Fish Weight from Length Calculator
Estimate a fish's weight from its length using species-specific formulas, plus the girth method for unusually fat or thin fish.
Length-only estimates use the classic species formulas (length³ ÷ species factor); adding girth switches to length × girth² ÷ 800, which handles unusually fat or lean fish far better. Both assume measurement to the tail tip. Real fish vary ±20% with season and condition — a pre-spawn female outweighs the chart, a post-spawn fish runs under it.
The species list is a girth table wearing a disguise
This page offers two formulas. Length-only is W = L³/K with a species divisor; the girth formula is W = L·G²/800 and works for anything that swims. They look like alternatives. Set them equal and the divisor turns out to be something else entirely: G/L = √(800/K).
| Divisor | Girth, as a share of length | Species |
|---|---|---|
| 1200 | 81.6% | Panfish (bluegill/crappie) |
| 1600 | 70.7% | Largemouth bass, Smallmouth bass |
| 2200 | 60.3% | Striped bass |
| 2700 | 54.4% | Walleye, Trout (stream/rainbow) |
| 3500 | 47.8% | Northern pike, Muskellunge |
That ranking is exactly deep-bodied to slender, and it is not a curve fit — it is what the divisors mean. Picking "northern pike" is not selecting species knowledge; it is asserting the fish is about half as thick around as it is long. The bass figure lands on exactly 1/√2, and the species choice alone spans 2.92× in weight at any given length.
So the length-only estimate is an assumption, not a measurement
A 20-inch largemouth comes out at 5.00 lb, and the girth that number assumes is 14.14 inches. Your fish will not be exactly that, and the error does not pass through gently — girth enters squared.
| Girth | Measured | Weight | Against the length-only figure |
|---|---|---|---|
| -20% | 11.31″ | 3.20 lb | -36% |
| -10% | 12.73″ | 4.05 lb | -19% |
| 0% | 14.14″ | 5.00 lb | +0% |
| +10% | 15.56″ | 6.05 lb | +21% |
| +20% | 16.97″ | 7.20 lb | +44% |
Ten percent fat is 21% heavy, not ten. A fish in heavy pre-spawn condition against the same fish in August is easily a 20% girth swing, which is the difference between a 5 lb estimate and a 7.2 lb one. If the number matters, measure the girth — the dropdown cannot know what condition your fish is in.
The cube, in the terms people get wrong
| Longer by | Heavier by |
|---|---|
| 5% | 16% (×1.16) |
| 10% | 33% (×1.33) |
| 20% | 73% (×1.73) |
| 50% | 238% (×3.38) |
| 100% | 700% (×8.00) |
A fish twice as long is eight times the weight. The practical version is that the last inch on a big fish is worth far more than the first: 19 to 20 inches on a largemouth adds 0.71 lb, while 9 to 10 inches adds 0.17 lb — the same inch, 4.2× the weight. That ratio is identical for every species on the list, because the divisor cancels out of it.
And the page's own examples check out
Each species note carries a worked example. Running all five back through the formula they annotate:
| Species | Length | The note says | The formula gives | Off by |
|---|---|---|---|---|
| Largemouth bass | 20″ | 5 lb | 5.00 lb | 0.0% |
| Walleye | 25″ | 5.8 lb | 5.79 lb | -0.2% |
| Northern pike | 40″ | 18 lb | 18.29 lb | 1.6% |
| Muskellunge | 50″ | 35.5 lb | 35.71 lb | 0.6% |
| Striped bass | 36″ | 21 lb | 21.21 lb | 1.0% |
The worst disagreement is 1.6% and four of the five are inside one percent. They were computed rather than remembered — and a test now pins them, so an edited divisor cannot quietly orphan its own example.
How to use
- Measure length accurately, nose to tail fork or tip.
- Pick the species.
- Add girth if the fish is notably heavy or lean.
- Release quickly — an estimate beats weighing a fish you intend to return.
Frequently asked questions
How can weight be estimated from length alone?
Because weight scales roughly with the cube of length for a given body shape, so each species has a characteristic coefficient fitted from measured fish. It works well for typical individuals and less well for anything unusually proportioned.
When should I use the girth method?
Whenever the fish looks fatter or leaner than typical — a pre-spawn female, or a fish in poor condition. Length alone assumes average girth, so it will underestimate a heavy fish substantially. Including girth captures the actual body volume and is considerably more accurate.
How accurate is the estimate?
Usually within about 10 per cent for a typical fish of a known species, and worse for outliers. It is an estimate rather than a measurement, and it should be described as one — reporting an estimated weight as a weighed figure is how records get disputed.
Where exactly do I measure?
From the snout to the fork of the tail for most species, or to the tip for those with rounded tails. Which convention applies varies by species and by the body keeping the record, so it is worth confirming before claiming anything. Measuring on a flat surface with the fish straight matters more than people expect.
Why estimate rather than weigh?
Because it is far kinder to the fish. Hanging a fish by the jaw on a scale can damage its jaw and internal structures, and time out of water matters. A quick length measurement on a wet mat, a photograph and a prompt release gives the fish the best chance.
Will an estimate count for a record?
No. Record claims require a certified scale, witnesses and a specified procedure, which varies by organisation. An estimate is for your own log and for telling the story honestly — and saying it was estimated is part of telling it honestly.
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