How to Measure Fish
Total Length Measurement
The total length is the maximum length of the fish, with the mouth closed and the tail fin pinched together. The best way to obtain this length is to push the fish's snout up against a vertical surface with the mouth closed and the fish laying along a tape measure, then pinch the tail fin closed and determine the total length. Do NOT pull a flexible tape measure along the curve of the fish. Prior to getting a final measurement the caudal (tail) fin will be pinched shut.
Conversely, most marine (saltwater regulations) refer to the "fork length", and scientists often use "standard length" which is to the end of the fleshy part of the body. "Standard length" has the advantage of not being affected by minor damage to the tail fin, nor does it give too much credit to a fish for the relatively light weight tail when calculating a fish's condition.
Girth is best measured with a fabric ruler, such as tailors use. It can also be determined by drawing a string around the fish at its widest point marking where the string overlaps and then measuring the distance between the overlapping points on a conventional ruler. The measurement should be taken perpendicular to the length of the fish. This measurement is analogous to measuring the circumference of someone's waist. Knowing the girth is important when trying to certify a fish for a record, and provides useful information to biologists about the relative condition of a fish.
How to estimate a bass' weight
Although it cannot be used to certify an official weight, use of the length and girth can give you a good estimate of a bass' weight. Scientists use a rather complex formula to attain the greatest accuracy. The equation is: Log (weight in grams)= -4.83 + 1.923 x Log (total length in millimeters) + 1.157 x Log (girth in millimeters). A 22" long bass with a girth of 15" weighs about 6.0 pounds using this formula.
Fortunately there are several other easy formulas that you can use, although they are not as accurate, they will give you a rough estimate. A quick, though very rough, estimate of torpedo shaped fish like young bass can be obtained by using: Total Length (in inches)-squared, times girth (in inches) divided by 1200. A 22" long bass with a girth of 15" weighs about 6.1 pounds using this formula.
Another common option used for estimating bass weights is: Girth (in inches)-squared, times length (in inches) divided by 800. A 22" long bass with a girth of 15" weighs about 6.2 pounds using this formula.