Fish Weight Calculator (estimate weight from length & girth, by species)

Understanding the fish weight calculator — concept and principle

Anglers who release their catch often want to know its weight without a scale. This calculator estimates a fish's weight from its length and girth, using a general rule or species-specific formulas for bass, northern pike, sunfish, trout and walleye published by state natural resources agencies.

Girth squared times length over 800 estimates 3.6 lb.

How it is calculated

Any fish: weight (lb) = girth² × length ÷ 800

Bass: length² × girth ÷ 1,200

Trout: length × girth² ÷ 800

Northern pike: length³ ÷ 3,500

Walleye: length³ ÷ 2,700

Sunfish: length³ ÷ 1,200

Length from the tip of the jaw to the fork of the tail and girth around the thickest part, both in inches; centimetres are converted with 1 in = 2.54 cm.
Results in pounds, kilograms (× 0.45359237) and ounces (× 16).

The principle

A fish is roughly a cylinder, so its volume, and therefore its weight, grows with length times the square of its thickness. Girth captures thickness directly, which is why the general rule uses girth squared times length. The divisor folds in the fish's shape and density and converts cubic inches to pounds. For long, slim species with fairly consistent body proportions, such as pike and walleye, girth tracks length closely enough that length alone, cubed, gives a usable estimate.

Worked example

Default inputs: general rule, length 20 in, girth 12 in.

Girth² = 12² = 144.
Weight = 144 × 20 ÷ 800 = 2,880 ÷ 800 = 3.6 lb.
In kilograms: 3.6 × 0.4536 = 1.63 kg.
In ounces: 3.6 × 16 = 58 oz.
Length used = 20 in; girth used = 12 in.

How to read the result

The estimate assumes a fish in average condition. A fish heavy with eggs or very well fed will weigh more; a thin or spawned-out fish will weigh less. Formula estimates can differ from a scale reading noticeably, so use them for personal records and not for official claims. Some fishing records require a certified scale weight.

Common mistakes

Measuring total length to the tip of the tail instead of the fork when the formula expects fork length.
Measuring girth behind the thickest part.
Using the general rule for very long, thin species, which tends to overestimate.
Mixing inches and centimetres.

Keep in mind: Estimates vary with species, season and condition. Handle fish carefully if you plan to release them, and follow local fishing regulations on size and handling.

Frequently asked questions

How do I measure girth without hurting the fish?

Use a soft tape quickly around the thickest part while supporting the fish, and keep it wet.

Why do pike and walleye formulas use only length?

Their slim, consistent shape lets length alone predict weight reasonably well.

How accurate are these formulas?

They give a reasonable estimate for an average fish, but individual fish can differ noticeably.

Can I use centimetres?

Yes; the calculator converts them to inches before applying the formula.

Sources: Wisconsin Department of Natural Resources, "Estimating fish weight" · South Carolina DNR Marine Resources, "Estimating Fish Weight"

Checked 9 Nov 2026. Wisconsin Department of Natural Resources, “Estimating fish weight” (bass, pike, sunfish, trout, walleye formulas); South Carolina DNR Marine Resources, “Estimating Fish Weight” (girth² × length ÷ 800, jaw-to-fork length).

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