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.
- Enter the Guide Number (GN), in feet or meters, for firing the flash at full power (1/1), at ISO 100.
- Select the VariPower ratio and ISO you want to use.
- Place tick next to Aperture to compute the aperture to use, or a tick next to Distance to compute maximum distance.
- 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.
- Click on «Compute». It will now compute Aperture to set on the camera (if you ticked “Aperture”), or maximum Distance (if you ticked “Distance”).
- Click on «Clear» to clear both Aperture and Distance to recompute (GN remains).
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.