A released fish cannot step on a scale — but it can be measured in ten seconds, and two measurements carry its weight. This Fish Weight Calculator applies the standard length-and-girth formulas used by fisheries agencies and tournament circuits to estimate weight for bass, trout and walleye, so a catch can be weighed, recorded and released in under a minute.
The Formula and the Worked Example
The standard estimator is W = (L × G²) / K: length times girth squared, divided by a species constant — 800 for largemouth bass, 750 for walleye, 900 for trout in the widely published tables. A bass of 20 inches with a 15-inch girth: 20 × 225 / 800 = 5.63 lb — a fine fish, honestly estimated. The formula’s shape is its own explanation: girth is squared because a fish is roughly a tapered cylinder, and its cross-sectional area (which girth measures, via circumference) times its length is its volume; the constant absorbs the average taper, the density of fish flesh (close to water) and the unit conversions into one divisor calibrated per body plan. Deep, compressed bodies (bass) carry more volume per length than torpedoes (trout) — hence the bass’s smaller divisor (÷800 divides less, weighs more per measurement) and the trout’s larger one. A length-only fallback exists for fish you could not girth — bass run near W ≈ L³ / 1600 in the roughest tables (a 20-incher: 8,000 / 1600 = 5.0 lb) — but the girth measurement is where the accuracy lives, because two 20-inch bass, one spawned-out and hollow, one pre-spawn and bursting, are different weights wearing the same length, and the tape around the middle is what tells them apart.
How Accurate Is It — and What Moves the Number
Against actual scales, the length-girth formulas land within roughly ±10% for typical fish in typical condition — the studies behind the published constants show most individuals inside that band, with the tails belonging to the tails: egg-laden females before the spawn (estimates run low, the fish is heavier), post-spawn and late-winter fish (estimates run high), and the extremes of each population. Anglers can tighten the estimate with two disciplines. Measure the girth at the widest point — usually just ahead of the dorsal fin — with the tape snug but not compressing; an inch of girth error at these sizes moves the weight by about 13% (the square again), so the girth tape deserves more care than the length board. Measure length the way the formula expects: total length, jaw closed, tail pinched to its longest point, on a flat board rather than along the body’s curve — a curved tape over the flank adds phantom inches. The calculator prints the estimate to two decimals out of arithmetic politeness; report it to the nearest quarter pound in the logbook and your records will be as honest as the method allows. For tournament and record purposes the formula never replaces a certified scale — its kingdom is the other 99% of fish: the released, the photographed, the ones whose weight is a story rather than a claim, and a principled estimate serves a story better than a guess inflated by the telling.
Measure, Photograph, Release: The Workflow This Page Serves
The formulas exist because catch-and-release needed a scale that works on a living fish in a wet hand, and the workflow is worth stating in the order that protects the fish. Keep the fish in the water or on a wet measuring board; support it horizontally with two hands (a big fish hung vertically by the jaw can be injured by its own weight — happily, the very quantity being estimated). Take length on the board, girth with a soft tape or a piece of line marked against a ruler later, a photograph for the record, and the fish goes back — total air time under a minute, and the calculator can be run on the bank afterward from two numbers in a phone note; it does not need the fish present, which is rather the point. Logbooks kept this way (length, girth, estimate, lure, conditions) become genuinely useful data across seasons: the estimates share one method, so their comparisons are trustworthy even where an individual figure carries its ±10%. And the practice has a conservation dividend beyond the released fish: length data shared with fisheries programmes feeds the growth curves from which the next generation of constants — and the slot limits that keep a fishery producing 20-inch bass — are computed. The tape measure, in the end, is a scale the fish survives.
A Length-Only Bass Table, and the Pike Variant
For the fish that allowed a length but not a girth — the quick board measurement before release — the bass’s L³/1600 fallback makes a pocket table worth memorizing: a 12-inch bass ≈ 1.08 lb, 15 inches ≈ 2.11, 18 ≈ 3.65, 22 ≈ 6.66, and a 25-inch fish ≈ 9.77 lb. The cubes punish optimism about small differences: an inch at the 20-inch class is worth about three-quarters of a pound, which is the honest scale of the one-that-grew-in-the-telling. Long, lean predators take a different length-only form in the published tables — pike and muskie run near W = L³/3500: a 36-inch pike estimates to 46,656/3500 ≈ 13.3 lb — the larger divisor pricing the torpedo’s slimmer cross-section, the same body-plan logic as the girth constants in reverse. Keep the variants straight by remembering what each prices: girth formulas price the individual fish in front of you, length-only formulas price the average fish of that length in that species — the first is an estimate of a fish, the second an estimate of a statistic, and the logbook deserves to know which one it received.
Condition, Priced: Relative Weight
Fisheries science turns this page’s estimate into a health index for the fish itself. Relative weight (Wr) compares a fish’s actual (or estimated) weight with the published standard weight for its length — the weight of a bass in good condition in a good fishery. A 20-inch largemouth’s standard weight is about 5.0 lb — the same figure the length-only table gives — so the worked example’s plump 5.63 lb fish scores Wr = 5.63 / 5.0 × 100 ≈ 113: a fish carrying 13% more than standard, the signature of a forage-rich system or a female near the spawn. A bass scoring under 90 is thin — overcrowded, underfed, or post-spawn — and a fishery whose surveys average below the mid-90s is telling its managers something about balance. The angler’s version of the same instrument needs nothing but this calculator and the length table: estimate with girth, compare with the length-only standard, and the ratio grades the fish in your hand as honestly as the agency’s spreadsheet does. Log the Wr beside the weight and a season’s worth of released fish becomes a condition survey of your own water.
Common Mistakes
Measuring girth at the tail or behind the widest point, or pulling the tape tight enough to compress the fish (girth is squared — its errors are doubled in effect). Measuring length along the body’s curve instead of flat. Using another species’ constant (a trout run through the bass formula gains phantom weight; the bodies differ, which is the whole reason for the table). Reporting the estimate to the hundredth in a record claim — certified scales settle records; formulas settle stories. And weighing by jaw-hanging a large fish for a photograph when a supported cradle or the tape would do: the estimate exists so the fish does not have to pay for the number.
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Frequently Asked Questions
How do I estimate a fish’s weight without a scale?
Use W = (length × girth²) / K, with K = 800 for largemouth bass, 750 for walleye, 900 for trout. A 20-inch bass with a 15-inch girth: 20 × 225 / 800 ≈ 5.63 lb. Accuracy is about ±10% for fish in normal condition.
Why is girth squared in the formula?
Because weight follows volume, and a fish’s volume is its length times its cross-sectional area — the area grows with the square of the girth. That is also why girth must be measured carefully: an inch of girth error moves the estimate by roughly 13%.
Where do I measure the girth?
Around the widest part of the body, usually just in front of the dorsal fin, with a soft tape snug but not squeezing. On a deep fish the widest point is unmistakable; measure twice if the fish allows it.
Can I estimate from length alone?
Roughly — bass run near L³ / 1600 (a 20-inch fish ≈ 5.0 lb) — but condition hides in the girth: a pre-spawn and a post-spawn fish of the same length differ by a pound or more, and length alone cannot see it. The two-measurement formula is worth the extra five seconds.
Why do the species constants differ?
Body shape. The constant converts a cylinder-ish volume into a real fish: deep-bodied bass pack more volume per length (÷800) than streamlined trout (÷900). Using the wrong species’ constant systematically over- or under-estimates by the shape difference.
Is a formula estimate valid for records?
No — records and tournaments require certified scales. The formula’s job is the released fish: a principled ±10% estimate for the logbook, the photograph caption and the story, obtained without keeping the fish out of water.