How to Calculate a Fish’s Weight Without a Scale
By taking some simple measurements of a fish before it is released and employing the formula below, you can get a reasonably accurate weight for the fish. While no formula can ever replace an actual accurate scale, the following calculation will provide an estimate that’s within about 10% of the actual weight.
Fish Weight = (Length * Girth * Girth) / 800
Example:
Length = 20”
Girth = 15”
Weight = (20 * 15 * 15)/800 or
4500/800 = 5.63 pounds
The goal is to take your measurements without placing undue stress on the fish. Keep it in the net and water and NEVER place the fish on dry ground such as rocks or grass as this will remove the protective slime and can allow the fish to be attacked by fungi or bacteria that can kill it.
We strongly recommend you use a cloth or flexible measure tape (like a sewing tape) rather than a metal style. In fact, a simple string will do as well. You can always take the string and lay it against a tape at your car or when you get home.
How to Measure Length and Girth
Find the length (“A”) of the fish. Use as flat a surface as possible but realize that laying the fish on the ground will cause it harm. Take the tip of the tape to the top of the nose of the fish and extend it to the longest tip of the tail when pinched (do not use the lower jaw on male trout with a kype). This will be your length number for the calculation.
Of all the measurements the girth is the most important. To take the girth (“B”) measurement. Find the widest part of the trout - which will usually be in front of or behind the dorsal fin - .and wrap the measuring tape around the fish. This will be your girth number for the calculation.
If you want, you can print off the following chart and carry it when you’re on the water.
