Fish Weight Calculator (Length & Girth)

How fish weight formulas work

Weight tracks volume, and volume for a roughly tube-shaped animal is close to cross-sectional area times length. Girth is the perimeter of that cross-section. That is the whole idea behind the length-and-girth family: length² × girth and length × girth² are both crude stand-ins for the volume of a fish-shaped solid, with a divisor that converts cubic inches into pounds and absorbs the difference between a fish and a tube.

The formulas this calculator uses, with the agency that publishes each (lengths in inches, results in pounds):

• Bass, with girth: length² × girth ÷ 1,200 — Wisconsin DNR publishes this as "bass weight = (length x length x girth) / 1,200". Florida FWC publishes the same expression as its "typically the most accurate estimate" for black bass.
• Trout, with girth: length × girth² ÷ 800 — Wisconsin DNR publishes this as "trout weight = (length x girth x girth) / 800". Florida FWC publishes the identical expression for full-bodied bass and, unusually, names its origin: "Rough estimate for bass, per Outdoor Life and Hal Schramm."
• Length only: length³ ÷ C. Wisconsin DNR publishes three of these: walleye ÷ 2,700, northern pike ÷ 3,500, and sunfish ÷ 1,200. The divisors this calculator uses for musky, catfish, crappie and striped bass are not agency-published — they are general heuristics, and every result on this page labels them as such rather than dressing them up as a source.

Where an agency girth formula exists, the calculator uses it as soon as you enter a girth. Otherwise it falls back to the length-only constant, and the result panel names the formula and the source that produced it.

Where these formulas actually came from

This is worth being honest about, because almost nobody is. Wisconsin DNR publishes its five formulas on a public page with no derivation, no sample size, and no error term. Florida FWC publishes three formulas and credits exactly one of them to a named person and a magazine. Nothing we could find from a state agency, an extension service or the American Fisheries Society documents where the length³ divisors originated, how many fish they were fitted to, or what water they came from.

So treat them as what they are: rules of thumb that agencies find good enough to hand to anglers, not calibrated instruments. The genuinely scientific length-weight framework is the standard-weight work further down this page, and it is a different tool answering a different question.

A worked example you can check

Say you land a largemouth, tape it at 20 inches, and get 14 inches around the shoulders. Here is every step, no rounding until the end.

• Wisconsin DNR bass formula. 20 × 20 = 400. 400 × 14 = 5,600. 5,600 ÷ 1,200 = 4.67 lb (4 lb 11 oz).
• Florida FWC "torpedo-shaped" variant. Same 5,600, divided by 1,000 = 5.60 lb.
• Florida FWC "full-bodied" variant. 14 × 14 = 196. 196 × 20 = 3,920. 3,920 ÷ 800 = 4.90 lb.
• Length only, no girth. 20 × 20 × 20 = 8,000. 8,000 ÷ 1,600 = 5.00 lb.
• Standard weight for a 20-inch bass. The published Ws curve gives 4.69 lb, and Florida FWC's own printed condition table lists 4 lb 11 oz at 20 inches — the same number.

Five published routes, one fish, answers from 4.67 to 5.60 lb — a spread of nearly a pound, about 20%. None of the five is wrong. They are different approximations fitted to different fish, and FWC says so on its own page: the estimates "will be less predictive for disproportionally shaped larger bass." The practical reading is that you caught a four-and-a-half to five-and-a-half pound bass. Quoting "4.67 lb" to two decimals implies a precision that does not exist.

Where this goes wrong, with numbers

1. A half inch costs more than you think

These are power laws, so errors multiply rather than add. On length² × girth, a 1% error in length moves the answer about 2%; a 1% error in girth moves it about 1%. On the length³ formulas, a 1% length error moves it about 3%. On length × girth², a 1% girth error moves it about 2%.

Back to the 20 × 14 bass, using the Wisconsin formula:

Half an inch is a wet fish on a bump board, a tail that is not quite squeezed, or a tape that rides up over the dorsal. It is not sloppiness; it is normal. It is also 6.6 ounces on a five-pound fish.

2. Total length, fork length and standard length are three different numbers

Every formula on this page expects total length. There are three standard ways to measure a fish, and University of California ANR defines them like this:

• Total length (TL): "the distance from the tip of the snout or lower jaw to the end of the longest ray or longest point formed when the lobes of the caudal fin are brought together."
• Fork length (FL): "the distance from the tip of the snout or lower jaw to the middle of the fork of the caudal fin (whichever juts out further)."
• Standard length (SL): "the distance from the tip of the snout or lower jaw to the end of the vertebral column."

On any fish with a forked or notched tail — bass, crappie, trout, walleye — FL is shorter than TL, and SL shorter still. South Carolina DNR is explicit that for total length the "tail should be pinched," meaning the lobes squeezed into a point. A fanned tail reads short; a fish measured to the fork reads shorter again.

