About the Fish Weight Calculator
This calculator estimates how much a fish weighs from a few tape-measure readings, so you can record a catch and release it quickly without hauling out a scale. Anglers, tournament competitors, and biologists have used length-and-girth formulas for decades because they are accurate enough for records and catch logs while keeping the fish out of the water for as little time as possible.
The girth-length formula
The standard, widely used estimate is Weight (lb) = Girth² × Length ÷ 800, where both girth and length are measured in inches. Girth is the distance all the way around the fattest part of the fish's body, and length is measured from the tip of the snout to the fork or tip of the tail. Because a fish is roughly a cylinder, its volume — and therefore its weight — scales with the square of its girth times its length, which is exactly what this formula captures. For example, a fish 20 inches long with a 12-inch girth estimates to 12² × 20 ÷ 800 = 2,880 ÷ 800 = 3.6 lb.
Length-only estimates by species
When you can't measure girth, weight can be estimated from length alone using Weight (lb) = Length³ ÷ shape factor, where the shape factor depends on how deep- or slender-bodied the species is. Common factors are roughly 800 for deep-bodied panfish, sunfish and crappie; about 1,050 for trout and walleye; about 1,200 for largemouth and smallmouth bass; and about 900 for long, slender fish like pike and muskie. A 20-inch bass estimates to 20³ ÷ 1,200 = 8,000 ÷ 1,200 ≈ 6.7 lb. Length-only numbers are rougher because they assume an average girth for the species — a fat, pre-spawn or well-fed fish will out-weigh the estimate.
Why girth matters so much
Two fish of identical length can differ 30 percent or more in weight depending on body condition, and the girth-length formula captures that because weight rises with the square of girth. A largemouth bass full of eggs in spring, or a trout that has been gorging on a heavy hatch, carries far more mass than its length suggests. Whenever accuracy matters — a personal-best, a tournament weigh-in estimate, or a fisheries record — measure girth.
Converting to kilograms
To convert the pound estimate to metric, multiply by 0.453592. So the 3.6 lb example above is 3.6 × 0.453592 ≈ 1.63 kg. The calculator shows both units automatically.