How this calculator works
Fish weight scales roughly with the cube of length, because a longer fish is also taller and wider. Adding girth improves the estimate by capturing body depth. The general formula multiplies length by girth squared and divides by 800, which comes from treating the fish as a stretched cylinder. Bass are deeper-bodied, so the Wisconsin DNR formula uses length squared times girth divided by 1,200 instead. When only length is available, a species constant replaces the girth term: divide the length cubed by 1,200 for sunfish, 2,700 for walleye or 3,500 for pike and muskie. All three formulas take inches and return pounds, so the page converts metric measurements to inches first and then converts the result to kilograms.
Formula
General: weight in pounds = length in inches x girth in inches squared / 800. Bass: weight in pounds = length in inches squared x girth in inches / 1,200. Length only: weight in pounds = length in inches cubed / species constant, with 1,200 for sunfish, 2,700 for walleye and 3,500 for pike. Kilograms = pounds x 0.45359237.
Worked example
A 50 cm fish with a 40 cm girth is 19.685 inches long and 15.748 inches around. The general formula gives 19.685 x 15.748 squared / 800 = 19.685 x 248 / 800 = 6.10 pounds, which is 2.77 kg. A 70 cm pike measured without a girth is 27.559 inches, and 27.559 cubed / 3,500 = 5.98 pounds, or 2.71 kg.
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Frequently asked questions
Where exactly should I measure girth?
Measure around the widest part of the body, usually just ahead of the dorsal fin. Keep the tape snug but not tight, and do not compress the fish. Measure length from the tip of the snout to the end of the tail with the tail closed.
Why do different species need different constants?
The constants encode body shape. A pike is long and slender, so its length cubed divided by a large constant gives a small weight. A sunfish is deep and round, so the same length gives a larger weight with the smaller constant.
How accurate is the estimate?
For a healthy fish measured carefully, the girth formula is typically within about ten percent. Length-only estimates can be further out because condition varies with season, food supply and spawning state. A certified scale is the only exact measure.
Can I use this for a released fish?
Yes. Keep the fish in the water or in a wet measuring cradle, take the measurements quickly and release it without squeezing. The calculation is only an estimate, so do not delay release to refine it.