Fish Weight Estimator

This calculator estimates fish weight two independent ways and reconciles them. The primary method is the classic length-times-girth-squared formula (Weight = Length × Girth² / K), which works because girth squared approximates the fish's cross-sectional area, and multiplying by length approximates its volume — divide by a species-specific constant K (ranging from 800 for trout and catfish to 1200 for bass, reflecting how slender or stocky each species' typical body shape is) and you get a reasonable weight estimate without a scale. The second method uses a length-only power curve (Weight = a × Length^b) fitted per species as a cross-check, useful when you can't measure girth accurately or at all — setting girth to 0 in this calculator switches to this method exclusively.

When both girth and length are provided, the calculator compares the two estimates: if they diverge by more than 50% in either direction, it assumes the girth measurement was likely off (a common field error, since girth is harder to measure precisely than length) and averages the two results for a more realistic figure rather than trusting the girth-based number outright. Results are also shown against a trophy-weight threshold and an approximate all-tackle world record for the selected species, both of which are rough benchmarks, not authoritative records. Because these are body-shape-based estimates rather than direct measurements, actual weight can vary with the fish's fat reserves, whether it's gravid with eggs, and how tightly the tape was pulled — treat the output as a solid catch-and-release estimate, not a certified weight.