Because of the exponents above, every 1% you shave off the length quietly removes about 2% from a length²-girth estimate, 3% from a length³ estimate, and 3.3% from the bass standard-weight curve. Nothing warns you. The number just comes out light, consistently, on every fish you measure that way.

3. A second agency disagrees with these divisors

North Dakota Game and Fish publishes a length-weight table of "the average weights of select fish statewide." Comparing it against the length-only divisors this calculator uses is uncomfortable, so here it is:

Wisconsin's walleye divisor and the North Dakota table agree to within a rounding error. The pike and catfish gaps are large, and the crappie gap runs the other way. North Dakota's own caveat explains part of it: "The true weight of an individual fish may vary due to the sex of the fish, time of year (e.g. spawning) when it is caught, health of the fish and the water body from which it is caught." A pike from a cold prairie lake is a different animal from one in a fertile southern reservoir, and no single divisor covers both.

We left the constants alone rather than quietly retuning them, because changing a published Wisconsin DNR figure to a number we averaged ourselves would make the source label a lie. Take it as a regional correction instead: on the northern plains, expect this calculator to read heavy on pike and catfish and light on crappie.

Quick length-to-weight reference (length-only formulas)

Computed exactly from the length-only constants above. Adding girth gives a better estimate for bass and trout. Individual fish vary; see the North Dakota comparison above.

Relative weight: what pond owners actually use

Estimating weight from length answers "how big was it." Relative weight answers a more useful question for anyone who owns the water: is this fish fat or thin for its length, and what does that say about the pond?

Fisheries biologists built standard-weight (Ws) curves for common species from large samples across each species' range. Florida FWC describes the method plainly: "scientists first come up with a standard weight based on averages from thousands of measurements of fish collected throughout their geographic range. They then divide the actual weight of a fish by the standard value found from the averages and multiply it by 100."

The curves this page uses, with total length in millimeters and standard weight in grams:

• Largemouth bass: log₁₀(Ws) = −5.528 + 3.273 × log₁₀(TL), applicable from 150 mm (about 6 in) up.
• Bluegill: log₁₀(Ws) = −5.374 + 3.316 × log₁₀(TL), applicable from 80 mm (about 3 in) up.

Note that both exponents are above 3. That is not a rounding artefact — it means these fish get proportionally heavier as they get longer, rather than simply scaling up. A bass is a different shape at 20 inches than at 10. This is exactly why a single length³ divisor cannot be right across a whole size range, and why the Ws curve and the shortcut formulas drift apart at the extremes.

We checked the equations against published tables rather than trusting the constants. Georgia's Department of Natural Resources gives a standard weight of 401 g for a 12-inch largemouth; the equation above returns 400.1 g. Florida FWC's printed condition table gives 0 lb 14 oz at 12 inches, 4 lb 11 oz at 20 and 8 lb 8 oz at 24; the equation returns 0.88, 4.69 and 8.53 lb. Every row matches. On the bluegill side, FWC's table gives 0 lb 14 oz at 10 inches and 1 lb 10 oz at 12; the equation returns 0.88 and 1.61 lb.

The part almost every page gets wrong: Ws is not the average fish

This is the single most useful thing on this page, and it is missing from nearly every fish-weight calculator on the internet.

Standard weight equations are not fitted to the average fish. They are built with the regression-line-percentile (RLP) technique, and the whole point of that technique is stated directly in the fisheries literature: "For each length interval, the 75th percentile was then calculated from the predicted weights of all the populations in the data set." The RLP method "is currently favored for developing Ws equations because it weights each population equally and produces Wr estimates of low variance and free of length bias."

In other words, Ws is deliberately set near the upper quartile of surveyed populations. A relative weight of 100 does not mean "average fish." It means "as heavy for its length as fish in the better three-quarters of the populations biologists sampled."

That reframes the whole scale. A Wr of 90 is not a fish that is 10% underweight; it is a fish 10% below a deliberately high benchmark, which is why Alabama Extension (ANR-1193) says relative weights "between .8 and 1, while not ideal, are well within the range found in healthy populations." Extension does treat under 80 as a real problem — "Fish that have a relative weight less than 0.80 or 80 percent of the standard are considered severely thin" — and notes that "in balanced ponds, the relative weights for most fish will be greater than 0.9." Georgia DNR gives the same practical band: "In a well-managed body of water, most fish have a relative weight between 90 and 100."

So do not panic at 92. Panic at 78.

Standard weight by length

Computed from the equations above. If your fish weighs more than the figure in this table, its Wr is over 100.

Measurement error hits Wr harder than it hits weight

The bass Ws exponent is 3.273, so a 1% length error changes the standard weight by about 3.3% — and because your measured weight does not move, the entire error lands on the Wr.

