Free tool
Fish Weight Calculator
Released a fish without weighing it? Measure length and girth, pick the species, and get a solid estimate of its weight.
Estimated weight — Largemouth Bass
3.27 lb
≈ 3 lb 4 oz · formula: (length × girth²) ÷ 1,200
How to measure length and girth
- Length: lay the fish flat on a ruler or bump board with its mouth closed. Measure from the tip of the snout to the end of the tail, gently pinching the tail lobes together.
- Girth: wrap a soft tape (or a piece of string you measure later) around the thickest part of the body, usually just in front of the dorsal fin.
- Keep the fish wet and work quickly if you plan to release it.
Formulas we use
These are widely used angler formulas with lengths and girths in inches and results in pounds:
- Largemouth Bass: (length × girth²) ÷ 1,200
- Smallmouth Bass: (length × girth²) ÷ 1,200
- Trout: (length × girth²) ÷ 800
- Catfish: (length × girth²) ÷ 800
- Northern Pike: length³ ÷ 3,500
They're estimates. A fish's real weight depends on its body shape, season (pre-spawn fish are heavier) and condition. Expect results within roughly 10–15% of true weight for typical fish. For records or tournaments, always use a certified scale.
Worked example
You catch a largemouth bass that is 20 inches long with a 14-inch girth.
20 × 14 × 14 = 3,920. Then 3,920 ÷ 1,200 = 3.27 lb — about 3 lb 4 oz.
Largemouth bass weight chart (lb)
Estimated weight by length (rows) and girth (columns), using (length × girth²) ÷ 1,200.
| Length ↓ / Girth → | 10" | 12" | 14" | 16" | 18" |
|---|---|---|---|---|---|
| 12" | 1.0 | 1.4 | 2.0 | 2.6 | 3.2 |
| 14" | 1.2 | 1.7 | 2.3 | 3.0 | 3.8 |
| 16" | 1.3 | 1.9 | 2.6 | 3.4 | 4.3 |
| 18" | 1.5 | 2.2 | 2.9 | 3.8 | 4.9 |
| 20" | 1.7 | 2.4 | 3.3 | 4.3 | 5.4 |
| 22" | 1.8 | 2.6 | 3.6 | 4.7 | 5.9 |
| 24" | 2.0 | 2.9 | 3.9 | 5.1 | 6.5 |
What the result means and how it is calculated
Short answer: the number shown is an estimate of the fish's weight in pounds, calculated from the length and girth you enter. It is not a measured weight.
Method: the calculator applies the widely shared angler rule-of-thumb formula listed for each species above, using inches as input. Bass and trout/catfish formulas use length × girth² divided by a species constant; the pike formula uses length only.
Limitations: these formulas are generic approximations, not species studies from a state agency. Body shape, season and spawning condition change real weight, so estimates can differ noticeably from a scale reading. Do not use the estimate for record claims, tournament results or legal size and bag decisions — check your state's official regulations for those.
Where to catch these species
- Largemouth Bass: see our best fishing spots in Florida
- Smallmouth Bass: see our best fishing spots in Michigan
- Trout: see our best fishing spots in Pennsylvania
- Catfish: see our best fishing spots in Texas
- Northern Pike: see our best fishing spots in Minnesota
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