Fish Weight Estimator
Estimate a fish's weight from length and girth for bass, trout, pike, muskie, and walleye using the standard length/girth formulas.
This assumes a fish in typical condition. A post-spawn fish runs light for its length, and a pre-spawn female heavy.
How it works
Pick a species, measure length from nose to tail fork, and for bass or trout add a girth measurement (wrap a tape around the fattest part of the body). The calculator runs one of the classic length/girth formulas anglers have used for decades to turn a tape measure into a weight estimate without a scale.
Worked example: a 20 inch largemouth bass with a 14 inch girth. The bass formula is (length times girth squared) divided by 1200, so that's (20 times 196) divided by 1200, which comes out to 3.3 pounds. Pike and walleye use a simpler length-only formula since their body shape is more consistent along the length, so a 40 inch pike lands at 18.3 pounds using length cubed divided by 3500, no girth needed.
If you didn't get a girth measurement before releasing the fish, check "estimate it for me" and the tool uses a rough girth of about 0.58 times the length, close enough for a ballpark number on a fish you already let go.
FAQ
How accurate is this compared to an actual scale?
These formulas run within about 10% of an actual weight for a fish of typical, healthy proportions. They're a field estimate, not a certified reading, so don't use them to settle a tournament weigh-in bet. For that you need a real scale.
Why does a fat spring fish weigh more than the formula says?
A pre-spawn female carrying a full load of eggs can run noticeably heavier than the formula predicts for her length, since she's wider than a "typical" fish of that size. A post-spawn fish runs the other way, thinner and lighter than the estimate. Girth is what catches most of that difference, which is why measuring it (rather than estimating it) matters most in spring and early summer.
Why don't pike and walleye use girth at all?
Their bodies are long and fairly uniform in cross-section along most of their length, so length alone predicts weight almost as well as length and girth together would for a rounder-bodied fish like a bass. It also means you can get a solid estimate from a length measurement alone, useful if you're trying to handle the fish as little as possible.
Is this better than weighing a fish on a scale hooked through the jaw?
For a fish you plan to release, yes. A quick length measurement (and girth if you can get it without much handling) gets you a weight estimate without hanging the fish by the jaw on a scale, which stresses it more and risks jaw damage on a big one.
For more on getting a fish back in the water in good shape, see catch and release done right. For species-specific playbooks, check out catching your first largemouth bass and how to catch northern pike.