A worked case. You weigh a bass at 1 lb 4 oz and tape it at 14.0 inches. Ws at 14 inches is 1.46 lb, so Wr = 1.25 ÷ 1.46 × 100 = 86: below average, nothing alarming. Now suppose the tail was fanned and the fish was really 14.5 inches. Ws at 14.5 is 1.64 lb, and Wr = 1.25 ÷ 1.64 × 100 = 76 — across the line Alabama Extension calls severely thin.

Half an inch flipped the management conclusion. Measure to the quarter inch, pinch the tail, and never read anything into a single fish.

Reading the pond from the numbers

A single fish tells you almost nothing. The value comes from weighing a dozen or more across a spread of sizes and reading the pattern between the two species. Alabama Extension advises owners to "measure the relative weight of bass and sunfish for as many sizes as possible throughout the season," and publishes this interpretation matrix:

Two of those rows come with extra detail. For the bass-crowded pond: "In bass-crowded ponds, the bass will have low relative weights with bass between 10 and 14 inches typically being very thin." For the other imbalance: "In ponds with too many bluegills (bluegill crowded), the bass will usually have high relative weight with bluegill in poor condition."

Only one combination — both species high — reads as well managed. Fat bass on their own are not a clean bill of health; paired with thin bream they point at a bluegill problem, not a success. And if both species come in low, the question is pond-wide rather than predator-prey. That is a conversation for your state extension fisheries specialist, who will want to see your numbers first.

Working out what your pond can support in the first place is the other half of this. Our pond calculator covers volume in gallons and acre-feet, surface acres, stocking numbers, and liming.

Why the same fish weighs different amounts in different months

Condition is seasonal, and for females the swing is mostly eggs. Michigan DNR's fecundity study of largemouth bass (Fisheries Research Report 1931) measured ovaries as a share of total female weight and found them running 6.6% to 10.9% in normal years — and in one poor year, "Their ovaries were a smaller proportion of the total female weight than in other years — 4.7% compared to 6.6-10.9%."

Read that as a weight swing. A pre-spawn female carrying a full ovary mass is roughly a tenth heavier than the same fish after she drops them. On a 20-inch bass with a standard weight of 4.69 lb, that is around half a pound moving in and out over a few weeks, independent of how well she has been feeding — and about ten points of Wr, the width of an entire interpretation band.

This is why biologists sample at a consistent time of year. Compare your pond to itself, in the same season, year over year.

What this calculator does not know

The inputs are two tape measurements and a species name. Here is everything they leave out:

• Sex, and where the fish is in the spawn. Worth roughly 10% on an adult female.
• What it ate this morning. A bass that just swallowed a six-inch gizzard shad is carrying that shad's weight and none of its length.
• What water it came from. Fertility, forage base and growing season all shift a population's length-weight relationship. The North Dakota table above is the whole argument.
• Which length you measured. The calculator assumes total length and cannot tell.
• Whether the species is one of the published ones. Musky, catfish, crappie and striped bass fall back on unsourced heuristics, and the result panel says so.
• Saltwater fish. Similar formulas exist for marine species, but the divisors differ and none of ours were fitted to them.
• Whether the fish is unusual. Giants, runts, deformities and heavily post-spawn fish are exactly where the formulas perform worst — FWC says as much about "disproportionally shaped larger bass."

When a scale is the only honest answer

Three cases where no formula will do. Records: every state record program requires a certified scale, generally witnessed. Relative weight: feed Wr a length-based estimate and you have compared one formula against another, which returns near the same answer for every fish you catch. Any decision you plan to act on: if you are about to start harvesting bass out of your own pond on these numbers, weigh them.

The other direction matters too. Wisconsin DNR's stated reason for publishing formulas at all is that fish "are sometimes damaged when weighed." If you are releasing the fish and the number is only for a photograph, two tape measurements are the kinder trade. Measure over water or a wet mat, keep the fish horizontal and supported, and get it back.

Frequently asked questions

Why is my estimate different from the scale? Condition, mostly. A pre-spawn female full of eggs outweighs the formula by roughly a tenth; a skinny post-spawn fish comes in under. Girth-based estimates track condition much better than length-only ones, which is why the calculator prefers them when you supply a girth.

Which of the three bass formulas should I use? The one your state publishes, if it publishes one, so your number is comparable with everyone else's in the same water. Failing that, ÷1,200 with girth — both Wisconsin DNR and Florida FWC point to it, and FWC calls it "typically the most accurate estimate."

Do these work for saltwater fish? Similar formulas exist for many saltwater species, but the divisors differ and ours were not fitted to them. The species list here is freshwater.

What is a good relative weight for largemouth bass? In a balanced pond most fall between 90 and 100. Under 80 is severely thin. Consistently low Wr in the 10-to-14-inch bass you catch, alongside fat bluegill, is the classic bass-crowded pond, and the usual answer is to harvest small bass.

