Fish Weight from Length Calculator
Estimate fish weight from length and girth measurements.
Useful for catch-and-release fishing without a scale.
Works for bass, trout, pike, and more.
Fish weight from length is estimated using a length-weight relationship formula established through population studies by fisheries biologists. Since weighing a live fish in the field is difficult, this formula lets anglers estimate weight from a simple length measurement.
Standard formula (bass, walleye, pike):
Weight (lbs) = (Length^3 × Girth^2) / 800
For largemouth bass specifically, a simpler body-condition formula is widely used:
Weight (lbs) = Length^3 / 1,200
Where Length is measured in inches (tip of mouth to tip of tail, held straight).
Worked example — largemouth bass: Length = 18 inches Weight = 18^3 / 1,200 = 5,832 / 1,200 = 4.86 lbs
Species-specific divisors:
| Species | Divisor |
|---|---|
| Largemouth bass | 1,200 |
| Smallmouth bass | 1,350 |
| Walleye | 2,700 |
| Northern pike | 3,500 |
| Muskellunge | 3,500 |
| Rainbow trout | 3,500 |
When girth is available (most accurate): Weight = (Girth^2 × Length) / 800
- Measure girth at the widest point (just behind the pectoral fins)
- This formula works well across many species
Reference — largemouth bass weight by length:
| Length | Estimated Weight |
|---|---|
| 12 in | 0.72 lbs |
| 15 in | 1.41 lbs |
| 18 in | 2.43 lbs |
| 20 in | 3.33 lbs |
| 22 in | 4.44 lbs |
Fish that weigh significantly more than the formula predicts are considered above average condition for their population — a healthy sign in well-managed waters.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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