Fish Weight Calculator
Estimate a fish’s weight from its length and girth — the catch-and-release formula, so the fish goes back in the water.
The girth formula, weight = girth² × length ÷ 800 with inches and pounds, works for most fish shapes; leave girth blank and a typical girth of 0.
How the fish weight calculator works
The girth formula, weight = girth² × length ÷ 800 with inches and pounds, works for most fish shapes; leave girth blank and a typical girth of 0.58 × length is assumed, which suits trout, bass and bream but underestimates deep-bodied fish. Measure the length nose to tail fork and the girth at the widest point.
Formula: weight (lb) = girth² × length ÷ 800 (inches)
Worked examples
| Inputs | Estimated weight | Note |
|---|---|---|
| 24 in long, 14 in girth | 5.88 | 5.88 lb |
| 60 cm, girth estimated | 2.51 | 2.5 kg |
| A metre-long fish with a 50 cm girth | 8.65 | 8.7 kg |
FAQFrequently asked questions
How accurate is the length-and-girth formula?
Usually within 10% for typical fish shapes, and better with a real girth measurement. Very deep-bodied species (carp, snapper) run heavier than the formula; long thin ones (pike, barracuda) run lighter.
How do I measure a fish’s length and girth?
Length from the tip of the snout to the fork of the tail (or the tip, for round-tailed species); girth around the thickest part of the body just in front of the dorsal fin. A soft tape, or a length of line measured afterwards, works.
Why divide by 800?
It is an empirical constant for a generic fish shape in inches and pounds; species-specific formulas use 750–900. Fisheries agencies publish constants for local species.
Does it work in metric?
Yes — the calculator converts centimetres to inches for the formula and gives the weight in both kilograms and pounds.
Where these figures come from
- World Handicap System — Rules of Handicapping — the score differential and handicap index formula
- International Cricket Council — Playing Conditions — over structure and run-rate definitions
- Cricket Australia — the national body for the sport these statistics most often describe here
Last checked: September 2026. Formulas follow the governing bodies where one publishes them — the World Handicap System for golf, the ICC playing conditions for cricket run rates.