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Fish Weight Calculator

Professional-grade fish weight estimator using scientific growth coefficients for 100+ species. Features relative weight analysis, catch logging, and CSV export.

User Rating 0.0 ★★★★★
Total Usage 4 times
Category Sport & Fitness
DB Loaded: 108 Species
Largemouth Bass
in
Normal (100%)
Lean Standard Fat
Measure Total Length (Pinched Tail)
ESTIMATED WEIGHT
0.00 lbs
Standard Ws 100% Wr
Standard

Session Log

0 Fish | 0.00 lbs
Time Species Length Weight Cond. Action
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About

This tool utilizes standard fisheries science allometric growth formulas to estimate the mass of a fish based on length. Unlike generic calculators, this engine processes data using species-specific coefficients (a and b values) derived from peer-reviewed biological surveys (USGS, FishBase). It is designed for conservation-minded anglers who wish to minimize handling time and stress on the catch.

Accuracy depends heavily on the measurement method. The calculator adjusts for Body Condition (seasonal spawning weight vs. post-spawn) and provides a Relative Weight (Wr) analysis, comparing your catch to the statistical average for that length. This provides context: is your 20-inch Bass a standard specimen or a trophy-class heavy?

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Formulas

The core calculation utilizes the power law equation common in fisheries biology. All internal calculations are processed in metric units for precision, then converted.

W = a × Lb × Cf

Where:

  • W = Weight in grams (converted to lbs).
  • L = Total Length in centimeters.
  • a = The intercept of the regression line (shape factor).
  • b = The slope of the regression line (growth rate).
  • Cf = User-defined Condition Factor (0.85 to 1.15).

The Relative Weight (Wr) is calculated to benchmark the catch:

Wr = WWs × 100

Reference Data

SpeciesRef Length (in)Std. Weight (lbs)Growth Type (b)
Largemouth Bass15.01.83Allometric (3.20)
Smallmouth Bass15.01.65Isometric (3.05)
Walleye20.02.75Allometric (3.12)
Muskellunge40.017.9Elongated (3.35)
Northern Pike35.011.8Elongated (3.08)
Rainbow Trout18.02.15Standard (3.00)
Striped Bass30.012.4Allometric (2.98)
Red Drum28.07.90Robust (2.91)
Tarpon60.088.5Robust (2.85)
Atlantic Salmon30.010.5Standard (3.02)

Frequently Asked Questions

Fish of the same length can vary in weight due to season (spawning), food availability, or health. The "Condition" slider allows you to adjust the formula by ±15%. Slide right for "Fat/Gravid" fish, slide left for "Lean/Post-Spawn" fish.
Most freshwater equations (Bass, Walleye, Pike) use "Total Length" (pinching the tail lobes together). Many marine species (Tuna, Jacks) use "Fork Length" (snout to the center fork of the tail). The dynamic diagram in the tool updates to show the correct method for the selected species.
Old paper charts often used a single generic formula for all fish (e.g., L x L x G / 800). This tool uses specific coefficients (a, b) for over 100 species, acknowledging that a 30-inch Pike and a 30-inch Catfish have vastly different body densities.
Yes. All data and your catch log are processed in your browser. No data is sent to a server, and your log persists even if you refresh the page (via LocalStorage).