How accurate are length-to-weight estimates?

Length-weight formulas are statistical averages for a species and are typically within 10 to 15 percent of true weight for a fish of average condition. A heavy, well-fed fish weighs more than the estimate; a lean one weighs less. Adding girth improves accuracy.

What is the W = a × L to the b formula?

It is the standard length-weight regression used in fisheries science. Length is raised to a power b (usually near 3, since weight scales with volume) and multiplied by a species constant a. Each species has its own a and b fitted from sampled fish.

When should I use the girth formula instead?

If you measured girth, the girth-based formula (length times girth squared, divided by a constant) captures the individual fish's body thickness, which the length-only formula cannot. Use girth for trophy fish or unusually fat or thin specimens.

Should I measure total length or fork length?

These formulas assume total length, measured from the tip of the snout to the end of the tail with the tail squeezed. Fork length (to the fork of the tail) is shorter and will under-estimate weight, so use total length where possible.

Does this work for catch-and-release records?

Yes. Estimating weight from a quick length and girth measurement lets you release a fish unharmed instead of putting it on a scale in a net. Many catch-and-release tournaments and personal-best logs accept formula-based weights.

Fish Weight Estimator by Length & Species

Putting a fish on a scale stresses it and costs precious release time. This estimator uses each species’ own length-weight regression to turn a quick length measurement (and optional girth) into a weight, so you can record the catch and release it fast.

How it works

The length-only estimate uses the standard fisheries formula with species constants a and b:

weight (lb) = a × length^b          (length in inches)

When you provide girth, the tool switches to the shape-based formula, which accounts for how fat or lean the individual fish is:

weight (lb) = (length × girth²) / shapeFactor

The shape factor is 800 for most fish and 1200 for slender, long-bodied fish like pike and muskie. Because the girth formula measures the actual fish rather than a species average, it is more accurate for unusually heavy or thin catches.

Example and tips

A 20-inch walleye with the walleye regression estimates near 2.7 lb; the same fish measured at a 12-inch girth gives (20 × 144) / 800 ≈ 3.6 lb if it is a fat, pre-spawn female. Always measure total length with the tail pinched, and take girth at the thickest point just ahead of the dorsal fin. Treat the result as a solid estimate, not a certified scale weight.