Fish Weight Formula

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Posted by Marty Seldon on 2005-04-15 21:34:39 in reply to Estimated permit weight? posted by Nassim Joaquin on 2005-04-10 11:27:44

After talking to a number of anglers including Dan, I posted the following formula on the FFF web site eight years ago.

Using 800 as a constant Nassim's permit is 19 pounds,13 ounces. With K = 900, its a 14# 10oz fish. The problem, is that the formula uses Girth not width and the aspect ratio of a Permit is quite different from a fat bodied fish.

I'd like to see a series of Permit Boga's and measurements to test the formula.

Federation of Fly Fishers,
Formula for Determining the Weight of Fish
25 February, 1997, Marty Seldon

The use of formulas to make estimates is common in medical practice and many other fields. They generally use scattered data points and yield approximate results. In determining the weight of fish, the value of the constant (K) is adjusted dependent
on the species of fish and how fat or skinny it is. The formula for fish weight is:

The (Girth) squared in inches, times the (Length) in inches, divided by a (Constant = K) is equal to (=) the weight of the fish in either pounds or grams.

Using inches and pounds:

For slender fish similar to trout: K = 900
For fat bodied fish such as a permit or a fat black bass; K = 800

Fat Black Bass Example: Girth = 18 in and the Length = 17 in.

18 x 18 = 324
324x 17 = 5,508
5,508/800 = 6.9 pounds, the weight of the fish

Trout Example: Girth = 13 in. and Length = 16 inches

13 x 13 = 169
169 x 16 = 2704
2704/900 = 3 pounds, the weight of the fish

Using the Metric System in centimeters and grams:

Girth and the Length are in centimeters (1 in = 2.54 cm) and the weight is in grams (1 pound = 453.6 grams). The formula is the same except:

For a slender fish similar to a trout, K = 31.1
For a fat fish such as a largemouth bass or a permit, K = 27.8

Fat Black Bass Example: Girth = 45 cm, Length = 43 cm

45 x 45 = 2025
2025 x 43 = 87,075
87,075/27.8 = 3132 gms = 3.13 kg = 6.9 pounds

Trout Example: Girth = 32 cm, Length = 41.5 cm:

32 x 32 = 1024
1024 x 41.5 = 42,496
42,496/31.1 = 1366 gms = 1.37 kg = 3 pounds.

Well known angler and author Dan Blanton advises that the Constant K= 800/27.8 should be used for bonefish, tarpon, redfish, striped bass, and similar species. Barracuda, pike, and trout that are not excessively fat or girthy are more accurately described, as are Pacific bonefish that have less girth by using K = 900/31.1.


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