Largemouth bass weight calculator

Why does a long bass weigh so much more than you expect?

The exponent in the largemouth equation is 3.273. A perfect cube would be 3.0. That difference sounds trivial and is not: it means the fish gets proportionally deeper and thicker as it lengthens, so weight outruns the cube law by a widening margin.

Put numbers on it. Double a bass from 12 inches to 24 inches and a cube law predicts eight times the weight — 0.88 lb becoming 7.0 lb. The published equation says 8.53 lb. That extra pound and a half is entirely the shape change, and it is why bass anglers who apply a walleye rule of thumb consistently lowball their fish.

Where does this bass weight data come from?

The length-to-weight curve is Henson's (1991) largemouth bass standard-weight equation as held in the FSA standard-weight database — the same reference Wisconsin and Minnesota crews use when they score condition. Relative-weight presets follow the published Wr bands. Every number on this page fails the build without a citation; the audit trail is on ourresearch methodology page.

What works for largemouth on the Mississippi River?

On Pools 8 and 9 the fish you want are in the flooded timber and weed edges of the backwaters, not in the main-channel current where smallmouth and walleye hold. A soft-plastic or frog around wood inPool 9 around Lansing is a different game from the dam face, and the tape matters once you decide whether a fish is a photograph or a slot you are about to release. If the jaw and dorsal say smallmouth, stop — this curve will overstate the weight.

Largemouth or smallmouth?

Both live in the Upper Mississippi, and the pools mix them: smallmouth on the current seams and rock, largemouth back in the sloughs and around wood. Two checks settle it. Channel cats from the same backwaters need thecatfish weight calculator, not this curve.

• Jaw. Close the mouth and look at the corner. On a largemouth the upper jaw ends behind the back edge of the eye. On a smallmouth it stops level with or in front of it.
• Dorsal fin. A largemouth's spiny and soft dorsal fins are nearly separated by a deep notch. A smallmouth's join in a shallow curve.

Smallmouth have their own published standard-weight equation, and it is not the one running here. We have not added smallmouth to this site yet rather than approximate it with the largemouth curve.

What does a largemouth bass weigh at each length?

Read the 20-inch row carefully. Typical condition puts that fish at 4.7 pounds and heavy condition at 5.4. A single 20-inch bass can legitimately be either side of the five-pound line depending on the month and what it has been eating, which is why a chart with one column per length settles no arguments.

Two worked examples from the backwaters

A 15-inch bass off a Pool 8 weed edge in June

15 in is 381 mm. The log of 381 is 2.58092; multiplied by 3.273 that is 8.44735; subtract 5.528 and you have 2.91935, so Ws = 831 g, which is 1.83 lb. Post-spawn June fish in the backwaters are often thin, so the lean setting at 1.56 lb may be closer, with a range of 1.37 to 1.74.

A 22-inch bass from a Lake Onalaska slough in October

22 in is 558.8 mm, log 2.74726, times 3.273 is 8.99078, minus 5.528 gives 3.46278, so Ws = 2,909 g, or 6.41 lb. October fish loading up before winter usually sit above the curve: heavy condition returns 7.38 lb with a range of 6.73 to 8.02. That is the difference between a good fish and a personal best, and the tape alone will not tell you which one you are holding.

What do bass anglers get wrong?

Applying the girth formula is the most common. Girth² × length ÷ 800 is everywhere on bass forums, and no one posting it can point to the study behind it. It is a saltwater rule of thumb that migrated. Our approach is to use relative weight instead, because relative weight is a published, validated metric with a paper trail.

Second is confusing smallmouth for largemouth on a phone photo, which reliably overstates the smallmouth by a few ounces at every length.

Third is measuring a bass with its mouth open. Total length is measured with the mouth closed and the tail pinched; an open-mouthed measurement on the board can add half an inch, which on a 20-inch fish is about four ounces of fictional weight.

Fourth is comparing an estimate against a tournament scale and concluding the calculator is broken. It is not measuring the same thing. If you want a like-for-like check, estimate the fish before you weigh it, then log both — over a season you will learn whether your local bass run above or below the curve, which is more useful than any single comparison. Themulti-species version of this calculator makes that easier when the day produces walleye and catfish alongside the bass.