by Rex Gibson
We often hear the expression “That’s a good fish”. But is it really? There are recognised measurements that give an angler the answer.
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 = Condition Factor Formula
Weight is in grams and length is in centimetres. 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.
Fulton’s condition factor calculator
Calculates trout condition (K-factor), length or weight from the two other numbers.
Factor (K) 
= 100 * 
grams / 
centimetres ^3 (cubed)
Fill out any two and Calculate.
K=100xW/L3
A factor 1 indicates a normal condition fish.
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.
Cms Inches Slim fish, K= 0.8 Normal fish, K= 1 Fat fish, K-factor 1.2
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 kilo 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
In New Zealand you often see the Condition Factor expressed differently as a much larger number. It is really just a variation on the Fulton Formula. Check out the superfly.co.nz website if you want more info.
The condition factor is arrived at by dividing its weight (in grams) by its cubed length (in centimetres) from the tip of its nose to the fork of its tail) as above and multiplying the result by 3612.8. This may sound complicated, but in this day and age of pocket calculators, it is very simple and takes little time.
There is not much chance of most of us carrying a calculator whilst fishing, other than our phones, so sticking with the internationally recognised Fulton Factor is probably best. All you need is the weight, length and access to the table above.
Check out the two fish below. Both caught on the same day in the Mackenzie lakes. Both were jacks and both weight in at 5 lbs (2268 grams). The first was 59 cm long = 23.5 in (with a girth of 30 cm); the second was 56 cm long= 22 inches with a girth of 33 cm.
The girth is important for the finer points of visual appraisal of condition but is not usually included in calculations.
This fish (above) has a K Factor of 1.1 (39.9 rating in Superfly)
The fish below has a K Factor of 1.3 (46.7 rating in Superfly)
Rex N. Gibson