Fish Weight

Fish Weight From Length and Girth

This Fish Weight Calculator estimates the weight of a fish from its length and girth using species-specific formulas. It is useful when you catch a fish without a scale. Before your trip, check line capacity with our Fishing Reel Line Capacity Calculator.

The Weight Formula

For most species, weight in pounds equals (Length × Girth²) / K, where K is a species constant. Trout uses K = 900, largemouth bass uses K = 800, and northern pike uses a cubic formula: Length³ / 2700. If girth is unknown, it is estimated as length × 0.58.

Measuring Length and Girth

Measure fish length as a straight line from snout to the start of the tail fin. Girth is the maximum circumference around the body, not the vertical body depth. Accurate measurements improve weight estimates significantly.