
Koi gain the most amount of weight at the age of 3 years, when they can gain almost a 1/4 pound (4 ounces) a month. Many of us would like to know how much our Koi weigh, without killing our Koi or ourselves in getting the result. This becomes very important for knowing how much medicine to inject into a sick Koi. We want to know if there is a Koi weight length relationship?
It turns out that fish anglers have been working on this problem for some time. Their formula is: Girth x Girth x Length / 800 = pounds (G^{2} x L)/800; or (G^{2} x L)/50 = ounces. This formula can be found in many places including Canadian, South Dakota and Minnesota's Fish and Game departments, etc. Obviously, the result will represent the mean, so there will be some fish lighter and others that are heavier. One fellow that specializes in catching "Musky" in Wisconsin has refined the divisor to 754.5 instead of 800, but he says the formula falls apart with Musky over 55 pounds. The Girth measurement is taken just in front of the dorsal fin. The length is from the tip of the nose to the tip of the tail. Because the Girth measurement turns out to be too much trouble, the anglers devised another formula where they assume the Girth is 0.58 x Length. Using this assumption the formula becomes; Length x Length x Length / 2378 = pounds; (L^{3}/2378). Other refinements to the divisor, allow for different Girth proportions for different shapes of fish; they change the 2378 to 1600 for Bass and Carp (that's the Koi species), for Pike 3500, for Perch 2700, and for Crappie 1250. This allows a weight calculation based solely on the fishes length. The range where this formula works is unknown. It may fall apart on very small or very large fish. Click on the graph below to see how this formula predicts a generalized fish weight based on the 2378 divisor (see further down for more accurate Koi data): Here is a table based on the same formula using the 2378 divisor (see further down for a more correct Koi chart):
I weighed a 5.0" Koi at 34.5 grams, or 1.22 ounces, or almost 50% heavier than the above formula predicts, which is a weight of 0.87 ounces. So that means the divisor should be 1650 for this particular Koi, which is pretty close to the anglers' divisor of 1600 for Carp. Here is a table based on the 1650 divisor (L^{3}/1650)=pounds; (L^{3}/103)=ounces:
Obviously, we can't rely on just a few measurements from one Koi hatchery. If you have any weight and length measurements for your Koi, we would appreciate your sending them to us, so we can further refine the charts and tables. For instance, how do butterfly koi fit this table? 
Phone: 3038838000 email: ColoradoKoi@ymail.com Postal Address: 5082 E. Hampden Ave, Suite #138 Denver, Colorado 80222 Copyright © 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009,2010, 2011, 2012, 2013, 2014, 2015 Colorado Koi, Inc. All Rights Reserved. 