Fish Weight Calculator

Background.

Most good fish are now photographed and released, and a fish lying on a wet measuring board is far better off than one hanging from a scale. That leaves the angler with two numbers — length and girth — and a question about weight. The long-standing answer among anglers is a single line of arithmetic: square the girth, multiply by the length, and divide by 800. A 30-inch fish with a 20-inch girth comes out at 15 pounds.

The rule works because a fish is mostly water, so its weight follows its volume, and volume grows with the square of girth but only in proportion to length. That is why a short, deep-bellied fish can outweigh a longer, slimmer one. If a fish were a perfect cylinder of water, its weight would be length times girth squared divided by about 348; the divisor 800 allows for the taper towards the head and tail, treating a typical fish as filling a little over two-fifths of the cylinder drawn around its widest point.

Because the rule is not tuned to any one species, expect it to be closer for some fish than others. Laterally flattened, deep-bodied species and long, thin ones depart from the average shape the divisor assumes, and a fish heavy with eggs or fattened up for winter carries weight that the girth only partly reflects.

Measurement matters more than most people expect. Girth enters squared, so a tape pulled an inch too loose on a 20-inch fish adds about 10% to the weight estimate. A soft tailor's tape, wrapped snugly around the thickest part of the body without squeezing, gives the most consistent results.

What is fish weight calculator?

A length–girth weight estimate predicts how heavy a fish is from two tape measurements instead of a scale. Length is the total length from the tip of the snout to the tip of the tail; girth is the circumference around the deepest part of the body. The traditional general formula, weight in pounds equals girth squared times length divided by 800, is applied here with both measurements in inches, and the answer is also converted exactly to kilograms.

How to use this calculator.

1. 01Lay the fish flat on a measuring board or tape with its mouth closed and measure total length in inches from snout tip to tail tip. If local rules specify how to hold the tail, measure the same way.
2. 02Wrap a soft tape around the thickest part of the body, usually just ahead of the dorsal fin, snug against the skin but not compressing it, and enter that as the maximum girth in inches.
3. 03If you measured in centimetres, divide by 2.54 before entering; 76.2 cm is 30 inches.
4. 04Read the weight in pounds, with the kilogram figure beneath it. Treat it as an estimate within a few pounds for large fish, not a record-book weight.

The formula.

Weight (lb) = G² × L ÷ 800, where G is the maximum girth in inches and L the total length in inches. G² × L has units of cubic inches and stands in for the fish's volume. A cylinder of fresh water with that length and circumference would weigh about G² × L ÷ 348 pounds, because a cubic inch of water weighs about 0.0361 lb and the cross-section of a circle is its circumference squared divided by 4π. Dividing by 800 instead scales that down to roughly 43% of the cylinder, which accounts for the tapering body. Girth has twice the influence of length: a 10% change in girth moves the estimate by 21%, while the same change in length moves it by 10%. Kilograms are pounds × 0.45359237.

A worked example.

Frequently asked questions.

References& sources.

1. [1]New York State Department of Environmental Conservation. Angling measurement guidance current 2026. Fish measurement and weight-estimation context. Retrieved 2026-08-06.
2. [2]U.S. National Library of Medicine. PubMed record current 2026-08-06. Indexed derivation and validation literature. Retrieved 2026-08-06.
3. [3]World Health Organization. Patient safety resources current 2026. Clinical decision-support safety context. Retrieved 2026-08-06.