- 1). Measure the girth of the fish with a flexible tape. The girth is defined as largest distance around the body of the fish and is usually near the dorsal fin. Note this measurement in inches.
- 2). Measure the length of the fish. Use the flexible tape to measure the distance from the center of the tail to the point of the nose. The exception is billfish. Take that measurement from the center of the tail to the end of the bottom jaw. Note this measurement in inches.
- 3). Square the length of the girth by multiplying it by itself. For example, multiply a girth of 20 inches by 20 to yield a value of 400. Multiply the value from the previous step times the length. Continuing our example with a length of 100 inches, you would multiply 400 by 100 for a value of 40,000. Dividing this value by 800 yields the weight of the fish. In our example, the estimated weight of the fish is 50 pounds. Divide the weight in pounds by 2.2 to determine the estimated weight in kilograms.
Source...