For smallmouth I found girth X girth X length divided by 800, so unless you got the girth it's anybody's guess. I carry a piece of string in my pocket. It's much simpler and more accurate than a cloth tape measure. You are supposed to measure the girth at the largest part of the fish, which on a smallmouth is usually about 2/5 of the way back on the dorsal fin vertically around the belly. Since you have to mash down the dorsal fin to take the measurement, I just measure around from just in front of the dorsal fin and just slightly angle back around in back of the pectoral fins. Since I throw all bass back I don't want to handle the fish any more than necessary. I get the same results unless it is an unusually "footballish" like fish. Good luck.