Fish Weight from Length Calculator
Estimate the weight of a fish from its length using the standard fisheries biology length-weight formula W = a * L^b. Select the species, enter the total length in inches, and the calculator returns the estimated weight in pounds. Constants are derived from published US Fish and Wildlife Service and state agency data.
Length-weight formula
W = a × Lb
Where W is weight in pounds, L is total length in inches, and a and b are species-specific empirical constants from fisheries biology surveys. The exponent b is typically close to 3, reflecting the cubic relationship between linear dimension and volume (and hence mass) for fish of uniform shape.
Species constants used in this calculator
| Species | a | b | Source |
|---|---|---|---|
| Largemouth Bass | 0.00890 | 3.128 | USFWS |
| Smallmouth Bass | 0.00832 | 3.131 | USFWS |
| Walleye | 0.000228 | 3.180 | USFWS |
| Northern Pike | 0.000180 | 3.096 | USFWS |
| Rainbow Trout | 0.000522 | 3.095 | USFWS |
| Channel Catfish | 0.00370 | 3.168 | USFWS |
| Bluegill | 0.01800 | 3.106 | USFWS |
| Crappie | 0.00870 | 3.124 | USFWS |
Fish weight from length: frequently asked questions
How is fish weight estimated from length?
Fish weight is estimated using the standard fisheries biology length-weight relationship: W = a * L^b, where W is weight, L is length, and a and b are species-specific constants determined from field data. This formula is published by state fish and wildlife agencies and the US Fish and Wildlife Service.
Why does the formula use different constants for different species?
Fish body shape varies by species. A chunky bass has a different length-to-weight ratio than a slender pike or trout. The constants a and b are empirically derived from thousands of fish measurements for each species, so the formula accounts for these shape differences.
How accurate is the length-weight estimate?
The formula gives an expected weight for an average fish of that length and species. Individual fish vary based on body condition, season, and food availability. Estimates are typically accurate to within 10 to 20 percent for healthy fish in normal condition. A fish in poor condition will weigh less than estimated; one in excellent condition may weigh more.
Should I measure total length or fork length?
Most fisheries agencies in the US use total length (tip of the mouth to tip of the tail, measured straight). Fork length (tip of the mouth to fork of the tail) is used for some pelagic species. This calculator uses total length for freshwater species. Check your state agency guidance for the species you are measuring.
Why would I estimate weight rather than weigh the fish directly?
Catch-and-release anglers often estimate weight to minimise handling time and stress on the fish. It is also useful when you do not have a scale on board and want a quick reference. Weight estimates are used in fishing tournaments where live-release is practised after a quick measurement.
Official sources
- US Fish and Wildlife Service: USFWS Fisheries Program.
- Anderson, R.O. and Neumann, R.M. (1996). Length, weight, and associated structural indices. In Murphy, B.R. and Willis, D.W. (eds.), Fisheries Techniques, 2nd ed. American Fisheries Society, Bethesda, MD.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.