Fish Age from Length Calculator

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:

These are the seven species in the dropdown, with the two numbers each one feeds into the model:

A higher K means the fish approaches its maximum length faster, which is why a yellow perch is most of the way there by age four while a pike is still adding length at fifteen. The t₀ value is fixed at −0.5 for every species, which is the usual placeholder when a population-specific fit is not available.

Both numbers vary between populations more than they vary between these species. A rainbow trout in a cold, food-poor mountain stream may top out at 35 cm, while the same fish in a tailwater below a dam can pass 70 cm. Treat the answer as a bracket of a year or two either side.

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.