Fulton's condition factor calculator
A simple tool to calculate trout condition (K-factor), length or weight from the two other numbers.
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.
The tool first appeared in this article about an amazing fish.
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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