Free tool
Fish age calculator
| Age | Length | Weight |
|---|---|---|
| 1 year | 5.9 in | 0.10 lb |
| 2 years | 10.3 in | 0.61 lb |
| 3 years | 13.6 in | 1.5 lb |
| 4 years | 16.2 in | 2.6 lb |
| 5 years | 18.1 in | 3.7 lb |
| 6 years | 19.5 in | 4.7 lb |
| 8 years | 21.4 in | 6.4 lb |
| 10 years | 22.5 in | 7.5 lb |
| 12 years | 23.2 in | 8.2 lb |
From a von Bertalanffy growth curve. Fish in warm, food-rich water grow faster than this and fish in cold or crowded water grow slower, so treat every row as a rough middle.
An honest note. Age from length is a rough estimate. The same species grows at very different speeds in different water and different parts of the country, so treat the answer as a ballpark with a year or two of slack either way.
How the age is worked out
Fish grow fast when they are young and slow down as they get close to their top size. The standard way to describe that is the von Bertalanffy growth curve:
Length at age t = L∞ x (1 − e^(−K x t))
L∞ is the length a typical fish of the species tops out at, and Kis how fast it gets there. Each year the fish closes the same share of the gap between its current length and L∞, so the gains shrink as it grows. The calculator runs that curve backwards to turn a length into an age.
It is the same curve fishline uses for the growth projection on a released fish: where it might be in one, two and three years if it goes back in the water.
Close to L∞ the curve flattens out, and a fish can sit at nearly the same length for years. When you enter a length near or past the top of the curve, the calculator says so instead of making up a number.
Growth curves by species
The numbers behind the calculator. A higher K means a species reaches its top size sooner.
| Species | Typical top length | K | Half of top length at |
|---|---|---|---|
| Largemouth bass | 24 in | 0.28 | 2 to 3 years |
| Smallmouth bass | 21 in | 0.26 | 2 to 3 years |
| Spotted bass | 19 in | 0.3 | 2 to 3 years |
| White bass | 18 in | 0.35 | about 2 years |
| Striped bass | 44 in | 0.15 | 4 to 5 years |
| Walleye | 30 in | 0.2 | 3 to 4 years |
| Sauger | 22 in | 0.25 | 2 to 3 years |
| Yellow perch | 14 in | 0.35 | about 2 years |
| Northern pike | 44 in | 0.18 | about 4 years |
| Muskellunge | 54 in | 0.15 | 4 to 5 years |
| Chain pickerel | 26 in | 0.25 | 2 to 3 years |
| Black crappie | 16 in | 0.35 | about 2 years |
| White crappie | 17 in | 0.35 | about 2 years |
| Bluegill | 11 in | 0.4 | 1 to 2 years |
| Redear sunfish | 13 in | 0.38 | 1 to 2 years |
| Channel catfish | 36 in | 0.15 | 4 to 5 years |
| Blue catfish | 54 in | 0.1 | about 7 years |
| Flathead catfish | 50 in | 0.12 | 5 to 6 years |
| Rainbow trout | 26 in | 0.3 | 2 to 3 years |
| Brown trout | 28 in | 0.25 | 2 to 3 years |
| Brook trout | 17 in | 0.4 | 1 to 2 years |
| Lake trout | 40 in | 0.1 | about 7 years |
| Chinook salmon | 42 in | 0.35 | about 2 years |
| Coho salmon | 32 in | 0.4 | 1 to 2 years |
| Common carp | 36 in | 0.2 | 3 to 4 years |
| Freshwater drum | 30 in | 0.2 | 3 to 4 years |
| Redfish | 42 in | 0.25 | 2 to 3 years |
| Speckled trout | 27 in | 0.35 | about 2 years |
| Flounder | 24 in | 0.3 | 2 to 3 years |
| Snook | 38 in | 0.25 | 2 to 3 years |
| Tarpon | 72 in | 0.1 | about 7 years |
| Mahi-mahi | 55 in | 0.9 | under 1 year |
Questions
How old is a 20 inch largemouth bass?
On an average growth curve, 6 to 7 years. Bass in warm southern water often get there sooner and bass up north often take longer, so a 20 inch fish can be a few years either side of that.
How old is a 20 inch walleye?
The curve puts a 20 inch walleye at 5 to 6 years. Walleye in big, fertile lakes grow faster than walleye in cold, clear water, so the real age can differ.
How do biologists tell a fish’s age?
They count yearly growth rings, much like tree rings, on scales, fin rays or the otoliths (small ear stones). That is the only way to know for sure. Length gives a fast, rough estimate.
Why is age from length so rough?
Growth depends on water temperature, the length of the growing season, food, and how many other fish are competing for it. Two fish of the same length from different lakes can be years apart, and once a fish nears its top size it barely grows, so length stops telling you much.
Need the weight as well? Try the fish weight calculator.
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