Results use two decimals.

Worked example

A flash has a guide number of 40 m at ISO 100 and the lens is set to f/5.6. The farthest subject for correct exposure is 40 ÷ 5.6 = 7.14 m. If the subject stands 5 m away, the aperture required is 40 ÷ 5 = f/8.00. A subject closer than 7.14 m at f/5.6 would be overexposed unless the flash power is lowered.

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Common mistakes

  • Using a guide number quoted in feet with a distance in metres. The tool expects metres.
  • Ignoring ISO. The guide number is for ISO 100, so at a higher ISO the real reach is greater than shown.
  • Reading the distance as a minimum. It is the maximum reach at full power for that aperture.
  • Confusing the f-number with the focal length or a shutter speed. Enter only the number after f/, such as 5.6.
  • Trusting a headline guide number measured at the longest zoom setting when the flash head is at a wide angle, where the number is lower.

About this calculator

This calculator uses a flash's guide number (GN) to find the correct flash-to-subject distance for a given aperture (f-stop), or the aperture needed for a given distance, using the relationship GN = aperture × distance (so distance = GN / aperture, and aperture = GN / distance).

Photographers using manual flash units — especially in studio or off-camera lighting setups without TTL metering — use it to get correctly exposed shots without trial and error, since guide numbers are rated for a specific ISO and distance unit (meters or feet) printed by the manufacturer.

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Comments (1)

  • Editorial team

    The guide number is quoted for ISO 100, and the tool does not adjust for ISO. Quadrupling ISO to 400 doubles the effective number, so a GN of 40 behaves like 80: at f/8 that reaches 10 m instead of 5 m. Enter the adjusted figure yourself if you shoot at higher ISO. Share which flash model and ISO you rely on if the reach looks off in practice.

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