Published Mar 20. 2017 - 1 month ago
Updated or edited Mar 20. 2017

Fulton's condition factor calculator

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

A well conditioned sea run brown trout
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 cannon ball, 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 fin the numbers in a table below.

Fulton's condition factor calculator

Calculates trout condition (K-factor), length or weight from the two other numbers.

Factor (K)  =
 100 *  g / 
cm ^3


Fill out any two and press Calculate. A factor 1 indicates a normal condition fish.

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

Sea trout
Sea trout
Sea trout
Sea trout
Sea trout
Henning Eskol - Nils Jorgensen

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

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