How many times have I landed a nice trout or smallmouth bass and only once I had landed the fish discovered I had left behind the scale? Or, after a long, tiring struggle with a big, but delicate brook trout, not dared to remove him from the water to weigh him up.
There is a way, I discovered, after a contentious conversation with a fishing buddy over the weight of a thick 17" native squaretail that I had released without weighing, a fairly consistent method to estimate the weight of a fish, if for any reason the conditions are not right for a weigh-in.
Some fisherman will call this the girth-squared method, others will call it the "800 Method".
This method is consistent for normal and cylinder shaped fish, it will not work with some very long, slender fish (northern pike and gar come to mind, but I will show you how to adjust for them below.)
You will still need to measure the fish, and what I would recommend you do for the length is what saltwater fisherman would call a "short-length". Measure the fish from the end of the lower jaw to the beginning of the fork in it's tale. This is your length.
Next you will need girth. You can either wrap the tape around the fish at its widest point to attain the girth, or simply measure one side of the fish and double it, then measure its thickness and double that also.
(For instance, might land a 14" smallmouth whose body is 5" deep and laid on his side, 2" thick. To determine girth, you would double the 5" body depth to make 10", then double the 2" thickness to 4" and add it to the 10". The girth of your small mouth is 14" as is his length.)
Now that we have learned a little about length and girth, we'll get to the formula.
The formula is as follows: Length x Girth Squared / 800 = Weight
Now, take the smallmouth bass we discussed above and punch in his dimensions. 14 inch length X 196 inch girth squared (girth multiplied by itself) = 2744
Divide this total by 800. 2744/800 =3.43.
Congratulations intrepid fisherman, you have landed a 3 1/2 pound small mouth. What a fattie!
For more slender fish, (northern pike, gar, muskie) and their cousins, substitute a 900 for the 800, and you will receive a very good estimate of your fish's weight!