Why does my fish's Wr change through the year? Eggs, mainly. Pre-spawn females read high and the same fish post-spawn reads well below — enough to cross a band boundary. Compare your pond to itself in the same season rather than to a number from a different month.

Can I claim a record with a formula estimate? No. Records require a certified scale. This is for bragging rights and quick releases.

How do you estimate a fish's weight from its length? Fisheries agencies publish length–weight formulas. Wisconsin DNR's bass formula is length² × girth ÷ 1,200 and its trout formula is length × girth² ÷ 800. Without girth, species-specific length³ formulas are used, such as walleye ≈ length³ ÷ 2,700, northern pike ≈ length³ ÷ 3,500 and sunfish ≈ length³ ÷ 1,200. Lengths are in inches, results in pounds.

Why do two published formulas give different weights for the same fish? Because they are different approximations of body shape, fitted to different fish. On a 20-inch bass with a 14-inch girth, Florida FWC's three published formulas give 4.67 lb (TL × TL × G ÷ 1,200), 5.60 lb (TL × TL × G ÷ 1,000) and 4.90 lb (TL × G × G ÷ 800). That is a spread of nearly a pound on the same fish. FWC itself notes the estimates 'will be less predictive for disproportionally shaped larger bass.'

How much does a small measurement error change the estimated weight? More than people expect, because the formulas are power laws. On length² × girth ÷ 1,200, a 1 percent length error moves the answer about 2 percent and a 1 percent girth error moves it about 1 percent. On a 20-inch, 14-inch-girth bass, reading a half inch long on both tape measurements adds about 8.8 percent — roughly 6.6 ounces on a 4.7 pound fish.

What is relative weight (Wr), and why isn't 100 the average fish? Relative weight is a fish's measured weight divided by the standard weight for its length, times 100. Standard weight equations are built with the regression-line-percentile technique, in which 'for each length interval, the 75th percentile was then calculated from the predicted weights of all the populations in the data set.' So Ws is deliberately set near the upper quartile of surveyed populations, not the middle. A Wr of 90 is not a fish 10 percent below average; it is a fish 10 percent below a high-end benchmark, which is why Alabama Extension calls 80 to 100 'well within the range found in healthy populations.'

Can I get relative weight without a scale? No. Relative weight compares a measured weight against a length-based standard. If you substitute a length-based estimate for the measured weight, you have compared one length formula against another and the answer comes out roughly the same for every fish you catch, regardless of condition. A cheap digital hanging scale is the whole tool.

Sources

• Wisconsin Department of Natural Resources — Estimating fish weight (bass ÷1,200, trout ÷800, pike ÷3,500, walleye ÷2,700, sunfish ÷1,200; note on fish being damaged when weighed).
• Florida Fish and Wildlife Conservation Commission — Bass Weight Calculator (three published bass formulas; the Outdoor Life/Hal Schramm attribution; the caveat on disproportionally shaped larger bass).
• Florida Fish and Wildlife Conservation Commission, Fisheries Research — Condition (definition of relative weight; printed standard-weight tables for largemouth bass and bluegill used to cross-check the equations on this page).
• Alabama Cooperative Extension System, ANR-1193 — Relative Weight: An Easy-to-Measure Index of Fish Condition (the under-80 / 0.8–1.0 / above-0.9 bands; the bass–bream interpretation matrix; the 10–14 inch bass observation; sampling advice).
• Georgia Department of Natural Resources, Wildlife Resources Division — How Healthy Is My Fish? (401 g standard weight for a 12-inch largemouth; the 90–100 band).
• Liao, H., C. L. Pierce, D. H. Wahl, J. B. Rasmussen and W. C. Leggett (1995). "Relative Weight (Wr) as a Field Assessment Tool: Relationships with Growth, Prey Biomass, and Environmental Conditions." Transactions of the American Fisheries Society 124:387–400 (the 75th-percentile basis and the regression-line-percentile technique).
• North Dakota Game and Fish Department — Fish Length-Weight Table (statewide average weights used for the cross-check table; the caveat on sex, season, health and water body).
• University of California Agriculture and Natural Resources, California Fish Website — Conversions and Glossary (definitions of total, fork and standard length).
• South Carolina Department of Natural Resources — How to Properly Measure a Fish (total length with the tail pinched).
• Laarman, P. W. and J. C. Schneider (1985). Maturity and Fecundity of Largemouth Bass as a Function of Age and Size, Michigan Department of Natural Resources Fisheries Research Report 1931 (ovary weight as 4.7% to 10.9% of total female weight).
• Standard-weight equations as compiled in the fisheries literature: Henson (1991) for largemouth bass; Hillman (1982) for bluegill.

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