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Guide number calculator

Most data sheets for flash units lists the guide number (GN) for the flash at full power (1/1) for ISO 100 for meters and feet. This calculator let you compute the GN for other varipower settings and other ISO values.

The calculator requires JavaScript. To use the calculator, you need to know the GN at ISO 100 for the flash you are interested in. You will usually find this in the manufacturer's specifications for the flash (usually found on the manufacturer's website). You may also use the DPAnswers flash finder to look up the specifications for the flash. Our product database contains the guide numbers for many popular flash units. Note that for flashes with a zoom head, the GN vary with the position of the head. For comparison purposes, use the GN given for the 35 mm (FX) position.

Detailed instructions:

  1. Enter the Guide Number (GN), in feet or meters, for firing the flash at full power (1/1), at ISO 100.
  2. Select the VariPower ratio and ISO you want to use.
  3. Place tick next to Aperture to compute the aperture to use, or a tick next to Distance to compute maximum distance.
  4. Below this, enter the value (either Aperture to use or Distance from the flash to the main subject) in the field that is not ticked.
  5. Click on «Compute». It will now compute Aperture to set on the camera (if you ticked “Aperture”), or maximum Distance (if you ticked “Distance”).
  6. Click on «Clear» to clear both Aperture and Distance to recompute (GN remains).
Guide Number:
VariPower:
1/1
1/2
1/4
1/8
1/16
1/32
1/64
1/128
ISO:
100
200
400
800
1600
3200
6400
12800
Aperture:Distance:

The calculator works equally well for meters and feet, provided the Guide Number and Distance is given using the same unit (feet or meters) of meaurement.

To learn more about using GNs to determine exposure and maximum reach (distance) of flash, take a look at our manual flash guide.

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