There are several calculations for estimating the weight of a fish. One of the most popular—and accurate—is a function of length and girth. Another method is based purely on length. Both methods incorporate a divisor (or “shape” factor) that can be adjusted for a particular fish species, region, or body of water to provide a more accurate estimate.

Generic formulas for estimating the weight of a fish based on length and girth, simply length:

• (length x girth x girth) / 800
• (length x length x length) / 1200

Formulas for estimating weight using both length and girth tend to be most accurate because they account for how plump (or “fat”) the fish is. And as fish, even of the same species, may be more “skinny” or “fat” depending on the body of water, incorporating girth as a factor provides a more consistent and accurate estimate of weight regardless of water conditions and location.

When only length is available, estimating weight accurately may require adjusting the divisor (“shape” factor) to account for regional differences in fish size. As an example, for a bass pulled from O.H. Ivie Lake, where forage is plentiful and bass tend to be “fat”, you might use a divisor of 750. When estimating the weight of a bass out of Utah Lake, where conditions are less favorable, and bass are “skinny”, a divisor of 800 will provide a more accurate weight estimate.

However, even weight formulas incorporating girth don’t account for differences in shape across different fish species. While a bonefish and mackerel may have the same girth, they have inherently different body shapes. This impacts their relative girth to length to weight ratio. Finding the right divisor for (“shape” factor) may necessary, even when using formulas that incorporate girth.

The following charts provide length to weight (in lbs) formulas for the most common freshwater and saltwater fish species, along with average length, average weight and maximum size data for adult specimens. The most common “shape” factor for “girth” formulas is 800. It also tends to be the most accurate across species. The most common “shape” factor for “length” formulas is 1200, but I have not found it to provide consistently accurate weight estimates across species.

All divisors (“shape” factors) presented below have been tested against verified length to weight species tables to ensure accuracy across species, as well as consistency across weight groups within species. You can expect that actual fish weight may variety between 3-5% from estimated weight. Remember, these are just formulas. They produce weight estimates along a linear plane. For many fish species, the relationship between length and weight runs along a curve, so formulas for estimating weight for some fish species may not work as well for very small, or very large specimens.

Measuring Fish Length

Estimating a fish’s weight requires accurate length measurements. There are three important length measurements for determining fish length: Total length (TL), Fork Length (FL) and Lower Jaw Fork Length (LJFL).

Total length (TL) is used for estimating the weight of most fish species. Total length is determined by measuring the distance from the tip of the nose to the tip of the tail.

Total length (TL) species: trout, bass, panfish, catfish, salmon, pike, snapper, grouper, snook, drum, seatrout, weakfish, bonefish, flounder, sheepshead, etc.

Fork Length (FL) is used for estimating weight for certain saltwater fish species, such as tarpon and sharks. Fork length is determined by measuring the distance from the nose to the fork in the tail.

Fork length (FL) species: mackerel, amberjacks, permit, pompano, bluefish, mullet, cobia, rudderfish, mahi mahi, hogfish, mullet, tuna

Lower jaw fork length (LJFL) is used for estimating weight for billfish, such as sailfish and marlin. LJFL is determined by measuring the distance from the tip of the lower jaw to the fork in the tail.

LJFL species: sailfish, swordfish, marlin, spearfish

Measuring Fish Girth

Estimating a fish’s weight requires accurate girth measurements. Girth is determined by measuring the circumference at the thickest portion of a fish’s midsection.

Additional Notes

From my experience standard length-to-weight formulas have a tendency to over estimate the weight of actual fish populations. They typically represent a fish that is well fed and “above average”. Without adjusting the divisor (“shape” factor) in the formulas presented, you can assume that for every 10 fish you catch about 4 of them will be slightly above the estimated weight, and 6 below.

For the (length x length x length) / divisor formulas, the divisor (“shape” factor) was determined by calculating estimated fish weights in increments of 1 pound across all relevant weight classes for a species and coorelating the estimate with standard length-weight species tables generated using a statistically signficant sample size.