How to Measure Fish

photo of bass on a measuring board All freshwater Florida Fish and Wildlife Conservation Commission regulations and the "Big Catch" program depend on "total length."

The image below depicts the most commonly used measurements for fish. For freshwater fish, the measurements that you need to use are total length and girth.

Common Measurements

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. The photo to the right shows a bass on a measuring board with the mouth held shut. Prior to getting a final measurement the caudal (tail) fin will be pinched shut.

Lay fish flat
Pinch mouth shut and align with front of tape
Pinch tailfin together to get total length.

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 Measurement

"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.

Gently lift fish up and slide a piece of fishing line or a flexible tape measure under fish.
Lay fish flat with line or tape under deepest part of fish. Wrap it around, fold fins down if needed, line should be perpendicular.
Gently release fish. Remember minimize the fish's time out of water. Lay marked line on tape measure, pull tight, and read girth.

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.

How to release a freshwater fish

Please remember that if you are going to release your catch, it is very important the fish be properly handled and released as quickly as possible. Also don't forget if you catch a quality-sized fish that you can receive a free angler-recognition certificate and sticker from the "Big Catch" program.

(Note: Marine [saltwater] fishes are sometimes measured using other techniques that are also used by freshwater fisheries scientists.