Free Fish Weight Calculator

Fish Size Calculator serves as a practical tool for approximating a fish's weight based on its length, girth, and species. Acting as both a fish length to weight converter and a fishing weight estimator, it helps anglers who may not have a scale on hand. By entering just a few measurements, you can obtain a reliable estimate of your catch's weight without needing specialized equipment.

How to Use the Fish Size Calculator

To estimate fish weight by length, you need three pieces of information: the fish's length, optionally its girth (the circumference at the widest part), and the species (or a closely similar type). The species matters because fish shapes vary significantly—a thick-bodied fish like a bass will weigh more than a slender fish of the same length.

Measuring Your Catch Accurately

Length can be recorded as either standard length (from the tip of the snout to the start of the tail fin) or total length (including the tail fin). Always measure in a straight line along the fish's side, avoiding any body curvature. Girth is the maximum circumference around the body. This is not the same as body depth, which is the vertical distance from the dorsal to the ventral side. If you cannot measure the girth, the calculator automatically estimates it using:

$$\text{girth} = \text{length} \times 0.58$$

Fish Length to Weight Formulas

The general formula used by this fishing weight estimator is:

$$\text{weight} = \dfrac{\text{length} \times \text{girth}^{2}}{\text{constant}}$$

where the constant varies by species. Common constants include:

• Trout: constant = 900, so $\text{weight} = \dfrac{\text{length} \times \text{girth}^{2}}{900}$
• Largemouth Bass: constant = 800, so $\text{weight} = \dfrac{\text{length} \times \text{girth}^{2}}{800}$
• Pike: constant = 2700, and the formula uses only length: $\text{weight} = \dfrac{\text{length}^{3}}{2700}$

All measurements should be in inches to obtain weight in pounds.

Practical Examples

• For a 20-inch trout, the estimated girth is $20 \times 0.58 = 11.6$ inches. Plugging into the trout formula gives $\dfrac{20 \times 11.6^{2}}{900} \approx 3.0$ pounds.
• A 27-inch largemouth bass has an estimated girth of $27 \times 0.58 = 15.66$ inches, yielding $\dfrac{27 \times 15.66^{2}}{800} \approx 8.28$ pounds.
• A 30-inch pike, which uses only length, gives $\dfrac{30^{3}}{2700} = 10$ pounds.

These examples highlight how easily the fish size calculator converts length to weight for different species.

Additional Facts

The heaviest fish known is the whale shark, with adults reaching up to 70 feet in length and weighing as much as 47,000 pounds. (Blue whales are mammals, not fish.)

Regarding consumption, fish are rich in omega-3 fatty acids and low in saturated fat, but some species may contain mercury. It is advisable to check local advisories, especially for pregnant women, nursing mothers, and young children.

Always ensure you have the appropriate fishing permit for your location, as regulations differ by region and season.

The Fish Size Calculator is a convenient companion for any angler who wants to quickly estimate the weight of their catch using simple measurements.