Fish Weight Calculator

Estimate a fish's weight without a scale, using length and girth.

How the Math Works

The Fish Weight Calculator uses a simple yet powerful mathematical formula to estimate a fish's weight: weight (lb) = length × girth³ 2 ÷ 800. This formula works because fish weight scales with body volume, which grows exponentially with size. By multiplying the fish's total length by the square of its girth (the measurement around the thickest part of the body), we approximate the fish's three-dimensional volume. Dividing by 800 converts this volume estimate into pounds using a species-adjusted constant that accounts for typical fish density and body composition.

Practical Applications

To use this calculator practically, first measure your fish accurately from the tip of its nose to the fork of its tail for length, then measure the girth at the widest part of its body. For example, if a fish is 20 inches long with a 12-inch girth, the calculation would be: 20 × 12² ÷ 800 = 20 × 144 ÷ 800 = 3.6 pounds. This method works for most freshwater game fish like bass, pike, and trout, giving anglers a reliable estimate without needing to kill or harm the fish for weighing.

Day-to-Day Use

This calculator helps anglers make informed decisions in everyday fishing scenarios. Whether you're keeping a fishing journal, determining if you've caught a trophy fish, or deciding which fish to release based on size regulations, this quick estimate saves you from having to gut and weigh your catch. It's especially useful when fishing in remote locations without a scale, helping you practice ethical catch-and-release by providing accurate size information to ensure compliance with local fishing limits and to judge whether a fish has spawned enough to be kept.

Worked example

20 in long, 12 in girth → 3.6 lb.

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