Some Koi can get to be a meter long which is 100 centimeters, 3.28 feet or 39.37 inches.
Obviously their size depends on the environmental conditions such as pond size, oxygen concentration, water temperatures, water quality, amount and type of food. Vitamins and exercise may also play an important role.
In the mid 1980's in Lake Biwa in Japan they netted a 6.5 foot Cyprinus carpio, but it died in a public aquarium in Kyushu. There are other examples of carp reaching enormous sizes in large bodies of water.
It appears that depending on their age and under optimum conditions Koi can grow over 2 centimeters (almost an inch) per month or faster, remember there are 2.54 cm in an inch. Check out the growth rates in the following graphs.
Just click on the charts to expand them - then click on your browser's back arrow to return.
Graph A shows length in cm versus age in months.
Graph B shows length of Koi divided by age which is a rough gauge of growth rate.
Graph C is of an empirical equation that shows length in centimeters and inches versus age in months.
Graph D is the 1st derivative of the above equation (Graph C) which is another measure of the rate of growth versus age. It is only as accurate as the above equation which although questionable can be improved with more data.
The true rate of growth in centimeters per month as a function of the Koi's age is probably somewhere between Graphs B & D, both of which show fish need to be fed well particularly during their prime growth period, when under 2 years of age.
We would appreciate any comments and age versus length data you can send us to make these graphs more accurate.