Fish Weight from Length Calculator
Estimate the weight of a fish from its length and girth so you can release it fast without a scale.
How it works
with girth: weight (lb) = girth² × length ÷ 800 · length only: weight (lb) = length³ ÷ species factor (inches)
The most accurate no-scale estimate uses girth, because a fish’s weight tracks its cross-sectional area much more closely than length alone. The classic formula divides girth squared times length by 800 for length and girth in inches. When you cannot measure girth, a length-cubed rule with a species-specific factor works because weight scales with the cube of a similarly shaped body — but a fat pre-spawn fish or a lean post-spawn one can deviate 20% or more.
Worked example
A 20-inch fish with a 14-inch girth: 14² × 20 ÷ 800 = 3,920 ÷ 800 = 4.9 lb. With girth unknown, a 20-inch bass at factor 1,600 gives 20³ ÷ 1,600 = 8,000 ÷ 1,600 = 5.0 lb.
Frequently asked questions
Why is girth better than length alone?
Two fish the same length can weigh very differently depending on body condition. Girth captures how “fat” the fish is, so the girth formula is far more accurate than any length-only estimate.
How should I measure the fish?
Measure total length from nose to tail tip lying flat, and girth as the distance around the widest part of the body. A soft tape or a piece of string laid against a ruler works well.
How accurate is the estimate?
The girth method is usually within about 10% for typical body shapes. Length-only estimates carry more error because they cannot see condition, so treat them as a ballpark for catch-and-release.
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This calculator is for educational purposes. Double-check important results before acting on them.