The Colorado Koi web site has a couple of fairly detailed articles on koi growth and koi weight koi weight .

Based on their discussion, a estimation of the weight in pounds of a koi with average build would be the koi’s length in inches from the tip of head to tip of tail cubed and divided by 1650.

W = L³/1650

Based upon this formula:

Of course, the weight of a particular koi might easily vary from the formula by 50% one way or another. Very young koi or and certain slender-built males may weigh a good bit less than the formula indicates. Some robust adults and egg-laiden females may way a good bit more than the formula indicates. So the formula is a useful tool to estimate the weight of a single koi, but even more useful to estimate the total weight of a group of koi.

There are some who think that the formula above overestimates the weight of the average koi, and that a typical 12 inch koi weighs closer to 3/4 of a pound than a whole pound. If you think that is more accurate, you can modify the formula above by simply changing the "1650" factor to 2300.

On a related topic, the same site referenced above has some good information of typical koi growth rates in percent of the koi's adult length at various ages:

1 year: 29%
2 years: 50%
3 years: 64%
4 years: 75%
5 years: 82%
6 years: 88%
7 years: 91%
8 years: 94%
10 years: 95%
14 years: 99%

Now on to the pond stocking level discussion.

Ray Jordan used to have an article ( that I can no longer find on the internet) recommending a conservative approach to pond stocking levels, saying,"With a very good filter system circulating your water every 1-1/2 to 2 hours, you should still stay well under 1-lb. Of fish per 100-200 gallons of water." If a 6000 gallon pond were stocked at 100 g/lb, it would have 60 lb of koi. This could be about 60 - 12" koi or 12 - 20" koi or about 7 - 24" koi or 3 - 32" koi using the Colorado Koi calculation.

Larry Lunsford has some detailed information on his site. Of particular interest on this topic are the pages on koi weight, stocking density, and a MAKC survey. His weight tables assume koi are a bit thinner than the Colorado Koi site does. On his pond stocking density page, Larry speaks positively of a hypothetical 5000 gallon pond with 50 to 150 lb of koi. This is a ratio of 33 to 100 g/lb.

The MAKC survey, provides some survey results for 82 koi pond owners. In that survey the maximum stocking rate of anyone was 30 g/lb. If a 6000 gallon pond were stocked at that level, it would have 200 lb of koi. This could be about 200 - 12" koi or 40 - 20" koi or 25 - 24" koi or 10 - 32" koi. The median stocking rate of anyone was 110 g/lb. If a 6000 gallon pond were stocked at that level, it would have 55 lb of koi. This could be about 55 - 12" koi or 11 - 20" koi or 6 - 24" koi.

In his 1973 book, "Koi of the World", Dr. Herbert Axelrod provides photos of many ponds for which he provides the surface area, depth, number of koi, and size range. For the ponds with very broad ranges of sizes, like 10 cm to 90 cm, the reader cannot estimate the stocking level in gallons per pound with any reasonable accuracy. But there are several with a more narrow size ranges, like 50 cm to 75 cm or 50 cm to 100 cm, for which one can calculate a reasonably accurate stocking level.

For example, the pond at the bottom of page 28 is 52 inches (4.33 feet) deep with 1089 square feet of surface area. It has 50 koi ranging from 50 cm to 100 cm. By approximating the length of the koi as if they were all 75 cm (29.5"), we get an average koi weight of (29.5 x 29.5 x 29.5)/1650 = 15.6 lb. Multiplying by 50, that is 780 lb of fish. The pond volume is 1089 x 4.33 x 7.5 = 35,365 gallons. The estimated stocking level is then 35,365/780 = 45 g/lb. If one were to assume that the koi only average 27", the stocking level works out to 59 g/lb.

Below are the stocking levels of some of the other ponds in the book. I did not include those that were said to constantly get water changes from natural springs or wells. The filtration systems of these ponds were not described.

I suspect most of the ponds chosen were the most eye-catching for book. Thus they probably tended to be the more heavily stocked ponds. And just because the pond looks good, does not necessarily mean it is stocked to a safe level.

I have no recommendation on stocking levels here. I am just providing some information on the topic. As you can see, the opinions vary widely. For what it is worth, as of July 2006, I have about 100 lb of koi in a two pond system with a total of about 6000 g. So it is now at about 60 g/lb, but the stocking density is increases with every meal.

[July 2013 update: I have since moved and now have a 12,000 g pond with about 200 lb of koi. This also works out to about 60 g/lb.]