Fish Weight from Length Calculator

Fish weight from length is estimated using a length-weight relationship formula established through population studies by fisheries biologists. Since weighing a live fish in the field is difficult, this formula lets anglers estimate weight from a simple length measurement.

With girth, which is the accurate one:

Weight (lbs) = (Girth² × Length) / 800

Measure girth at the widest point, just behind the pectoral fins, and length from the tip of the mouth to the tip of the tail with the fish held straight. Both in inches. Girth is what tells a fat fish from a thin one, so this version works across most species with only a small adjustment to the divisor: 800 for bass, pike and walleye, 900 for the more slender trout and salmon, 750 for round-bodied catfish and carp.

Worked example: an 18 inch bass with a 13 inch girth. (13² × 18) / 800 = (169 × 18) / 800 = 3,042 / 800 = 3.80 lbs

Without girth, from length alone:

Weight (lbs) = Length³ / divisor

Less accurate, because it assumes an average-conditioned fish, but useful when the fish is already going back in the water.

Leave the girth field blank and this calculator uses the length-only version instead.

Reference, largemouth bass by length:

Those match what a tape and a scale agree on for a bass in ordinary condition. A pre-spawn female in April can carry a pound more than the table says at the same length, and a post-spawn fish in June a pound less. That is the whole reason to measure girth when you can: length tells you how old the fish is, girth tells you how it has been eating.