Fish Age from Length Calculator
Estimate the age of a fish from its length.
Uses species-specific growth models for trout, bass, walleye, and common freshwater species.
Fish age can be estimated from length using species-specific von Bertalanffy growth models. This is an approximation — actual age varies significantly between populations, water temperatures, food availability, and geographic location.
Von Bertalanffy growth model: L(t) = L∞ × (1 − e^(−K × (t − t₀)))
Where:
- L(t) = length at age t
- L∞ = asymptotic maximum length
- K = growth coefficient
- t₀ = theoretical age at length zero
Rearranged to estimate age from length: t = t₀ − (1/K) × ln(1 − L/L∞)
Typical maximum lengths and growth rates:
| Species | Max Length | Growth Pattern |
|---|---|---|
| Rainbow Trout | 70–100 cm | 10–15 cm/year early |
| Brown Trout | 80–100 cm | 8–12 cm/year early |
| Largemouth Bass | 60–80 cm | 8–15 cm/year |
| Walleye | 80–100 cm | 10–15 cm/year |
| Northern Pike | 100–150 cm | 15–25 cm/year |
| Atlantic Salmon | 100–120 cm | Variable |
Why age matters for conservation: Older, larger fish are disproportionately important for reproduction. A 50 cm female trout may produce 3–5x more eggs than a 30 cm female. Catch-and-release of trophy fish preserves the breeding population and future fishing quality.
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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