Fulton's condition factor calculator

A simple tool to calculate trout condition (K-factor), length or weight from the two other numbers.

Published on Mar 20. 2017 - 8 years ago
Updated or edited 1 years ago

Trout

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Martin Joergensen

A well conditioned sea run brown trout

Henning Eskol

This small tool enables you to calculate the weight, length or condition of a fish using math.

The Fulton formula

It's possible to estimate the weight of a trout based on what's known as the K-factor or the condition – you could call it the BMI or health index of the fish. The factor is calculated using the Fulton formula. Fulton suggested this in the early 1900's:
K = 100 × weight/length^3
Weight is in grams and length is in centimeters.

So the weight of the fish divided by the length cubed will give you a number. Fulton multiplied the number with 100 to get it close to 1, and it so fits that the number 1 indicates a “normal” fish in OK condition. A really fat fish will be higher, like 1.2 or even 1.5 for a cannonball, while a skinny fish will be below 1, like 0.8 or less for a post spawning kelt or a really skinny winter fish.
You can utilize this to estimate the weight of a fish based on its length and condition.
I have made small tool for you to do these calculations. Fill in two of the numbers, and the math gives you the missing one. You can also find the numbers in a table below.

We are working on an improved version of the form. Please use the table below and return later.

The tool first appeared in this article about an amazing fish.

Sea trout

Henning Eskol - Nils Folmer Jørgensen

A table with lengths and weights for trout in different conditions

Use this to find the weight of your fish by finding its length in the table and then the weight depending on the condition of the fish.

Centimeters	Inches	Slim fish, K-factor 0.8		Normal fish, K-factor 1		Fat fish, K-factor 1.2
25	10	0.125 kilos	0.28 lbs	0.156 kilos	0.34 lbs	0.188 kilos	0.41 lbs
27.5	11	0.166 kilos	0.37 lbs	0.208 kilos	0.46 lbs	0.25 kilos	0.55 lbs
30	12	0.216 kilos	0.48 lbs	0.27 kilos	0.6 lbs	0.324 kilos	0.71 lbs
32.5	13	0.275 kilos	0.61 lbs	0.343 kilos	0.76 lbs	0.412 kilos	0.91 lbs
35	14	0.343 kilos	0.76 lbs	0.429 kilos	0.95 lbs	0.515 kilos	1.13 lbs
37.5	15	0.422 kilos	0.93 lbs	0.527 kilos	1.16 lbs	0.633 kilos	1.4 lbs
40	16	0.512 kilos	1.13 lbs	0.64 kilos	1.41 lbs	0.768 kilos	1.69 lbs
42.5	17	0.614 kilos	1.35 lbs	0.768 kilos	1.69 lbs	0.921 kilos	2.03 lbs
45	18	0.729 kilos	1.61 lbs	0.911 kilos	2.01 lbs	1.09 kilos	2.4 lbs
47.5	19	0.857 kilos	1.89 lbs	1.07 kilos	2.4 lbs	1.29 kilos	2.8 lbs
50	20	1 kilos	2.2 lbs	1.25 kilos	2.8 lbs	1.5 kilos	3.3 lbs
52.5	21	1.16 kilos	2.6 lbs	1.45 kilos	3.2 lbs	1.74 kilos	3.8 lbs
55	22	1.33 kilos	2.9 lbs	1.66 kilos	3.7 lbs	2 kilos	4.4 lbs
57.5	23	1.52 kilos	3.4 lbs	1.9 kilos	4.2 lbs	2.3 kilos	5 lbs
60	24	1.73 kilos	3.8 lbs	2.2 kilos	5 lbs	2.6 kilos	6 lbs
62.5	25	1.95 kilos	4.3 lbs	2.4 kilos	5 lbs	2.9 kilos	6 lbs
65	26	2.2 kilos	5 lbs	2.7 kilos	6 lbs	3.3 kilos	7 lbs
67.5	27	2.5 kilos	5 lbs	3.1 kilos	7 lbs	3.7 kilos	8 lbs
70	28	2.7 kilos	6 lbs	3.4 kilos	8 lbs	4.1 kilos	9 lbs
72.5	29	3 kilos	7 lbs	3.8 kilos	8 lbs	4.6 kilos	10 lbs
75	30	3.4 kilos	7 lbs	4.2 kilos	9 lbs	5.1 kilos	11 lbs
77.5	31	3.7 kilos	8 lbs	4.7 kilos	10 lbs	5.6 kilos	12 lbs
80	31	4.1 kilos	9 lbs	5.1 kilos	11 lbs	6.1 kilos	14 lbs
82.5	32	4.5 kilos	10 lbs	5.6 kilos	12 lbs	6.7 kilos	15 lbs
85	33	4.9 kilos	11 lbs	6.1 kilos	14 lbs	7.4 kilos	16 lbs
87.5	34	5.4 kilos	12 lbs	6.7 kilos	15 lbs	8 kilos	18 lbs
90	35	5.8 kilos	13 lbs	7.3 kilos	16 lbs	8.7 kilos	19 lbs
92.5	36	6.3 kilos	14 lbs	7.9 kilos	17 lbs	9.5 kilos	21 lbs
95	37	6.9 kilos	15 lbs	8.6 kilos	19 lbs	10.3 kilos	23 lbs
97.5	38	7.4 kilos	16 lbs	9.3 kilos	20 lbs	11.1 kilos	25 lbs
100	39	8 kilos	18 lbs	10 kilos	22 lbs	12 kilos	26 lbs
102.5	40	8.6 kilos	19 lbs	10.8 kilos	24 lbs	12.9 kilos	28 lbs
105	41	9.3 kilos	20 lbs	11.6 kilos	26 lbs	13.9 kilos	31 lbs
107.5	42	9.9 kilos	22 lbs	12.4 kilos	27 lbs	14.9 kilos	33 lbs