How do I determin the guide number of a flash?

I have a flash. It's a generic brand, no markings on it, but it's pretty darn powerful.

How do I determin the guide number of a flash?
To build on Dale's answer (sly old dog), set up the 12 foot target. Start at f16 (or smaller if you have it), and work, shot by shot, moving to the next larger f-stop.





WAIT! Make sure you're shooting at something realistic at 12 ft, not just a blank wall. Use a person (you and a self-timer?). And identify each shot with the f-stop written on a paper.





Shoot through the f-stops and look for the best exposure. Reversing Dale's formula can give you the proper GN for the flash.





PS If it's a really bright flash, make the distance out to 15 feet. you'll just change the math a bit to get the GN. I'm thinking 12 feet maybe too close, but you can test this with a digital camera in manual mode, too.
Reply:It should state the guide number in the manual or paper that came with it and it should be referenced to 100 ISO. You could sort of work backwards to figure it out, though, using the formula





Guide no. divided by distance (in feet), equals the correct aperture. Set your ISO to 100 and shoot at f8 a distance of 12 feet and if the exposure using the flash's manual setting yields a properly exposed image, the guide number is 100. If it is underexposed, it is less than 100 and if it is overexposed, it is over 100.