Fish Weight Calculator
Estimate fish weight by length and species using species-specific formulas and benchmark data.
How the weight estimate works
Fish weight and length follow a power law: W = a × Lb, where W is weight in pounds, L is total length in inches, and a and b are species-specific empirical coefficients. The exponent b typically falls near 3.0, which reflects isometric growth — a fish that doubles in length grows roughly eight times heavier. Species with deep, round bodies (bluegill, crappie) have a b slightly above 3.0; long, slender fish (northern pike, muskellunge) tend to sit slightly below.
L — total length (in)
a, b — species-specific coefficients
Each calculation also produces a rarity score (0–100) based on how that length compares to catch records in state creel surveys and tournament data for the species. A common-size fish scores near zero; a verified state-record-class fish approaches 100. The score is catch-relative, not population-relative — it tells you how unusual your fish is among reported catches, not how many of that size exist in the water.
Life stage and age range are derived from published mean growth curves. Age estimates are highly variable across geographic regions and water quality — the same species reaches 20 inches in 3 years in a productive southern reservoir and may take 7+ years in a cold, nutrient-poor northern lake.
27 supported freshwater species
Coverage spans the most commonly caught and targeted freshwater sport fish in North America. Each species has independently calibrated coefficients; using a generic formula across species can produce errors of 40% or more, which is why species selection matters.
How to measure your catch
This calculator uses total length: tip of the closed mouth to the farthest reach of the tail. For forked-tail species (salmon, walleye, bass), squeeze the tail lobes together before measuring — this is standard for total length and gives a longer reading than fork length, which stops at the fork.
Lay the fish flat on a wet measuring board or ruler. Keep the body straight — a curved fish overestimates length by 2–5%, which translates to a meaningful weight error. For live fish, wet the board or tape to reduce scale and slime damage.
If a regulation or tournament requires fork length (tip of snout to the fork of the tail), that measurement will read shorter than total length. The difference varies by species and tail shape, typically 3–8% shorter. Entering fork length in a total-length calculator will slightly understate the weight estimate.
When girth matters
The length-weight formula assumes average body condition for the species. Fish that are unusually fat or lean fall outside that assumption. An autumn walleye gorging before ice-up, a trophy largemouth from a high-nutrient impoundment, or a well-fed catfish can weigh 20–30% more than the central estimate at a given length. Post-spawn fish running lean may weigh noticeably less.
If you have a girth measurement (circumference at the widest point, typically just ahead of the dorsal fin), a more precise estimate is available:
G — girth circumference (in)
L — total length (in)
The divisor varies slightly by species and body shape (800 is the common default for most bass and walleye; some authorities use 900 for slender fish). This calculator does not require girth, but the ±20% confidence band accounts for normal body-condition variation in the length-only approach.
Accuracy and limitations
±15–25% of the central estimate under normal conditions. The displayed range represents approximately one standard deviation around the median.
Post-spawn fish run 10–20% lean. Pre-winter fish — especially northern pike, walleye, and catfish — run 10–20% heavy. Spring measurements consistently read lighter than fall measurements at the same length.
The same species from a productive, nutrient-rich system will often run heavier than the formula predicts. Oligotrophic, cold, or heavily pressured waters may produce fish that run lighter.
A half-inch error in a 24-inch fish translates to roughly 0.4–0.7 lb of weight error — larger than it sounds when documenting a personal best.
Age ranges are derived from published mean growth rates and can vary by 2–4 years depending on water temperature, food availability, and latitude. Treat them as orientation, not certification.
Where the data come from
Length-weight coefficients are calibrated from state fisheries agency survey reports (Wisconsin DNR, Minnesota DNR, Michigan DNR, Iowa DNR, Montana FWP, Illinois DNR, and others), supplemented by published peer-reviewed fisheries literature. Primary references include:
- Anderson, R. O., and R. M. Neumann (1996). "Length, Weight, and Associated Structural Indices." Fisheries Techniques, 2nd ed. American Fisheries Society.
- Wege, G. J., and R. O. Anderson (1978). "Relative weight (Wr): A new index of condition for largemouth bass." New Approaches to the Management of Small Impoundments.
- Blackwell, B. G., M. L. Brown, and D. W. Willis (2000). "Relative Weight (Wr) Status and Current Use in Fisheries Assessment and Management." Reviews in Fisheries Science 8(1):1–44.
- State DNR creel census records and tournament databases for rarity benchmarks and trophy-class thresholds.
Rarity thresholds are derived from species-specific length frequency distributions in publicly available creel survey datasets, normalized to reflect the probability of a catch exceeding a given length relative to all reported catches of the species.