The actual formula, for salmon and steelhead is:

LENGTH X GIRTH X GIRTH / 775

The length is in inches and weight in pounds.

For example Length = 30", Girth = 20"

30 x 20 x 20 / 775 = 15.48 pounds

I checked Bob's calculator and that appears to be the formula he uses. I've seen some formulas use 800 as the divisor but I have found 775 to be more accurate.