Fish Weight Calculator

The most common formulas use the fish's length and girth (circumference at the widest point). Popular methods include Length × Girth² ÷ 800 for general fish, and Girth³ ÷ 2300 for trout, salmon, and steelhead. These formulas approximate the cylindrical volume of the fish body and convert it to weight. Results are estimates and may vary a few pounds from actual weight.

Different species have different body shapes and densities, so a single formula doesn't fit all fish. Trout and salmon are often estimated with Girth³ ÷ 2300, while heavier-bodied fish like bass use a higher divisor (÷ 1200) compared to sleeker predators like pike (÷ 750). Choosing the right formula for your species gives a more accurate estimate.

Girth is the circumference around the widest part of the fish's body, typically just behind the pectoral fins. Use a flexible measuring tape and wrap it around the fish at its thickest point. For catch-and-release, keep the fish in the water or handle it gently while measuring.

Total length is measured from the tip of the mouth (closed) to the end of the tail fin. Lay the fish flat on a measuring board or ruler. Some species use fork length (to the fork of the tail) — check the specific formula requirements if precision is important.

These formulas give a reasonable approximation, typically within a few pounds of the actual weight. Accuracy depends on how closely the fish's body shape matches the formula's assumptions. A scale is always the most accurate method, but these estimates are great for catch-and-release situations where weighing isn't practical.

The general formula (Length × Girth² ÷ 800) can be applied to many saltwater species as a rough estimate. However, for marine species with well-documented length-weight relationships (such as red drum, flounder, or snook), species-specific scientific formulas from fisheries research will be more accurate.

The divisor accounts for differences in body density, shape, and species-specific proportions. Values like 800, 775, 900, and 1200 have been derived from actual weigh-ins of hundreds of fish across studies by wildlife agencies and research groups. Slight variations exist between published sources, and all are valid approximations.

Some length-only formulas exist for specific species, often derived from fisheries data. However, girth is a key factor in weight estimation since two fish of the same length can have very different girths (and therefore weights). If girth isn't available, use a species-specific length-weight table from your local fisheries agency for the best estimate.