How fish weight is estimated
Anglers commonly estimate a fish’s weight from its measurements so it can be measured, photographed, and released quickly without the stress of a scale. This calculator uses the standard length-girth formula when both measurements are available, or a length-only formula calibrated to typical body shapes when girth isn’t measured.
The formulas used
When girth is known: weight (lb) = length x girth^2 / 800, a formula widely used across freshwater species. When only length is known, weight is estimated as length^3 divided by a species-shape constant (1200 for standard-bodied fish like bass and trout, 2700 for slender fish like pike and musky, and 850 for girthy fish like catfish and carp).
These are estimates only and can vary meaningfully by individual fish condition and season. For catch-and-release fishing, minimize handling time regardless of the estimated weight.