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.