Measuring Your Fish

Length Limits

Length limits are an important tool in fish management. Length limits are used in Iowa to protect important predator species such as bass, Walleye, Muskellunge and trout from overharvest.

Length limits are also used to provide larger quality-size fish to catch. Length limits are not the cure-all to better fishing. Improved fishing opportunities exist where anglers obey the law and return fish outside the length limit unharmed to grow and be caught again.

Estimate Fish Weight

Estimate the weight of a released fish with the following formula: Sunfish, Walleye, Northern Pike, Muskellunge and Largemouth and Smallmouth Bass, take the length in inches and multiply it by itself 3 times.

Divide that total by the following number for each species: sunfish, divide by 1,200; Walleye, divide by 2,700; Northern Pike and Muskellunge divide by 3,500; Largemouth and Smallmouth Bass, divide by 1,600; catfish, divide by 3,300.

For example, a 20-inch Largemouth Bass:
First multiply 20 x 20 x 20 (20 x 20 = 400, 20 x 400 = 8,000) then divide 8,000 by 1,600 = 5
You have just released a 5-pound Largemouth Bass!

For trout, multiply the girth (distance around the body) by itself and then multiply that value by the fish’s length and divide by 800.

How to Measure a Fish

Measure total length from the tip of the snout to the end of the tail with the fish laid flat, mouth closed and tail lobes pressed together. See Fishing Seasons and Limits for Paddlefish and Shovelnose Sturgeon measurement.